fisheries and aquaculture

A DISTANCE COURSE MODULE

ON

Fisheries and Aquaculture

(Biol 421)

For Biology Students in Summer In-Service Program

By

Mulugeta Wakjira (M.Sc.)

Department of Biology

Jimma University

Editor

Tadesse Habtamu (M.Sc.)

Department of Biology

Jimma University

© CDE, Jimma University, Ethiopia

June, 2011

Jimma, Ethiopia

Mulugeta Wakjira, Department of Biology, Jimma University, June 2011, Page i

Table of Contents

Table of Contents ..................................................................................................... i

Module Introduction ............................................................................................ iii

Chapter 1 Diversity of Fishes ............................................................................... 1

1.1. Diversity by Taxonomic Groups ............................................................................ 2

1.2. Diversity by Size, Habitat Type and Age .......................................................... 10

1.3. Diversity by Sexuality and Brooding Behaviour .......................................... 11

1.4. Diversity by Feeding Behaviour ......................................................................... 12

Chapter 2 Fish Reproduction and Development .............................................. 14

2.1. Fish Reproduction .................................................................................................... 15

2.1.1. Patterns of Sexuality in Fishes ......................................................... 15

2.1.2. Sites of Embryo Development in Fishes .......................................... 16

2.1.3. Fertilization and Spawning in Fishes ............................................... 17

2.1.4. Reproduction in Cartilaginous Vs Bony Fishes ............................... 22

2.2. Fish Growth, Development and Recruitment ................................................ 23

Chapter 3 Capture Fisheries .............................................................................. 27

3.1. Introduction................................................................................................................ 28

3.2. Fishing Gears and Crafts ........................................................................................ 32

3.3. Fishing Methods ........................................................................................................ 38

3.4. Commercially Important Fishes ......................................................................... 43

Chapter 4 Aquaculture ....................................................................................... 46

4.1. Introduction................................................................................................................ 47

4.2. Types of Aquaculture .............................................................................................. 49

4.3. Aquaculture Systems .............................................................................................. 54

4.4. Establishment of Aquaculture ............................................................................. 57

4.5. Sustainability of Aquaculture .............................................................................. 61

Chapter 5 Status and Prospects of Capture Fisheries and Aquaculture ........ 65

5.1. World Case .................................................................................................................. 66

5.2. Ethiopian Case ........................................................................................................... 69

Chapter 6 Benefits of Fisheries .......................................................................... 77

Mulugeta Wakjira, Department of Biology, Jimma University, June 2011, Page ii

6.1. Nutrition and Socio-economic Benefits .......................................................... 78

6.1.1. Subsistence or Artisanal Fishing ...................................................... 78

6.1.2. Industrial and Recreational Fishing................................................. 83

Chapter 7 Processing and Products of Fish ...................................................... 85

7.1. Fish Processing Methods ....................................................................................... 86

7.1.1. Post-mortem Changes and Fish Quality .......................................... 86

7.1.2. Processing Methods .......................................................................... 92

7.2. Types of Fish Products ........................................................................................... 98

Chapter 8 Fisheries Management .................................................................... 100

8.1. Stock Assessment ................................................................................................... 101

8.1.1. Definitions and Stock Concept ....................................................... 101

8.1.2. Survey of Stock Population ............................................................. 103

8.1.3. Quantitative Estimation of Stock Population ................................ 106

8.2. Sustainable Exploitation of Fisheries Resources ....................................... 108

Chapter 9 Fish Parasites and Diseases ............................................................ 114

9.1. Fish Immunity .......................................................................................................... 115

9.2. Major Fish Parasites .............................................................................................. 116

9.2.1. Protozoan Parasites of Fishes ....................................................... 119

9.2.2. Helminthic Parasites of Fishes ....................................................... 122

9.2.3. Copepod Parasites of Fishes ........................................................... 138

9.3. Major Fish Diseases ............................................................................................... 140

9.3.1. Bacterial Diseases of Fishes ............................................................ 140

9.3.2. Viral Diseases of Fishes ................................................................... 144

9.3. 3. Fungal (Mycotic) Diseases of Fishes ............................................. 146

References .......................................................................................................... 148

Appendix 1: Data on the status of world fisheries production by FAO (2000-

Table1), FAO (2008-Table 2) and FAO (2010-Table3). ................................. 151

Appendix 2: Assignment Questions ................................................................. 154

Mulugeta Wakjira, Department of Biology, Jimma University, June 2011, Page iii

Module Introduction

Dear student, this course (Biol 421) follows the Aquatic Sciences and

Wetland Management (Biol 302) course. Therefore, it is necessary that you

recall your past knowledge of the course Biol 302 for better understanding of

the present course.

The course Fisheries and Aquaculture (Biol 421) introduces you to basic

ideas and concepts in fisheries and aquaculture. Much emphasis has not been

given to and only basic concepts have been addressed on fish biology due to

credit hour constraint. Much emphasis has been given to the fisheries and

aquaculture. A student taking this course is, thus, required to refer to chapter

6 of the course Aquatic Sciences and Wetland Management (Biol 302)

besides what is given in this module on aspects of fish biology.

The present module has been organized into nine chapters. The first and

second chapters deal with the diversity and reproduction of fish respectively.

These two chapters appear to provide you with some basic concepts on the

diversity and reproductive biology of fishes so that you would better

understand the subsequent chapters. Chapters three and four discuss about

the various aspects of capture fisheries and aquaculture, the two broad

categories of fisheries practices. The chapters give distinctions between the

two forms of fisheries and other underlying fundamental concepts. Chapter 5

addresses the status and prospects of capture fisheries and aquaculture on a

global and national (Ethiopian) scale. Here various limitations associated

with capture fisheries and aquaculture, and thus the prospect of both sectors

has been discussed.

Mulugeta Wakjira, Department of Biology, Jimma University, June 2011, Page iv

In chapters six and seven the benefits of fisheries and the methods of

processing fish products have been discussed respectively. Chapter six has

incorporated points on the various socio-economic benefits generated by

human being from the fisheries resources. Chapter seven discusses the

various methods employed to process fish and fish products given the easily

perishable nature of fish.

Chapter eight deals with fisheries management where the concept of

sustainable exploitation of fisheries resources and related issues are raised.

In the last chapter fish parasites and diseases that are important especially in

aquaculture practices, and thus are of major socio-economic concern, have

been widely addressed.

Moreover, presentation of the module is in such a way that "guiding

questions” are frequently posed at intervals so that a student can

understand and learn the concepts easily. Moreover, at the end of each

chapter come questions related to the chapter and course or module

objectives. Thus, after completing every chapter the student should test

him/herself of the mastery of the chapter by answering all the questions.

Finally, an assignment comprising of various items form the whole chapters

has been attached as Appendix 2 at the end of the module. Dear student,

you are thus expected to copy all the questions down on a separate sheet of

paper, work out all the questions and submit it to your course instructor

upon completion of the course. The assignment constitutes a minimum of 25

% of your total marks.

Mulugeta Wakjira, Department of Biology, Jimma University, June 2011, Page v

Course or Module Objectives

Upon successfully completing this course, you will be able to:

� Discuss the diversity of fishes

� Discuss the various modes of sexuality and reproductive behaviours

in fishes.

� Distinguish between capture fisheries and aquaculture

� Discuss the status and potential of fisheries and aquaculture both on

the global and Ethiopian scale

� Discuss the various socio-economic benefits of fisheries to human

being especially in relation to ensuring food security for the needy

community.

� Mention and discuss the various fish processing and preservation

methods

� Explain the idea of fisheries management

� Identify and discuss the important fish parasites and diseases

Mulugeta Wakjira, Department of Biology, Jimma University, June 2011, Page 1

Diversity of Fishes

Chapter Outline

1.1. Diversity by Taxonomic Groups

1.2. Diversity by Size, Habitat Type and Age

1.3. Diversity by Sexuality and Brooding Behaviour

1.4. Diversity by Feeding Behaviour

Chapter Objectives

Upon completing this chapter, you will be able to:

� Discuss the characteristics of a typical fish

� Discuss the various fish characteristics that make them more

diverse than any of the vertebrate groups

� Distinguish among the various classes of fishes

� List down the major characteristics of the various fish classes

� Distinguish between the extinct and extant fish groups

� Distinguish among the types of aquatic environments inhabited by

the various groups of fishes

� Discuss habitat diversity of fish

� Discuss the size diversity of fish

� Discuss diversity of fishes in their feeding behavior

� Discuss the diversity of fishes in their reproductive behavior


Fishes are very diverse and are categorized in many ways. For instance,

fishes are diverse with respect to species number, body sizes, habitat type,

age, sexuality, brooding behavior, feeding behavior, locomotion, toxicity,

vulnerability, etc. We will discuss some of these fish diversities as shown

above in the chapter outline.

1.1. Diversity by Taxonomic Groups

A typical fish:

� is ectothermic (i.e. has variable body temperature)

� has a streamlined body for rapid swimming

� extracts oxygen from water using gills

� has two sets of paired fins, usually one or two (rarely three) dorsal

fins, an anal fin and a tail fin

� has jaws

� has skin that is usually covered with scales

� is oviparous i.e. lays eggs.

However, it is important to note that there are many groups of fishes which

are exceptions to these characteristics. For instance,

• Lungfishes have lungs to breathe atmospheric air

• Lamprey and hagfish do not have the paired (i.e. pelvic and

pectoral) fins

• Lamprey and hagfish do not have jaws

Activity:

Dear student, as an introduction to this chapter, from your knowledge of

Aquatic Sciences and Wetlands Management (Biol 302) course module,

chapter 6, would you please mention some of the characteristic features

of a typical fish?


• Some fishes are either viviparous or ovoviviparous i.e. give

birth to live young.

Fish exhibit greater species diversity than any other class of vertebrates. For

instance, according to FishBase about 31,900 species have been discovered

and described by the year 2010. This makes fish to be more diverse than the

combined total of the rest of vertebrates such as amphibians, reptiles, birds

and mammals.

FishBase is a database or information system with key data on major

biological characteristics of fishes such as reproduction, ecology, feeding, etc.

It was developed in 1989 and funded by the European Commission (EC) until

2000 when a consortium of various organizations took over the

responsibility. You can access FishBase on the Internet at its website:

www.fishbase.org

Early fish taxonomic classifications may treat all fishes as monophyletic

group belonging to a single class namely “Class Fish”. Recent studies,

however, revealed that fishes are rather diverse and paraphyletic i.e. fishes

are collections of various groups such as ostracoderms, cyclostomes,

placodermi, acanthodi, chondrichthyes (cartilaginous fishes) and

osteichthyes (bony fishes) that are categorized into different classes.

Activity:

Dear student, before we continue with our discussion of fish species

diversity, can you explain what a FishBase is?

Activity:

Dear student, in the hierarchical classification of organisms, how many

classes of fishes can be recognized?


Therefore, we may rather recognize the taxonomic classification of fishes

into 11 classes as summarised in Table 1.1 though this too may not be

absolutely authoritative. All fishes, however, belong to Kingdom: Animalia,

Phylum: Chordata, and Subphylum: Vertebrata.

Table 1.1. Classes of Fishes

S. No. Class Status Remark Jaw condition

1 Pteraspidomorphi Extinct

Collectively

known as

Ostracoderms

Agnathan

(jawless) fishes

2 Anaspida Extinct

3 Conodonta Extinct

4 Cephalaspidomorphi Extinct

5 Thelodonti Extinct

6 Myxini

(commonly hagfish)

Extant

Collectively

known as

Cyclostomes

7 Pteromyzontia

(commonly lampreys)

Extant

8 Placodermi Extinct Commonly called

armoured fishes

Gnathostoman

(Jawed) fishes

9 Acanthodii Extinct Commonly called

spiny sharks

10 Chondrichthyes Extant Commonly called

cartilaginous

fishes

11 Osteichthyes Extant Commonly called

bony fishes


Agnathan Fishes

Dear student, as you can see from Table 1.1, ostracoderms are entirely

extinct while cyclostomes are the extant group of agnathan fishes. Agnathan

fishes are generally primitive group characterized by:

• Lack of jaws and the paired (i.e. pelvic and pectoral) fins which are

characteristic of more advanced fishes.

• Possession of notochord instead of vertebral column.

The term “cyclostome” has been derived from two words “cyclo” meaning

circular; and “stome” meaning mouth opening. Therefore, the group has been

named so because of its possession of a roughly circular mouth by its

members. Dear student, please refer to the Figures given in chapter 6 of your

Aquatic Sciences and Wetlands Management course module to observe the

circular mouth of agnathan fishes.

Activity:

Dear student, let’s now come to a brief discussion of agnathan fishes and

then proceed to that of the gnathostoman fishes.

Q1. Which of the agnathan fish classes are extinct (i.e. already disappeared)

and which are still extant (i.e. living)?

Q2. What are the major distinguishing characteristics of agnathan fishes?

Activity:

Dear student, why do you think that cyclostomes are named so?


Dear student, the two typical examples of cyclostome fishes with their

feeding habits and habitats they occupy are given in Table 1.2.

Table 1.2. Comparison of the two living groups of agnathan fishes.

Lamprey Hagfish

Feeding habit Suck blood from their hosts;

they are parasitic on other

larger and advanced fishes

Scavengers

Habitat Both freshwater and marine Marine

Gnathostoman Fishes

Dear student, let us now come to the discussion of gnathostoman fishes.

The gnathostoman fishes are generally advanced group over the agnathan

fishes. Unlike the agnathan fishes, these fishes possess paired fins and other

characteristics of fish.

The four groups or classes of gnathostoman fishes are Placodermi,

Acanthodi, Chondrichthyes and Osteichthyes. As you can see from Table 1.1,

Placodermi and Acanthodi are extinct whereas Chondrichthyes and

Activity:

Dear student, from your knowledge of Aquatic sciences and wetland

management course, can you give the two typical examples of

cyclostomes with their feeding habits and habitats they occupy?

Activity:

Q1. How do gnathostoman fishes differ from the agnathan fishes?

Q2. What are the four major groups or classes of gnathostoman fishes?


Osteichthyes are extant groups. Placodermi is assumed to be the first

vertebrate group to develop jaws and paired fins. In evolution it is also

assumed that a branch of placodermi probably gave rise to the two main

modern classes of fish: the Chondrichthyes and Osteichthyes.

Chondrichthyes

Chondrichthyes are commonly known as cartilaginous fishes due to their

possession of an endoskeleton made up of cartilage. Moreover, they are

characterized by their lack of swim bladders, different tail fin construction,

lack of a gill covering (operculum), and a skin covered with tooth-like

structures called denticles or placoid scales giving them a rough sandpaper

appearance. They are almost exclusively marine in distribution.

The cartilaginous fishes include two major subgroups: Elasmobranch or

Selaschii (e.g. sharks, rays and skates) and Holocephalans (e.g. chimaeras

or ratfish).

Osteichthyes

Activity:

Dear student,

Q1. What are the major characteristics of chondrichthyes?

Q2. What kind of habitat d o they occupy?

Q3. What are their major subgroups?

Activity:

Q1. How do osteichthyes differ from the cartilaginous fishes?

Q2. What are the major subgroups of osteichthyes?


Osteichthyes are commonly known as bony fishes because of their

possession of bony endoskeleton as opposed to the cartilaginous fishes.

Moreover, they differ from the cartilaginous fishes by their possession of

swim bladder. Swim bladder generally functions as a float or, in a few fishes,

modified to become lung. The bony fishes are divided into two major

subgroups:

� subclass Sarcopterygii (lobe-finned fishes)

� Subclass Actinopterygii (ray-finned fishes)

Sarcopterygian Bony Fishes

Sarcopterygian bony fishes are characterized by their possession of fleshy

fins with a central supporting bone and distantly located paired fins. These

bony fishes are subdivided into two groups: Dipnoi (lungfishes) and

Crossopterygii (e.g. coelacanth) as in Table 1.3.

Activity:

Q1. What are the major differences between sarcopterygian and

actinopterygian bony fishes?

Q2. What are the major subcategories of sarcopterygian and actinopterygian

bony fishes?


Table 1.3. Sarcopterygian bony fishes

Category Habitat

type

Example Occurre

nce

Remark

Dipnoi

(Lungfishes)

Mainly

freshwater

Lepidosiren America They can

breathe

atmospheric

air using lung

Protopterus Africa

Neoceratodus Europe

Crossopterygii

(Coelacanth)

Mainly

marine deep

seas

Latimeria chalmulae

Comoro

Archipel

ago

islands

(Africa)

L. chalmulae is

called “living

fossil” i.e. it is

the only living

representativ

e of its group

Dear student, please refer to the figures in chapter 6 of your Aquatic sciences

and Wetlands Management course module for observation of fin structure

and location. In evolution it is assumed that crossopterygians are the earliest

bony fishes that probably gave rise to the actinopterygian on one side and to

the tetrapods on the other hand.

Actinopterygin Bony Fishes

Actinopterygians are bony fishes having supporting structures known as rays

in their fins. In this group of bony fishes the paired fins are closely located in

contrast to those of the sarcopterygian bony fishes. These bony fishes are the

most highly successful and diverse of all the fishes and include over 95% of

all living fish species predominating in both the fresh and marine waters.

Thus, they represent an advanced adaptation of the bony fishes to strictly

aquatic conditions. Actinopterygian bony fishes are in turn subdivided into

three subgroups namely chondrostei, holostei and teleostei.


Chondrostei, Holostei and Teleostei

Chondrostei and holostei have are bony fishes that have soft cartilaginous rays

in their fins whereas teleostes have strong spiny rays in their fins. Examples

include (chondrostei: sturgeons, bichirs, paddlefishes, spoon fishes) and

(Holostei: bowfin, garpikes, gars, etc). Teleostei are the most advanced and the

most numerous groups of living fishes that are classified in to a large number of

orders, families, genera, etc. Teleosts are fish groups that are important as food

and thus important in fishery.

Dear student, please refer to chapter 6 of your Aquatic Sciences and Wetland

Management course module for more information on all of these fish groups.

1.2. Diversity by Size, Habitat Type and Age

Fishes come in various body sizes ranging from about 7.9 millimeters (e.g.

Paedocypris progenetica commonly called minnow fish) to about 20 meters long

(e.g. whale shark).

Fishes occur both in freshwater and marine environments, within which they

occupy various types of habitats. For instance, if we consider fishes inhabiting

marine environments such as seas and oceans, we find that some are littoral

(i.e. live closer to shore), some are pelagic (i.e. live in the open water), and some

are demersal or benthic (i.e. occupy near bottom of water). Pelagic fishes, in

turn, occur at various depths of water column. For instance, epipelagic fishes

occupy a depth of 0 to 200 m, mesopelagic fishes occupy 200-1000 m depth

and bathypelagic fishes inhabit a depth below 1000 m, which is very cold.

Refer to section 3.1.2 of your Aquatic Sciences and Wetland Management (Biol

302) course module for the various habitats within the marine environments.

Fishes also vary in terms of their age ranging from the shortest lived fishes (e.g.

goby fishes-whose age is of a few days) to the longest lived fishes (e.g. orange

roughly-whose ages can reach hundreds of years). The oldest fish in captivity,

the Australian lungfish, is also estimated to be more than 75 years old.


1.3. Diversity by Sexuality and Brooding Behaviour

With respect to pattern of sexuality some fishes are gonochores (i.e.

heterosexuals), some are hermaphrodites (i.e. bisexuals) and a few are

unisexual. Dear student, please refer to section 2.1.1 of this module for the

details of sexuality in fishes and for the description of each of the pattern of

sexuality.

Brooding refers to the behaviours of fishes in which they protect and care

their eggs and/or young employing various mechanisms. Most of the fishes

such as Nile tilapia (Oreochromis niloticus) are mouth brooders protecting

their eggs and young by keeping them in the mouth of the parents for

extended period of time. Other fishes (e.g. seahorses) are pouch brooders

nourishing their offspring in a pouch or sac like the mammalian Kangaroo do.

The parent (i.e. the male or the female) that broods or takes care of the eggs

and young fish varies among various species. In some fishes males are

responsible for brooding (this is called paternal brooding-e.g. Sarotherodon

melanotheron), in others females are responsible (this is called maternal

brooding-e.g. Oreochromis niloticus) and in other fishes both parents are

responsible for brooding (this is called biparental brooding-e.g. a catfish

species called Phyllonemus typus).

Activity:

Q1. What is brooding in fishes?

Q2.Can you mention of the various types of brooding behaviours

observed in fishes?


1.4. Diversity by Feeding Behaviour

Fishes have diverse feeding behavior. Fishes vary with respect to their

feeding habits, types of food taken, mechanism of food capture and

time of feeding.

According to their feeding habits some fishes are parasites (e.g. lampreys),

some are scavengers (e.g. hagfish), some are planktivorous feeding on

plankton (e.g. whale shark and some bony fishes), some are detritivorous

feeding on decayed organic matter known as detritus (e.g. some bony fishes),

some are piscivorous feeding on other fishes (e.g. a bony fish called Nile

perch), some are molluscivorous feeding on mollusks (e.g. some bony

fishes) and some are omnivores feeding on a little bit of everything (e.g.

some bony fishes).

According to type of food taken there are two major categories of fishes

namely generalists and specialists. Generalist fishes (e.g. omnivorous

fishes) feed on everything whereas specialist fishes feed on specific food type

(e.g. planktivorous fishes).

According to the mechanism of food capture some fishes are filter feeders

that obtain their food by filtering water, some are predators, some are

grazers and some are pickers. In relation to the time of feeding some fishes

are diurnal feeding during daytime, some are nocturnal feeding during night

or dark time, and some are crepuscular feeding during dawn and dusk.

Activity:

Dear student, can you give examples of fishes according to their

diversities in terms of their feeding behaviours mentioned above?


Chapter Review Questions

1. What are the characteristics of a typical fish?

2. Give some examples of fishes that are exceptions to:

(a) Gill breathing

(b) Possession of paired fns

3. What are the characteristics of fishes that make them diverse? List at

least five examples

4. What is the basic difference between agnathan and gnathostoman

fishes?

5. List down the agnathan fish Classes and distinguish between the

extinct and extant groups

6. List down the gnathostoman fish classes and distinguish between the

extinct and extant groups

7. What kind of aquatic environments does each of the following fish

groups inhabit?

(a) Cyclostomes

(b) Chondrichthyes

(c) Osteichthyes

8. What are the differences between the sarcopterygian and

actinopterygian bony fishes?

9. List down the three sub-groups of actinopterygian bony fishes and

their differences

10. What are the various types of aquatic habitats occupied by various

groups of fishes?


Fish Reproduction and Development

Chapter Outline

2.1. Fish Reproduction

2.1.1. Patterns of Sexuality in Fishes

2.1.2. Sites of Embryo Development in Fishes

2.1.3. Fertilization and Spawning in Fishes

2.1.4. Reproduction in Cartilaginous Vs Bony Fishes

2.2. Growth, Development and Recruitment in Fishes

Chapter Objectives

Upon completing this chapter, you will be able to:

� Distinguish between the various types of sexuality in fishes

(heterosexuality, hermaphrodite and unisexuality)

� Distinguish among oviparous, viviparous and ovoviviparous fishes

� Distinguish between the fishes undergoing internal and external

fertilization

� Defines spawning in fishes

� Gives examples of fish spawning grounds

� Distinguishes between fertilization in cartilaginous and bony

fishes

� Distinguishes between the various developmental stages of fishes

� Defines recruitment in fishes


2.1. Fish Reproduction

2.1.1. Patterns of Sexuality in Fishes

Various ranges of sexuality or sexual pattern are observed in fishes. These

include heterosexuality (gonochores), hermaphroditism (bisexuals) and

unisexuality.

Heterosexuality is the most common form which involves separate male

and female parents. However, there are some considerable variations among

the heterosexual fishes. For instance, in some bony fishes sperm cells remain

viable only for a few seconds once released, and in some live-bearing fishes,

the female is able to store sperm for up to 8 or even 10 months, and this

sperm is used to fertilize new batches of eggs as they develop. In others, a

female may carry sperm from several males at once.

Hermaphroditism takes various forms in fishes. Some fishes serve as both

male and female and thus can produce both egg and sperm. This situation is

often known as simultaneous hermaphroditism. In others there is a time

sequence of hermaphroditism whereby young fishes reverse their sex as they

grow older i.e. males become females and vice versa. This situation is called

either sequential hermaphroditism (if sex reversal happens only once) or

serial hermaphroditism (if sex reversal takes place more than once). This is

a manifestation of fishes’ sexual plasticity. Nevertheless, the simultaneous

hermaphroditic fishes often undergo cross-fertilization with other similar

hermaphroditic fishes. A few undergo external self-fertilization whereby egg

and sperm are shed simultaneously into the water from the same individual.

Still a few others may undergo internal self-fertilization.

Activity:

Dear student, what are the differences among these three major forms of

sexuality patterns in fishes?


A few fishes are unisexual i.e. consist only of female population (e.g. Poecilia

formosa, of the Amazon River). Such fishes, thus, undergo parthenogenetic

reproduction in which unfertilized eggs develop directly into embryos. Yet,

even though development proceeds without fertilization in these females,

mating with males of other species is still required just to stimulate egg

development. Nevertheless, in all cases of sexuality eggs are produced in

ovary (pl. ovaries) and the sperm (milt) in testis (pl. testes). As it is a case

with other vertebrates, ovary and testis constitute gonads in fish.

2.1.2. Sites of Embryo Development in Fishes

Based on “where” their embryos develop fishes can be categorized as

oviparous, ovoviviparous and viviparous. Most fishes are egg-layers (i.e.

oviparous), thus embryo develops and hatches outside the female’s body in

the environment (water). However, some fishes give birth to live young. Such

live-bearing fishes can be ovoviviparous or viviparous.

Activity:

Dear student,

Q1. Can you mention the various groups of fishes according to “where”

their embryos develop?

Q2. Which of these fish groups are predominant?

Activity:

Dear student, we have mentioned above that both ovoviviparous and

viviparous fishes give birth to live young. So, what is the difference between

the two groups?


In ovoviviparous fishes:

• Embryonic development takes place only partly within the female’s

body

• The embryo does not receive any additional nutrition from the

mother’s tissue.

• The mother’s body provides the eggs with respiration (gas exchange)

in most cases.

• Thus, development of embryo depends solely on a food supply from

the egg yolk sac.

The process of ovoviviparity in fishes takes any one of the following forms:

• In some ovoviviparous fishes the embryo develops in the egg while

the egg is still within its follicular covering within the ovary

• In other ovoviviparous fishes the eggs are released from the

protective follicles into the cavity of the hollow ovary, where

development continues.

In both cases the fishes bear premature young (embryo) whose development

relies on yolk for food supply.

In contrast to ovoviviparous fishes, in viviparous fishes embryo stays in the

female’s body, until borne, where it is supplied with all nourishment through

the mother's tissues.

2.1.3. Fertilization and Spawning in Fishes

Activity:

Dear student, how many modes of fertilization could be found in fishes?


There are two major forms of fertilization in fishes: internal fertilization

and external fertilization. In all live-bearing (i.e. ovoviviparous and

viviparous) fishes and in some egg-laying (oviparous) fishes fertilization

occurs internally. In contrast, external fertilization is a form of fertilization

occurring only in some of the oviparous fishes.

Fishes undergoing internal fertilization use various intromittent organs to

transfer sperm or milt from males into the females’ body. For instance, in the

male topminnows (an example of a bony fish) the anal fin has been modified

into an intromittent organ called the gonopodium, and in sharks the pelvic

fins of the male are modified into intromittent organs called myxoptergia or

claspers.

In external fertilization females lay eggs in water where the males discharge

their sperm (milt) onto the eggs i.e. the process of egg deposition in water

(i.e. oviposition) by the female fishes is followed by a release of sperm by the

male fishes onto the eggs.

Activity:

Dear student, in fishes undergoing internal fertilization the males

obviously should have intromittent organs for transferring sperm (often

called milt) into the female's body.

Q. What are these organs used by such male fishes to transfer sperm into

the female’s body?

Activity:

Dear student, we have discussed above about the modes of fertilization

occurring in fishes. Now let’s come to another reproductive process in fish:

spawning. What is spawning?


Spawning refers to the release of egg and sperm by the female and male

fishes for fertilization. Fishes that hatch from the same spawning event at the

same time are termed as cohort in fishery. Fishes often aggregate in pair or

group for spawning as in Fig. 2.1a and b respectively.

Fig. 2.1. Spawning in fishes (a) Pair Spawning, (b) Group Spawning

Group spawning is more efficient, in terms of ensuring fertilization, than pair

spawning since large number of sperms is released in the former. On the

contrary, pair spawning is more economical than group spawning in terms of

(a)

(b)

Activity:

Q1. How efficient are pair and group spawning in terms of fertilization?

Q2. Which of these two major patterns of spawning in fishes is more

economical in terms of sperm number released?

Q3. Which of these two patterns of spawning ensures more genetic

variability?


the number of sperm released. Genetic variability is ensured in group

spawning since large number of fishes takes part in the process.

Areas of water bodies where fishes spawn are known as spawning grounds.

Studies indicate that small streams in the watersheds of the rivers or lakes

often serve as spawning grounds for river and lake fishes as well as for some

marine fishes. For instance, fishes living in any of the large rivers may spawn

in small streams or tributaries upstream the river. Therefore, many fish

species undertake long distance migrations reaching hundreds of Kilometers

to arrive at their spawning grounds. One of the reasons for the fishes to

migrate upstream a long distance from their feeding grounds in search for

spawning grounds can be a need for well oxygenated sites to lay their eggs.

Obviously dam construction across a river has an impact of blocking fish

migration upstream for spawning. Therefore, during dam construction it is

necessary also to construct fish ladders and other bypass systems that would

help the fishes migrate past the dams.

Activity:

Dear student, studies indicate that fishes usually aggregate before they

migrate to a specific site for spawning. Taking river fishes or lake fishes as

an example, where do you think that these fishes migrate for spawning?

Activity:

Q1. Based on our above brief discussion about the fish spawning grounds,

what do you think could be the effects of dam construction across a given

river in relation to fish spawning?

Q2. How do you think that such effects of dam construction on fish

spawning can be mitigated?


In egg laying or oviparous fishes there is a possibility that a huge number of

the spawned eggs could be eaten by predators or destroyed otherwise. To

cope with the problem, however, each female produces thousands, even

millions, of eggs at a spawning. Moreover, fishes undertake various parental

care strategies to safeguard their eggs and young.

Parental care shows great diversity in fishes.

• Hiding: Some fishes, such as the Atlantic herring, form huge schools of

males and females and freely shed their eggs and sperm (milt), and

then hide the eggs.

• Nest Building: Other fishes build nests to care for both the eggs and

newly hatched young. The nests may be depressions in the beds of

streams and lakes. The eggs and young of nesting fish are commonly

guarded by one of the parents, usually the male.

• Mouth Brooding: Some fishes (e.g. Nile tilapia) carry the eggs and

young in their mouth (a condition known as mouth brooding) for

long time, fasting until the young are large enough to defend for

themselves.

• Pouch Brooding: Others have evolved methods of carrying the eggs

with them, such as in special pouches on the body.

Activity:

Dear student, in oviparous fishes there is a possibility that some of the

spawned eggs could be eaten by the predators or destroyed by other

factors.

Q. How do such fishes cope with such problems?

Activity:

Dear student, what different parental care strategies do fishes employ to

safeguard their eggs and young?


2.1.4. Reproduction in Cartilaginous Vs Bony Fishes

In cartilaginous fishes:

• Fertilization is mainly internal whereby eggs are fertilized inside the

females’ body.

• The males transfer sperm into the females’ genitalia through various

intromittent organs (e.g. myxoptergia or clasper in shark).

• The females have cloaca, between their pelvic fins, that contains three

openings namely: opening to rectum (at front), opening to vagina (at

middle) and opening to urinary duct (at back). Therefore, the males’

intromittent organ penetrates through the middle cloacal opening to

discharge sperm into the females’ vagina.

• In terms of the site of embryo development, most cartilaginous fishes

are oviparous and only a few are live-bearers (i.e. viviparous or

ovoviviparous).

• Live-bearing viviparous cartilaginous fishes generally produce small

number of offspring that are retained, protected and nourished

within the females’ body.

Bony fishes:

• Have well developed reproductive organs

• Mainly undergo external fertilization (i.e. oviparous)

• In bony fishes undergoing internal fertilization the males have got an

intromittent organ (e.g. gonopodium) to transfer sperm into the

female’s cloaca.

Activity:

Dear student, what are the major similarities and differences between the

reproduction in cartilaginous fishes and bony fishes?


2.2. Fish Growth, Development and Recruitment

Growth refers to an increase in body size (i.e. body length or weight)

whereas development refers to the various stages through which the fish

passes from the embryonic stage to death.

Unlike most other vertebrates, fish grow throughout their life time, though

they grow more slowly as they age. The rate of growth varies greatly, being

most rapid where food is most abundant. Growth is commonly quicker in

warm regions than in cold.

The terminologies used to describe the life history or developmental stages

of fishes could vary for various disciplines such as taxonomy, physiology,

fisheries management, etc. Generally we may recognize five stages such as

embryonic stage, larval stage, juvenile stage, adult stage and senescent.

• Embryonic stage starts with fertilization and continues until its hatching

(in oviparous fishes) or birth (viviparous fishes).

• Larval stage extends from the time of hatching until the differentiation of

fins. The larval period in oviparous fish is relatively short (usually only

several weeks). Larval fish is very different in appearance from the

juvenile and adult stages. However, larvae rapidly grow and change

appearance and structure, through a process called metamorphosis, to

become juvenile. Initially larval fish depends on egg yolk for

Activity:

What are the various fish developmental stages? How do they differ from

each other?

Activity:

How does fish growth compare with that of other vertebrates?


nourishment and gradually switches from its yolk sac to feeding on

plankton.

• Juvenile stage begins with the differentiation of fins and continues until

maturity. Therefore, juvenile fish has a more fish-like appearance.

• Adult stage starts with the sexual maturity and ends up with the

production of the first gametes. This stage includes reproductive

behaviours such as spawning.

• Senescent stage covers a period of a very slow growth during which rate

of reproduction can also be reduced. During this stage a body of fish may

undergo rapid degenerative changes.

Recruitment refers to the number of new juvenile fish or cohorts reaching a

size or an age where they represent a viable target for fishery i.e.

recruitment can be defined as the number of new juvenile fishes or cohorts

added to the existing fish biomass at a given age or size. Studies indicate that

variability in recruitment is a primary factor that drives changes in fish

populations. In turn, variability in fish recruitment is caused by density-

independent and density-dependent processes on all pre-recruit stages.

Activity:

Dear student, in our above discussions we have seen about the fish

growth and development. Before we come to wrap up on this

chapter, can you mention what recruitment is in fishes? Explain.

Activity:

Dear student, what are the density-independent and density-dependent

processes that affect recruitment?


The density independent factors that affect fish recruitment include

temperature and food conditions experienced by pre-recruits. The density-

dependent processes that affect fish recruitment include competition for

food or refuge with conspecifics or offspring of other species. In general

mortality rates during the pre-recruit stage are very high. This means a

population of faster growing individuals will experience a lower cumulative

mortality than a slower growing one.



1. Define heterosexuality, bisexuality and unisexuality in fishes

2. What are the differences among the simultaneous hermaphroditism,

sequential hermaphroditism and serial hermaphroditism in fishes?

3. What are the differences among the oviparous, viviparous and

ovoviviparous fishes?

4. What are the two forms of fertilization in fishes?

5. How is sperm transferred from male to female in fishes undergoing

internal fertilization?

6. What is spawning in fishes?

7. What is recruitment in fishes?

8. List down the various developmental stages in fishes


Capture Fisheries

Chapter Outline

3.1. Introduction

3.2. Fishing Gears and Crafts

3.3. Fishing Methods

3.4. Commercially Important Fishes

Chapter Objectives

Up on completion of this chapter, you will be able to:

� Define capture fisheries

� Define and distinguish between the fishing gears and methods

� List down some major fishing gears often used in capture fisheries

� Give examples of major fishing crafts

� Give examples of commercially important fishes in the world

� Give examples of commercially important species in Ethiopia


3.1. Introduction

Dear student, in this chapter and chapter 4 you will be able to distinguish

between two major and broad categories of fisheries. Before, proceeding to

the details and discussions of these two subjects it may be wise to take a

moment to define what is meant by fisheries.

Fisheries (sing. fishery) can be defined as human’s utilization of fish and

other aquatic organisms of certain values. This comes in two major forms:

capture fisheries (chapter 3) and aquaculture (chapter 4).

Capture fisheries (also known as natural fisheries or wild fisheries) can be

defined as catching of fish and/or other valuable aquatic organisms for food,

recreational, economic or commercial purposes from the natural water

bodies such as streams, rivers, lakes, seas and oceans. In contrast,

aquaculture is the growing or farming of fish (or other beneficial aquatic

organisms) in the natural or artificial water bodies mainly for food or

commercial purpose. For more details on aquaculture refer to chapter 4 of

this module.

Activity:

• Dear student, before you go on reading the following sections take a

moment and try to jot down the similarities and differences you

think would exist between capture fisheries and aquaculture. You

need to refer to chapter 6 of your Aquatic Sciences and Wetlands

Management (Biol 302) course module to refresh your memory on

the subject.


Capture fisheries come in various levels or scales. For instance, commercial

fishing versus artisan fishing. Commercial fishing is the activity of catching

fish and other aquatic organisms for commercial profit, mostly from capture

fisheries. Large scale commercial fishing utilizing advanced fishing

techniques and materials with high investment is known as industrial fishing.

In contrast, artisanal fishing is a term sometimes used to describe small

scale commercial or subsistence fishing practices. It uses traditional

techniques and traditional fishing boats. It is subject to difficulties in the

export process due to inadequate investment in refrigeration and processing

facilities. However, the most important goal of artisan fishing is domestic

consumption. Subsistence fishing is fishing for personal consumption.

Ethiopian fisheries are generally small scale and subsistence involving

mainly traditional fishing techniques and occasional local fishermen who

have alternative means of employment such as small-scale agriculture. As

described in sections 3.2 and 3.3 in most of the Ethiopian lakes and rivers

traditional fishing gears and fishing crafts are employed. Nevertheless, as

discussed in chapter 5 of this module, fisheries stand a potential economic

sector providing various socio-economic benefits to the society. Primarily it

is an important source of protein and other nutrients to the poor. It also

provides income, employment and even contributes to GDP (Gross Domestic

Product) particularly in developed countries with well developed industrial

fishing.

Activity:

Dear student,

Q1. Which scale of fishing do you think applies to Ethiopian fisheries?

Explain.

Q2. In general, what are the various socio-economic benefits of a

fisheries activity to the society?


Dear student, it is also important to note that in developed countries people

also catch fish as a leisure activity. Such kind of fishing is often known as

recreational or sport fishing or angling. The people catching fishes in

recreational fishing are thus called anglers. In recreational or sport fishing

anglers enjoy themselves by struggling to catch large fishes that are often

difficult to fish. In sport fishing, the anglers do not basically take the fish

home for consumption. They return them back into the water. This is not

practiced in Ethiopia.

Aquatic organisms that provide some socio-economic values to human being

include vertebrates such as finfish, sea turtles (aquatic reptiles), whales

(aquatic mammals), seals (aquatic mammals), dolphins (aquatic mammals);

invertebrates such as cnidarians (coelenterates), squids (mollusks), octopus

(mollusks), oysters (mollusks), clams (mollusks), lobsters (crustaceans),

crabs (crustaceans), shrimps (crustaceans); and algae (aquatic plants). Their

values are summarized in Table 3.1.

Activity:

• Dear student, can you mention some of the aquatic organisms

including finfish of certain socio-economic values to human being

along with their benefits?


Table 3.1. A summary of some of the aquatic organisms of certain socio-

economic values to human being.

Category Benefits or values to human

Finfish Food/Economic values

Sea turtles and Crocodiles Economic values

Aquatic mammals (e.g. whales) Economic values

Cnidarians (e.g. corals) Jewelry /Economic values

Shellfish (mollusks and crustaceans) Food/Economic values

Algae (e.g. sea weeds) Ice-cream making and agar-agar

production

Thus, exploiting these aquatic organisms that are naturally grown in their

natural environment can be termed as capture fisheries whereas growing

them under human controlled settings to generate more benefits from the

organisms is termed an aquaculture.

Although more than 31, 000 different species of finfish are known to exist

only a few species are important in capture fishery. Cyclostomes such as

lampreys are almost unknown in fisheries. Whereas the cartilaginous fish

Activity:

Dear student,

• From your Aquatic Sciences and Wetlands Management course (Biol

302), chapter 6, you have to recall that there are diverse groups of

finfish including cyclostomes (agnathan fish), chondrichthyes

(gnathostomatan fish) and osteichthyes (gnathostomatan fish).

Q1. Roughly, what percent of the fish species do you think are important in

food or commercial fishery?

Q2. Which of these fish groups are more important in food or commercial

fishery?


such as shark and rays are caught for their commercially important fins and

skins respectively. Bony fish, particularly the teleosts (actinopterygians) are

more important in both the food and commercial capture fishery worldwide.

3.2. Fishing Gears and Crafts

Fish and/or other valuable aquatic organisms can be captured using various

fishing gears and methods. We shall come to the discussion of fishing

methods in the subsequent section (section 3.3). Fishing gears refer to the

tools used to capture aquatic resources.

The Food and Agriculture Organization (FAO) of the United Nations (UN)

identifies the following major types of fishing gears: surrounding nets, seine

nets, trawl nets, dredges, lift nets, cast nets, gillnets and entangling nets,

traps (such as pots, stow or bag nets, fixed traps), hooks and lines (such as

handlines, pole and lines, set or drifting longlines, and trolling lines), and

grappling and wounding gears (such as harpoons, spears, arrows, etc). Some

Activity:

Dear student,

Q. If you might have had a chance to encounter some of the fishing

gears or have read about them from literature, would you please try

to list down some of them before you go on reading the following

sections?

Activity:

Dear Student, we have discussed above about the aquatic organisms

that are socio-economically important to human being in fisheries.

What various materials and techniques do you think can be used to

harvest the organisms in capture fisheries?


of these and the major fishing gears and how they are used are given figures

(Fig. 3.2, 3.3, 3.4, 3.5, 3.6). The classification of fishing gears, such as those

listed above according to FAO, is based on the combination of the principles

of how the fishes are captured and the way the gears are constructed.

The various fishing gears used in Ethiopia include gillnets, beach seines, long-

lines, hook and lines, scoop nets, cast nets, spears, etc. However, gillnet is

mainly used in most of the fisheries accounting for most of the commercial

fish production. It is, however, important to note that in some of the water

bodies such as in Gambella various types of the traditional gears account for

most of the fish capture.

It is important to note that the type of gear used and the way it is constructed

and used in catching the organisms may have some damaging effects both on

the aquatic organisms and the environment. In the past the technological

development of fishing gears was aimed at just increasing fish capture

production.

Activity:

Dear student, what do you think may be the side effects of such past

time gear development that just targeted increasing the quantity of

fish or organisms caught?

Activity:

Q. Which of the above mentioned fishing gears do you think are used in

Ethiopian water bodies (lakes and rivers)? Which one is predominant?


The consequences of such gear development include:

• Overfishing of certain fish, and under exploitation of others

• Damaging effect of some gears to the environment including the

untargeted catch (both fish and non-fish) and destruction of habitats

(e.g. when using dredge nets).

As a result, at present gear development is very much diverted from its past

emphasis of increasing capture production, and focused on selective fishing

gears with less impact on the organisms and the environment. Moreover, the

types of gear to be used and the mesh sizes of the netted gears are

determined through legislative laws.

Which fishing gears and methods to be used are based on factors such as

where to fish (e.g. small or big water body, shallow or deep water), species

and size of fish to be caught, weather condition during the fishing time or day

(e.g. sunny or rainy), cost of the gear, boat and fuel, and the market

requirements.

Activity:

Q1. Do you think that there exists any legislative law in Ethiopia that

addresses the type of gear and the mesh size to be used in capture

fisheries? Discuss with any students taking the same course.

Q2. In the following sections we will come to the discussions of fishing

methods and gears. Before that, please try to discuss with any of your

friends taking the same course what factors determine our choice of the

fishing gears and methods.


Fishing crafts are vessels of relatively small (e.g. rafts or boats) or large size

(e.g. ships) that are required to navigate on the water bodies while operating

the gears for fishing. Modern boats and ships are motorized. However, rafts

and traditional boats (e.g. reed boats, coracles and canoes) are not provided

with engines. Therefore, they are driven using paddles (see Fig. 3.1c). Such

traditional boats are often common with small scale and subsistence

fisheries.

Rafts are structures with a flat top that float and often used to fish in shallow

waters (Fig.3.1a). Reed boats are constructed from reeds (Fig.3.1b). Coracles

are light boats shaped like a bowl, typically with a frame of woven grass or

reeds covered with animal hides or tree barks (Fig.3.1c). Canoes are small

narrow boats, usually pointed at bow and normally open on top. It is often

constructed from timbers or dugout from a tree trunk (Fig.3.1d). A fishing

ship is also shown in Fig.3.1e.

Activity:

Dear student,

Before we proceed to the discussions of the major types of fishing

methods, lets us briefly raise some points about the fishing crafts. What

do you think are the fishing crafts? Can you give examples?


Fig 3.1. The various types of fishing crafts. (a)Modern raft, (b) Reed boats, (c)

Coracle boats, (d) Canoe boats, (e) Fishing Ship

Fishing crafts (boats or ships) can be used to set the gears in the water (e.g.

gillnets) so that the fishes are caught after the gear setting (Fig.3.2a).

Alternatively, fishing crafts can be used to actively tow or pull the gears in

the water to immediately catch fish (Fig.3.2b).

Fig.3.2 (a) A gillnet set in water at given point, (b) A fishing craft or vessel

towing dredge net in the water.

Fishing craft

Dredge nets

Gillnet

(a) (b)

(a) (b) (c)

(d) (e)

Paddles


The various types of fishing gears can be broadly categorized as static

(passive) and active gears. Static gears are set at a given position in the water

and left for some times until the fishes come to the gear to be caught. These

include gillnets and entangling nets, trap nets, long-lines, etc. On the other

hand, active gears are towed or pulled in water column using fishing crafts to

catch the fish. Therefore, in case of the static gears the probability of fish

being caught by the gears depends on the probability of the fish moving into

the gear; whereas in active gears it is the gear that should move to the

position where the fishes are. These include trawl nets, seine nets, dredge

nets, lift nets, scoop nets, cast nets, trolling lines, jigging lines, hand lines,

Spears, etc.

In developed countries, modern technology has yielded electronic navigation

and sophisticated fish-finding equipment that can locate the fish. For

example, Colour echosounder helps to locate the depth at which the fish are

Activity:

Dear student,

Q1. Among the various types of fishing gears we mentioned above

according to FAO, which ones are to be set at a given point in the

water body to catch fish after being set, and which ones are to be

towed or pulled in the water column by the boats or ships to catch the

fishes immediately?

Q2. In this respect can you categorize the various fishing gears into

some broad categories?

Q3. Which types of fishing crafts do you think are used in Ethiopian

fisheries? What can you say about the level of development of

Ethiopian fisheries in this respect?


located. Colour net recorder, used along with the echosounder, is used to

give information about the gear being used and the fish moving into it i.e.

the quantity and types of fish moving in to the gear.

3.3. Fishing Methods

Fishing methods refer to the various ways the fishing gears are employed in

capturing the organisms. It, thus, means that the same fishing gear can be

used in many different ways. The methods used to catch the finfish and

shellfish are basically similar. However, some differences exist. The major

types of fishing methods used for finfish include netting, lining, trolling,

trawling and seining. Trawling, dredging, jigging and pots are often used for

capturing shellfish (such as squids and crabs).

In netting method of catching fish fishing gears such as gillnets and

entangling nets are employed. These nets are typically long, narrow and flat

with diamond shaped meshes. The nets are set stretched, keeping their

meshes open, in the water column with “weights” at the bottom edge and

supporting “floats” at the top edge (Fig.3.3). Fishes are thus caught when

attempting to pass through the meshes which catch usually in their gills.

Hence, gillnets are named so because fishes are caught when their gills are

snared in the net’s mesh. Gillnets are static gears that are set at a given point

and the capture of fishes depends on the fishes moving into the gear

sometimes after setting of the net. One should wait at least overnight to get

Activity:

Dear student, try to think of the differences and similarities that exist

among the major fishing methods such as netting, trawling, seining,

lining, trolling and dredging before you pass on to read the following

sections.


could fish catch after setting the gillnets. For this reason they are said to be

the set-and-wait gears. Gillnets are easy to construct and often supplied by

the net makers. They are cheap and also relatively easy to operate, and

consequently have been extensively used in reservoirs and lakes fisheries

throughout the world. Limitations of gillnets, however, include their requisite

of a boat from which they are set and lifted and this imposes an additional

cost. Moreover, gillnets are vulnerable to theft and thus fishermen need to

frequently spend the night watching over their gear or devise ways to

conceal the floats.

Fig. 3.3. A gillnet with mesh, floats and weights

Activity:

Dear student,

Q1. What do you think are the purposes of putting “weights” and “floats”

on the edges of gillnets while setting them in water ?

Q2. What local materials do you think can be used as “weights” and

“floats”?

Weights

Mesh

Floats


The “floats” and the “weights” help the gear (gillnet) remain float in the water

column during setting. The gillnet mesh sizes matter in catching fish of

appropriate or desirable size. Too small mesh sizes are often not

recommended as they often target the smaller or young or larval fishes that

have not fully grown. In contrast, too large mesh sizes catch only large size

fishes. Thus, it is recommended to use an appropriate mesh size that does not

catch untargeted fishes or other organisms.

Trawling method of catching fish involves towing or dragging of the

especially designed trawl nets using one or two fishing vessels such as boats

(Fig.3.4a). Trawling is the most important commercial fishing method used

to catch a range of species especially the deep habiting ones. However, its use

is limited because of its high by-catch (i.e. untargeted catch). Dredging is

towing a rigid steel-framed dredge net along the bottom of the water body

with a fishing vessel to catch shellfish such as scallops and oysters (Fig.3.4b).

Therefore, unlike gillnetting, trawling and dredging are active fishing

methods. However, all the methods employ nets constructed in different

ways.

(a) (b)

Fig. 3.4. (a)Trawling method, (b) Dredging method

Fishing vessels

Trawl net

Dredge nets


By-catch refers to unwanted or untargeted organisms captured by the fishing

gears. It can be fishes of unwanted size (e.g. too small size or young fishes) or

other animals than fish unwanted in the process of fishing. By-catch is

considered a loss in fishery and it is environmentally deleterious.

Long-Lining involves the use of fishing gears known as long-lines. In this

method the long-lines consist of a main line with hooked and baited short

lateral lines (known as snoods) attached at intervals. The line is anchored at

each end and held at the surface by floats (Fig.3.5). This is a static or passive

gear method.

Fig. 3.5. A long-line with its baited and hooked lateral snoods

Activity:

Dear student, what do you think is a bait and its purpose in catching fish?

Activity:

Dear student, what do you think is a by-catch in the process of fish

capture? Can you give examples?

Floats

Main line

Sn

oo

ds

Weights


Bait is anything used as food by the target fish or lure attached to the ends of

the snoods or the lines to attract fish to the lines. The lured fishes are then

caught by the hooks at the tips of the snoods when attempting to eat the bait.

Trolling method only slightly differs from long-lining method in that many

parallel baited and hooked lines are towed behind a boat. The difference is

that there is no main line in this case and all the lines are of same size

(Fig.3.6a). Jigging is a method that involves continuously lowering and

retrieving lines provided with lures from the fishing vessel (Fig.3.6b). It is

mainly used to catch squid in marine fishery. Thus, unlike long-lining, trolling

and jigging are active methods of fishing. However, all utilize lines as fishing

gears in different ways.

(a) (b)

Fig. 3.6. (a)Trolling method, (b) Jigging method

Poison fishing involves the use of poisons or toxins extracted from some

plants. The poisons are added into the whole water body in order to

Activity:

Dear student, now we are coming to the conclusion of our discussion on

the fishing methods. Have you ever heard of a fishing method that

involves the use of fish poisons or toxins? Can you give an example in

Ethiopia?


intoxicate the fishes. The fishes are then easily collected when they become

unconscious because of the intoxication. Plants used to extract such fish

poisons in Ethiopia are mainly Milletia sp (Locally: Birbira); and generally in

Africa involve Euphorbia sp. (Locally called Quliqual in Ethiopia). However,

the use of such poisons is generally illegal due to the effects of the poisons on

the other aquatic organisms. The poisons might also be toxic to human if

consumed via fish.

3.4. Commercially Important Fishes

According to the Food and Agriculture Organization of the United Nations

(FAO, 2002, 2004, 2006, 2008, and 2010), a publication of the United

Nations, the commercially important marine finfish species worldwide

include Anchoveta, Alaska Pollock, Atlantic herring, Skipjack tuna, Japanese

anchovy, Chilean jack mackerel, Largehead hairtail, Chub mackerel, Capelin,

Blue whiting, Yellowfin tuna, Salmon, Groundfish and Tuna. The

commercially important freshwater finfish species important worldwide

include carps, barbells, (and other cyprinids), tilapias and other cichlids,

shads, salmon, trouts, smelts, catfish, salmon, and Nile perch.

Commercially important fish species in Ethiopia are summarized in Table 3.

2. Dear student, please refer to your Aquatic Sciences and Wetlands

Management (Biol 302) course module, section 6. 2. 1, for details on some of

the economically important families of fishes in Ethiopia.


Table 3.2. Commercially important fish species in Ethiopia (Source: Ethiopian

Institute of Agricultural Research, EIAR)

S. No. Scientific name Common name Vernacular name

1 Lates niloticus Nile perch Nech asa

2 Oreochromis niloticus Nile tilapia Qoroso/Chogofe

3 Barbus species Barbus Bilicha

4 Labeo species Labeo Barbo/Lebi

5 Clarias garipienus Cat fish Ambza

6 Bagrus dockmac Bagrus Kerkero

7 Polypterus bichir Nile bichir Eguwella

8 Gymnarchus niloticus Gymnarchus Wit

9 Malapterurus species Malapterurus

10 Crussian carp Carp Daba

11 Distichodus niloticus Distichodus Piro

12 Hydrocynus forskali Hydrocynus Weri

13 Heteroticus niloticus Heteroticus Ediwela

14 Citharinus citharinus Citharinus Ajaka

15 Synodontis species Synodontis Akok



1. What is fishery?

2. What are the two major forms of fisheries?

3. Define capture fisheries

4. List down some examples of fishing methods and give their

differences

5. What is a fishing gear?

6. List down some examples of fishing gears and how they are used

7. What is a fishing craft?

8. List down some examples of fishing crafts and their differences

9. Give examples of commercially important fishes in the:

(a) World

(b) Ethiopia


Aquaculture

Chapter Outline

4.1. Introduction

4.2. Types of Aquaculture

4.3. Aquaculture Systems

4.4. Establishment of aquaculture

4.5. Sustainability of aquaculture

Chapter Objectives


� Define aquaculture and distinguish between aquaculture and

pisciculture

� Discuss the importance of aquaculture practices

� Distinguish between the various types of aquaculture (pond, cage,

pen etc)

� Distinguish among the various types of aquaculture systems

(extensive, intensive, semi-intensive, etc)

� Discuss the factors to be considered (e.g. site and species

selection) when planning to establish an aquaculture

� Discuss the factors that should be considered in sustainability of

aquaculture


4.1. Introduction

Aquaculture can be defined as rearing or farming of aquatic animals and

plants in a natural or artificial water bodies under human controlled setting.

The usage of the term aquaculture is often universal referring to the farming

of all valuable aquatic organisms such as finfish (e.g. tilapia, perch), shellfish

(e.g. mollusks and crustaceans), coelenterates (e.g. corals), aquatic mammals

(e.g. whales and sea lions), aquatic reptiles (e.g. turtles and crocodiles), and

algae (e.g. phytoplankton and seaweeds)as described in section 3.1 of this

module. Therefore, strictly speaking a form of aquaculture that involves

rearing of only finfish is termed as pisciculture (pisc meaning fish; culture

meaning farming or rearing). However, it has become commonplace to use

the term aquaculture restrictively just to mean fish farming.

Aquaculture practices are important primarily for providing foods and

nutrition to the human being particularly in developing countries where

there is critical shortage of food. Moreover, aquaculture practice is important

in fishery management and biodiversity conservation. As discussed in

chapter 5 of this module, most of the world capture fisheries have already

peaked off and even started declining. In this respect, aquaculture can serve

as an alternative source of fish and shellfish in the face of a decline in capture

fisheries. In addition, it helps to rear and keep artificially fish species that

have been heavily exploited and whose status has been endangered through

natural fishery.

Activity:

Dear student, what do you think are the various benefits of aquaculture?


Other aquatic organisms than finfish can also be reared in aquaculture. For

instance, there is a Nile crocodile farm in Ethiopia, commonly known as

crocodile ranch, in Arba Mich on the edge of Lake Abaya, mainly for its skin

which is processed into leather products such as shoes and ladies handbags.

Moreover, the farm has been a source of tourist attraction. The farm,

however, suffers heavy flooding especially during the rainy seasons causing

the death of thousands of crocodiles. The farm can generate considerable

amount of foreign currency for the country, through export of the crocodiles

skin and tourist attraction, if properly managed. Other aquatic organisms

such as shellfish and algae are also reared for various socio-economic

reasons including food and commercial benefits.

The process of introducing fish seeds (often larvae) into a water body is

termed as stocking. The fish seeds required for stocking a water body can be

obtained from fish hatchery stations or alternatively they can be collected

from other natural water bodies. However, the latter case may have an

impact on the fish population in the natural water bodies and thus it is

Activity:

Dear student, as described above other aquatic organisms, than finfish,

can be reared in aquaculture.

Q. Is there any such farming in Ethiopia? What do you think is the socio-

economic benefit of such farming?

Activity:

Dear student, in our discussions above we have defined and described

what an aquaculture is. Assume that you have dug a pond and filled with

water for the purpose of fish farming. Where do you think that you can

get fish “seeds” to start rearing fish in your pond?


preferable to get fish seeds from the hatchery stations. In Ethiopia, the Sebeta

Fish Culture Station (currently organized as the National Fish and Other

Aquatic Life Research Center), located near Addis Ababa, used to serve this

purpose since its establishment in 1975.

4.2. Types of Aquaculture

Aquaculture establishment comes in various forms including pond

aquaculture, cage aquaculture, pen aquaculture, tank aquaculture, etc.

Pond aquaculture has been practiced since 4000 years ago in Asia and later

adopted to Europe. It has shown only little changes over centuries being limited to

fresh water pond system owned by a single household producing fish for

subsistence i.e. just to cover the daily food need. Aquaculture ponds can be

either with soil bottom (known as earthen ponds) or concrete-lined ponds.

The former are often used for extensive aquaculture system whereas the

latter are used for intensive and semi-intensive aquaculture systems. The

choice of species for stocking and rearing in all types of aquaculture systems

including ponds, pens and cages is more or less governed by principles,

including fast growth in confinement, good consumer acceptance, high

tolerance to a wide range of environmental conditions, resistance to disease,

ready supply of fish seed for stocking and ease of culture and management.

Accordingly, fish species commonly raised in freshwater ponds include

tilapia, catfish, carps, eel and salmonids. Similarly different species of fish are

reared in brackish and marine water ponds.

Activity:

Dear student, what is the difference between soil bottom ponds and

concrete-lined ponds?


Successful pond aquaculture requires us to regularly accomplish pond water

management and pond maintenance activities. Water in the pond should be

kept at certain levels and its quality should also be frequently monitored for

optimal fish growth. This is particularly important in intensive and semi-

intensive culture systems where large amounts of fish wastes are

continuously excreted into the pond and where excess, unconsumed fish

feeds add to the bottom causing water pollution. In general, in fish pond

culture, the pond water should be frequently freshened by the entry of new

water from the river or another water source through the supply canal while

old water is drained through the outlet or drainage gate, and dumped into

the receiving water bodies (e.g. river, seas) through the drainage canal.

Besides the water management procedures, pond maintenance procedures

should also be executed for healthy pond. These include regular application

of fertilizers to facilitate the growth of phytoplankton that serve as food for

planktivorous or herbivorous fish, addition of lime to regulate water pH at

alkaline or near-alkaline levels, application of pesticides to prevent pests, use

of screened gates to prevent entry of predators, monitoring of the fish stock

for growth rate determination as a basis of feeds and water management,

and regular pond repairs and maintenance. Intensive and semi-intensive

culture systems may not require addition of fertilization since they are

Activity:

Dear student, what should one do in order to have successful pond

aquaculture?

Activity:

Dear student, we have seen above that water management is one of the

important factors in successful pond aquaculture. What other factor is

required for the successful pond aquaculture?


dependent on externally supplied food than the natural food in the ponds. Up

on successful culturing procedure marketable-size fishes are harvested at the

end of the culture period by draining the pond or reducing the water level

and using harvesting nets to catch the fish.

Although pond culture has been a tradition in aquaculture practice for so

long time, fish culture in natural water bodies such as lakes, rivers, estuaries

and marine coastals, using enclosed materials such as cage, pen and tank, has

been started as recent as 1920s in Africa and Asia. Therefore, cage and pen

aquaculture represent the latest developments in the growth and

improvement of aquaculture practices. At present they account only for small

portion of the aquaculture despite their rapid growth.

Cage and pen are enclosures or confinements constructed from the

supporting frames (of various materials such as bamboo, wood, or metal)

and covered with nets. Although there are people who use the terms cage

and pen interchangeably, these are two different enclosures according to

FAO (1984). A fish cage is totally enclosed on all, or all but the top, sides by

mesh or netting (Fig. 4.1.), whereas in pen culture the bottom of the enclosure is

formed by the lake or sea bottom (Fig. 4.2). Therefore, fish pens are fixed but

fish cages can be either fixed or floating. Moreover, fish pens theoretically have

no limit to their size or area while cages cannot exceed 1000 m2 in area for

reasons of the quantity of net required for cage construction and manageability of

operation.

Activity:

Dear student, how do cage and pen aquaculture differ from pond

aquaculture? What is the difference between cage and pen?


Fig. 4.1. (a) A typical floating fish cage with fish inside, (b) Many floating

fish cages set in the water

Extra or more nets are required in cage construction, than in pen, because the

floor or the base of the cage should be close unlike in pens. Moreover, the

techniques used to harvest fish from the two systems are different. In cage culture

the cages should be lifted up and then the fish are scooped whereas in pen culture

nets are used to catch the fish from the enclosures or the pens.

Fig. 4.2. (a) A typical fish pen outside of water, (b) A fish pen set in a water body

(a) (b)

(a)

(b)


Cage and pen aquaculture systems have both advantages and limitations.

Cages and pens have several advantages over pond culture systems. These

include:

� They are easily applicable in existing types of water bodies such as

lakes, rivers, reservoirs and marine coastal waters.

� They provide higher productivity than the ponds with minimal inputs

and at lower costs to develop and operate.

� The phytoplankton from the main water body, in which the cage or

pen is set, serve as food for the fishes being reared in the cage or pen

� Therefore, they help to clean up eutrophic waters through the

culture of planktivorous or herbivorous species that feed on

phytoplankton.

Limitations of cage and pen aquaculture include various environmental impacts.

Wastes produced by the fishes in the cages or pens enter the water body and

cause various environmental problems such as eutrophication, depletion of

oxygen and other consequences. Moreover, there is a possibility of fish

disease transfer in both directions i.e. between the wild fish species living in

the natural water body and the reared fishes contained in the cages or pens.

Predators from the natural water body may also attack the culture fishes

within the enclosures.

Tank aquaculture involves the use of tanks (more or less similar to the water tank

used to temporarily store water) constructed from different materials in various

designs as shown in Fig.4.3. Fish tanks are constructed in various designs such as

rectangular or circular, and often used in intensive indoor fish rearing activities

such as in hatchery. See section 4.3 for the definitions of various aquaculture

systems such as intensive, extensive, etc systems.


Fig. 4.3. Circular fish aquaculture tanks

4.3. Aquaculture Systems

Various fish farming or aquaculture systems exist based on stocking density,

level of input, methods or techniques of farming, etc. These include extensive

farming, intensive farming, semi-intensive farming, integrated farming,

circulatory farming, monoculture farming, polyculture farming, monosex

farming and ranching.

In extensive farming low density fish are stocked and farmed in large areas

of water bodies. The fishes mainly naturally feed and grow only with less

Cross sectional view

Lateral view

Activity:

Dear student, as you read various literature you come to know that pond

and tank cultures are often referred to as land-based aquaculture

systems, whereas cage and pen cultures are called water-based

aquaculture systems. Why? Please discuss with any one of the students

taking this course.


input of capital and labour. This is, therefore, an old system that can be

applied only to the naturally fertile water bodies. It means that if the natural

fertility of the water body in which we are rearing the fish is less, the

productivity (i.e. the quality and quantity) of fish will be less. Intensive

farming is the opposite case of extensive farming. In intensive fish farming,

high densities of fish are reared in a water body and the fish rely almost

entirely on the feed externally supplied by the farmer. Therefore, this is a

costly system that requires high labour and capital input. It, however, gives

high fish production if the input required is properly provided.

Circulatory system is a system of aquaculture or fish farming whereby the

exhausted water is chemically treated and circulated to be re-used. This

system of farming is often practiced as an extreme case of intensive farming.

Semi-intensive farming represents an intermediate form of farming in

which the farmer provides some additional feeds and/or fertilizer to

supplement the naturally available food in the water. Fish farming in earth

ponds can represent such system of aquaculture.

In Integrated system fish farming is combined with other animal husbandry

such as poultry or cattle farming. The purpose of such integration is to use

excreta or faeces released from the animals husbandry that will serve as

manure to stimulate the growth of phytoplankton. This is because

phytoplankton are important food sources for the planktivorous

Activity:

From our discussion of extensive and intensive fish farming above,

what do you understand about the:

Q1. Differences between the two forms of farming in terms of stocking

density?

Q2. Advantages and limitations of each form of farming in terms of cost

and amount of production?

Q3. If you happen to invest on fish farming, which of these two systems

do you go for? Discuss your stand with any of your friends taking this

course.


(herbivorous) fishes. However, it is possible to use inorganic fertilizers to

increase fertility of the water if the integration is not possible. In fish

aquaculture it is possible to rear only a single species or many species of fish

in one water body. The former case is known as monoculture system and

the latter form is termed as polyculture system.

Monoculture system allows very high fish production with the use of

additional feeds since it reduces species competition for feed and other

resources. It is applicable in water bodies with less or no niche diversity

which the different species can occupy. In polyculture system the different

fish species should have complementary feeding habit and different niches in

order to effectively utilize the resource in the water body. It yields a higher

production than monoculture system if the water body in which the fish are

reared is fertile and has diversified niche. It is important to note, however,

that there are piscivorous (fish eating) fishes (e.g. Nile perch), and such

fishes don’t have to be kept in the same water body with the other non-

piscivorous fish species. This is because the piscivorous species eat and

destroy the other non-piscivorous fish species.

It is possible to keep both sexes of fish (male and female) in the same water

body in the process of fish farming. However, this may have a side effect of

causing unwanted breeding among the fish. In that case it is good to rear

Activity:

Dear student, what do you think is the difference between the

monoculture and polyculture systems of fish farming?

Activity:

Dear student, the monoculture and polyculture systems we discussed

above are based on the number of species farmed in a given water body.

What about a fish farming system according to fish sex?


male and female fishes in separate water bodies. Such system of fish farming

is known as monosex culture system. However, it can be a difficult task to

carryout especially in large scale farming.

Ranching is an exceptional form of aquaculture in which fish are reared in

hatcheries, and then released into the natural water bodies to naturally feed

and grow, and ultimately recaptured. According to FAO (1997) this form of

fish farming can also be called stock enhancement or culture-based fisheries.

Therefore, strictly speaking ranching may not be considered an aquaculture.

It is mainly practiced in marine water bodies by countries such as Japan.

In ranching, fish naturally feed and grow. It, thus, reduces input costs. It,

however, may be difficult to re-capture the fish back especially in big water

bodies such as in marine environments.

4.4. Establishment of Aquaculture

There are many factors to consider during aquaculture development. The

two important factors are species and site selections. Species selection is

one of the most important factors to be considered in establishing

aquaculture. Species are selected according to their biological

characteristics, economic and market conditions, and effects on environment.

In aquaculture one should go for the selection of species with desirable

biological characteristics such as feeding habit, reproduction rate, growth

rate, etc.

Activity:

What do you think are the advantage and limitation of such form of

aquaculture?


A good quality fish for aquaculture should be the one with a feed

requirement that can be easily supplied, and have faster reproduction and

growth rates so that production can be obtained in a relatively shorter period

of time. It is very expensive to supply the high-cost fishmeal that carnivorous

fish species require to grow; thus farming of the herbivorous fishes is both more

economical and easier to integrate with other conventional farm activities.

Fig.4.4. A pyramid of trophic relationship among the aquatic organisms

As you move from the base of the pyramid (i.e. from the primary producers)

to top of the pyramid, the amount of energy that flows from one level to the

other becomes less and less. It means that if you (as a human being) feed on a

kg of planktivorous fish you get more energy than from eating the same

amount of piscivorous fish (e.g. Nile perch). On the other hand, raising in

order to obtain a kg of carnivorous fish from aquaculture production, we

Activity:

Q. What kind of biological characteristics of fish can be considered as

desirable in aquaculture practices?

Phytoplankton

Zooplankton

Planktivorous fish

Piscivorous (carnivorous) fish


need to supply more than a kg of fish meal (as a feed) to the carnivorous

fishes, and which is more expensive. However, the raising of herbivorous or

planktivorous fishes is less expensive since it is possible to increase the

production of phytoplankton by increasing the fertility of the ponds as

discussed in sections 4. 2 and 4.3.

Moreover, a farmer planning for fish aquaculture should consider that the

fish species he/she is rearing are demanded on market and with good price.

While considering the biological and market conditions in species selection,

one should not undermine the effects the fish might produce to an

environment. For instance, common carp (Cyprnius carpio L.)has been

reported to have some environmental impacts. Carp contribute to poor

water quality by uprooting vegetation and stirring up sediments during

feeding, leading to increased turbidity. Carp have significant effects on the

aquatic plants both through direct grazing and uprooting in search of prey,

leading to a reduction in plant density and biomass (Sidorkewicj et al., 1998).

Turbidity of the water reduces light penetration, which can make it difficult

for the other fish that rely on sight to feed. Reduced light can also decrease

plant growth, and suspended sediments can smother plants and clog fishes'

gills. There have been suggestions that carp may increase the likelihood of

eutrophication or algal blooms by preying on animals that eat algae and

stirring up nutrients trapped in bottom sediments.

Site selection is another important factor of consideration in establishing

fish farming. In selecting site for both for the water-based and land-based

Activity:

Dear student, from our above discussion about species selection for

aquaculture establishment, what are the characteristics or conditions of

fishes that we need to take in to account? List them and write down how

each is important.


farms the limnological characteristics such as water quality and quantity

should be studied and determined if they are suitable for the fish species of

our choice.

The various limnological characteristics that need to be studied in this regard

include the amount of dissolved oxygen, water temperature, conductivity,

pH, amount of various nutrients such as nitrogen and phosphorus. The

measurement and determination of these characteristics are important

because various fish species have different requirements for the amount of

dissolved oxygen, water temperature, pH, etc. Moreover, both the land-based

and water based aquaculture systems should take into account of

accessibility and availability of man power for construction and operation.

The site should be preferably readily accessible by land or water transport;

close to sources of inputs such as fish seeds, feeds, fertilizers, and markets,

fish ports, processing plants, and ice plants; and linked by communication

facilities to major centres.

In addition, for a pond culture, the engineering and design of the pond need

to be carefully carried out following standard engineering procedures. For

instance, the pond should be constructed in such a way that it will not be

over flooded and it is also good to have a pond underground drain out or

spillway. The location of the pond should also be in such a way that it can

regularly obtain water supply. It is also better to not to have ponds

constructed in close proximity with big trees and thick vegetation which

entail large expense for clearing. The soil quality of a pond should be,

preferably, clay-loam or sandy-clay for water retention and suitability for

diking. The soil pH should be alkaline (≥7) to prevent problems that result

Activity:

Dear student, which characteristics of water bodies should be studied in

site selection for aquaculture?


from acid-sulphate soils such as poor fertilizer response, low natural food

production, slow growth of culture species and probable fish kills.

4.5. Sustainability of Aquaculture

Aquaculture is an important sector in providing fish especially to the poor.

Yet, its sustainability is confronted with some limitations. Studies indicate

that aquaculture practices compromise water quality, cause environmental

and public health problems, involves risks of introduction of exotic or non-

native species and genetic problems to the wild populations through escapes

from aquaculture installations, may compete with the land-based agricultural

activities for land, and with capture fisheries for water and species.

Water quality parameters such as water temperature, amount of dissolved

oxygen, pH, conductivity, amount of phosphorus and nitrogen, etc, of

Activity:

Dear student, in sections 4.1 through 4.4 we have discussed about

aquaculture and mentioned that it can be taken as an alternative source

of fish to the exhausting capture fisheries. Do you think that

aquaculture will continue to be sustainable source of fish? Explain.

Activity:

Dear student, in our discussion above about site selection in aquaculture

establishment we mentioned that factors such as water quality

parameters, site accessibility, availability of human power, design or

engineering of the pond, location of the pond and pond soil

characteristics need to be carefully studied. Please list down each of

these factors in your note book and then write down how each is

important.


aquaculture water in ponds or tanks can be affected as a result of the feeding

regime and metabolism of the fish. For instance, fresh water fish add

ammonia and salts to the water through their metabolic processes.

Management activities including cleaning of the tanks and disease treatment

using antibiotics may also affect aquaculture water quality. This in turn may

affect the aquaculture setting itself and the receiving water bodies (e.g.

rivers) to which the exhaust water is discharged. This in turn causes serious

environmental and public health problems. Moreover, in cage and pen fish

culture fish wastes collect on the bottom, damaging or eliminating bottom-

dwelling life. Fish wastes are organic and thus can also decrease dissolved

oxygen levels in the water column.

Aquaculture can be the cause of the introduction of non-native species from

one area to another. Such introduction of exotic or non-native fish species

can cause the loss of food, habitat, spawning or reproduction areas, etc of the

native or indigenous fish species. This ultimately leads to the decline or loss

of the native species, and thus reduces biodiversity.

Farming of fishes higher in food web trophic levels, such as carnivorous or

piscivorous fishes, requires us to feed them with other fish lower in food web

trophic levels (see Fig. 4.4). As discussed in section 4.4 of this module, in

relation to species selection for aquaculture, one has to give more fish meal

to such carnivorous fishes. This has two side effects: first, we deplete fishes

in the natural water because we get the fishes to be given as feed to the

carnivorous fishes by catching from the natural water bodies; second, it is

expensive to afford such fish meal to the carnivorous species and it is not

profitable.

Some aquaculture fish such as salmon and tilapia have been genetically

modified to improve growth rate and other characteristics. This has caused a

fear that farmed fishes escaping from aquaculture setting may reproduce

with the wild pollution causing genetic pollution such as production of


offspring with undesirable characteristics. Moreover, the effect of eating such

genetically modified fish on human health has not been confirmed.

In addition aquaculture can lead to the degradation of the physical habitat

through lowering of the ground water level of the coastal areas when

diverting water for the ponds, destruction of the coastal vegetation leading to

erosion.

So far, research and commercial feed improvements during the recent

decades (1990s and beyond) have lessened many of the problems associated

with farming of carnivorous fishes. Also in order to reduce aquaculture

competition with other land-based agricultural activities for land, it is

imperative to integrate both sectors to maximize synergistic and minimize

antagonistic interactions, towards the common benefits. Much of the

aquaculture problems, however, still remain unsolved. Therefore, the future

prospect of aquaculture, as a potential source of fish, is based on further

research undertakings in order to reduce or avoid the associated limitations

by improving the techniques.

Activity:

Dear student, in our discussions so far, I hope, that you have come to

know that aquaculture is an important source of fish, especially to the

needy, in the face of a decline in fish production from the capture

fisheries. However, its sustainability is full of challenges. How do you

think that it is possible for aquaculture continue sustainably?



1. What is aquaculture?

2. What is pisciculture?

3. What are the various importances of aquaculture practices?

4. What are the differences among pond culture, cage culture, pen culture

and tank culture?

5. What are the differences among extensive culture, intensive culture and

semi-intensive culture?

6. List down and discuss the factors to be considered when planning to

establish an aquaculture

7. What are factors that are related to sustainability of aquaculture?


Status and Prospects of Capture

Fisheries and Aquaculture

Chapter outline

5.1. World Case

5.2. Ethiopian Case

Chapter Objectives:

Up on completing this chapter, you will be able to:

� Discuss the status of capture fisheries production on a global scale

� Discuss the status of aquaculture production on a global scale

� Explain the trends in the fisheries productions coming from

capture fisheries

� Explain the trends in the fisheries productions coming from

aquaculture

� Discuss the trends in the fisheries productions coming from both

capture fisheries and aquaculture

� Discuss factors that are accounted for the decreasing trends in the

fisheries productions coming from the capture fisheries

� Tell the capture fisheries production potential of Ethiopia

� Tell the actual capture fisheries production of Ethiopia

� Discuss the past development history of Ethiopian fisheries

� Discuss the present development status of Ethiopian fisheries

� Discuss the important points about the Fisheries Legislation of

Ethiopia


5.1. World Case

Millennium Development Goals (MDGs) are eight objectives set to be

achieved by all the United Nation member states, which Ethiopia is a

member, by the year 2015. Eradication of extreme poverty and hunger is one

of those eight goals of MDGs. In this regard, capture fisheries and aquaculture

can be considered potential sectors that can contribute to the improvement

of the livelihood of many people in the poorest countries, and thus help in the

achievement of the MDG goal.

Data reported by FAO (2000, 2008, 2010) about the State of World Fisheries

and Aquaculture, are attached in Appendix 1.

Activity:

Dear student,

Q1. Have you ever heard about the Millennium Development Goals

(MDGs)?

Q2. Which of the millennium development goals may relate to fisheries?

Activity:

Dear student, what does the state of fisheries and aquaculture look like?

In other words, is the production from the fisheries and aquaculture

sectors increasing or decreasing?


Data from the three tables in Appendix 1indicate that most of the capture

fisheries production comes from marine fishing; whereas most of the

aquaculture production is obtained from inland waters.

Dear student, if you carefully observe the data given in all the three tables in

Appendix 1, you can learn that capture fisheries has shown an increasing

trend at a decreasing rate and thus has become plateau from 1990s onward

from 6% per annum in 1950s and 1960s, to 2% per annum in 1970s and

1980s, to almost zero growth rate in 1990s. In other words, production from

the capture fisheries has already reached its maximum potential and even

started declining. Careful observation of the data also shows that capture

Activity:

Dear student, carefully look at the trends in fish production from

total capture fisheries and total aquaculture of the world, both

from inland (e.g. lakes and rivers) and marine (e.g. seas and oceans)

water bodies, from the year 1996 – 2009 in the three tables given in

Appendix 1.

Q1. From which type of waters (inland or marine) is most of the fish

production generated in:

(a) Capture fisheries?

(b) Aquaculture?

Q2. What trend do you see in the total production coming from the:

(a) Capture fisheries?

(b) Aquaculture?

Q3. What can you say about the future prospect of capture fisheries as

a source of fish production on a global scale?


fisheries has peaked off both in the inland (e.g. lakes and rivers) and marine

waters (e.g. seas and oceans).

Many of the world’s capture fisheries have been pushed to the brink of their

productive capacity by overfishing or overharvesting due to increased

human pressure. For instance, studies indicate that approximately about one

half of the major marine fish stocks are already fully exploited. Moreover,

Pollution, degradation of aquatic habitats, and other forces compound the

pressure, with climate change posing a potentially enormous threat. The

expected impacts of climate change, such as greater shifts between drought

and flooding, higher water temperatures, and heavier storms in coastal areas,

can alter fisheries in ways that have major implications for ongoing

production of fisheries.

In order to reverse the declining trend of capture fisheries production proper

management actions need to be implemented. For instance, fishing of the

species and water bodies already fully or over exploited should be avoided

for some period of time so that the fish stocks and the environment will

regenerate. Otherwise, the effects are damaging not only in terms of socio-

Activity:

What factors do you think have caused such decline in capture fisheries

production on a global scale?

Activity:

Dear student, we have seen above that fish production from capture

fisheries has already leveled off and even started a decline.

Q1. Please list down four factors that are said to be causes for a decline

in the world capture fisheries.

Q2. What management measures do you think should be taken to

reverse the situations?


economy but also ecologically. Please refer to section 8.2 for better

understanding on fisheries management.

Dear student, from Appendix 1 you can see that aquaculture fishery

production keeps on increasing in contrast to the capture fisheries. The

sector has shown an increasing trend at an increasing rate from 5% per

annum in 1950s and 1960s, to 8% per annum in 1970s and 1980s, to 10%

per annum in 1990s.

There are some constraints to the sustainability of aquaculture as discussed

in section 4.6 of this module. Dear student, please refer to section 4.6 to

refresh your memory on the challenges to the sustainability of aquaculture.

In spite of all these constraints, it is, however, possible to conclude that

aquaculture stands a potential candidate in providing fish supply to human

being in the future if it is properly managed.

5.2. Ethiopian Case

Details of Ethiopian fisheries have been addressed in chapter 6 of your

Aquatic Sciences and Wetland Management (Biol 302) course module.

Ethiopia is a landlocked country without maritime access. The country is,

however, endowed with about 7000 km2 wide lakes, 250 km2 wide reservoirs

and ponds, and 7400 km long rivers. Therefore, the Ethiopian fishery comes

entirely from inland water bodies (i.e. lakes, reservoirs, ponds and rivers).

However, both the capture fisheries and aquaculture sectors of the country

are underdeveloped.


Scant and rough studies estimate the country’s annual capture fishery

potential to be between 30,000-50,000 tons of fish. There are also reports

that indicate the country’s capture fishery potential to be between 44, 000-

49, 000 tons of fish per year. Despite a little discrepancy in the estimates, it is

possible to conclude that the country has a considerable fishery potential

that could augment the livelihood of its people if properly utilized.

Out of the country’s total annual fishery potential 72 % is ascribed to lakes,

15 % to rivers, and 13 % to reservoirs and other small water bodies. Most of

the lakes and reservoir fisheries are located in Oromia, Amhara, and

Southern Nation Nationalities and Peoples (SNNP) Regions; whereas most of

the riverine fisheries come from the Benishangul and Gambela Regions.

Activity:

Dear students, some basic points about the Ethiopian fisheries have been

discussed in chapter 6 of your Aquatic Sciences and Wetland

Management (Biol 302) course module. From your knowledge of the

course:

Q. What is an estimate for the Ethiopian capture fishery production

potential per year?

Activity:

Q. What is an estimate for actual capture fishery production per year from

the Ethiopian water bodies?

Activity:

Dear student, which types of the country’s water bodies (lakes or rivers

or reservoirs), do you think that, have the highest capture fishery

potential?


The country’s annual average fish production is generally very low despite

the crucial need for food supply in the country. For instance, in 1994 only 25-

28 % of the total fisheries potential was harvested as summarized in Table

5.1. In other words, this means that about 72 - 75 % or 31, 740 – 35, 740 tons

of the country’s fishery resource has not been still utilized.

Table 5.1. Summary of the Ethiopian fisheries resource potential and actual

production for the year 2001/02 (Source: Ethiopian Institute of Agricultural

Research, EIAR)

Type of Water body Production potential Actual production

Tons Percent

Major Lakes 30,963-35,963 10,441 29-34 %

Major Rivers 7,000 700 10 %

Reservoirs & Ponds 6.067 1, 150 19 %

Total 44,030-49,030 12,291 25-28 %

Most of the Ethiopian actual Lake fisheries are derived from the rift valley

lakes (e.g. Abaya, Chamo, Awassa, Langano, and Zeway) and a high land lake

(i.e. Tana Lake), which are located in Oromia, Amhara and SNNP Regions. In

contrast, most of the country’s actual riverine fisheries originate from Baro-

Akobo Rivers in Gambella and Benishangul Regions. Dear student, please

refer to chapter 2 of your Aquatic Sciences and Wetlands Management (Biol

Activity:

Dear student, we have discussed above that most of the country’s fisheries

potential is predominantly attributed to the lakes followed by the rivers.

Q. From which of the lakes and rivers do most of the country’s actual capture

fisheries come?


302) course module for the discussion on various lakes and rivers of

Ethiopia.

So far some fisheries development projects, under the auspices of mainly the

overseas governmental and non-governmental organisations, have been

implemented hosted under the Ministry of Agriculture.

� 1973 - 1978: The Japan International Cooperation Agency under

the Japan Overseas Volunteers Service” sponsored the

establishment of the “Freshwater Fisheries Research Station”

project, which ultimately established the Fisheries research center

located in Sebeta town, near the capital Addis Ababa. Dear

student, please refer to section 5.1 of this module for more

information on Sebeta Fisheries research center.

� End of 1970s - end of 1980s: The Netherlands based non-

governmental organization (NGO) funded the small scale fisheries

development in Gambela, Tana Lake and Arba Minch areas.

� European Union (EU) supported the Lake fisheries development

project (LFDP) which was oriented to increase and improve fish

production and marketing from the rift valley lakes. LFDP was

implemented in two phases: LFDP phase I between 1981 - 1987

and LFDP phase II between 1992 - 1998.

� In 1980s a domestic or national NGO, namely the Ethiopian

Orthodox Church/Inter Kerk Urk (EOC/DICA) implemented a

Activity:

Dear student,

In our above discussions, we have had a look at the Ethiopian fisheries

potential and actual productions from its various inland water bodies.

Q. What did the past development activities of the fisheries sector of the

country generally look like?


development project in Lake Tana and southern lakes (such as

Chamo and Abaya), which supported a purchase of motorized

fishing boats and some equipment.

In recent times the country has come to recognize the socio-economic

benefits of its fishery sector more than ever before. This can be justified by

its endorsement of Fisheries Legislation 315/95, which is the first of its kind

in the country. Dear student, we will come shortly to a brief explanation on

the country’s Fisheries Legislation below, and now let’s focus on the sector’s

current state and future prospect.

The socio-economic benefits generated from the country’s fisheries sector

range from the people’s direct involvement in fishing, construction and

supply of fishing gears, and processing and retailing activities, which create

additional income and employment to a large number of especially the

coastal people. As such the fishery sector, thus, helps in ensuring food

security and poverty alleviation particularly to the low income earning

Activity:

Dear student, we have discussed above about the past development

activities of fisheries sector of the county.

Q. What do the current state and future prospect of the fisheries sector of

the country look like?

Activity:

Dear student, before we proceed to the discussion on the current state and

future prospect of the fisheries sector, can you mention some of the socio-

economic benefits of the sector?


people. Moreover, due to its high quality nutritional value it helps in

preventing deficiency diseases in people consuming fish.

Accordingly, the country has developed a couple of fisheries development

projects in recent years. For instance;

• Funded by the African Development Fund (AfDF), in October 2004, a

National Fisheries Development Study Project was prepared, with the

main objective of acquiring knowledge on the fisheries resource base,

and identifying development and management interventions.

However, the status and outcomes of the project have not been

reported.

• In April 2009, a National Aquaculture Development Strategy (NADS),

funded and supported by the Food and Agriculture Organization Sub-

Regional Office for East Africa, has been prepared. The NADS project

has been planned in short-term (2009-2012) and long-term (2010-

2019) bases with the following major objectives:

� Defining a regulatory framework in which the aquaculture

industry can be developed in an economically, socially and

environmentally sustainable manner.

� Integrating the aquaculture industry into the agricultural

sector and to facilitate development of viable aquaculture

plans.

The success of these and other projects, and thus the development of

fisheries sector of the country, requires commitment of the government and

the concerned executive bodies.


Generally fisheries legislations are required for the management and proper

utilization of fisheries resources. In the absence of such legislations, there

will be no legal basis to control and manage illegal fishery activities and

practices. Thus, Ethiopia, cognizant of the socio-economic benefits of its

fisheries sector, has endorsed a fisheries legislation numbered 315/95 to

ensure proper management and utilization of its fisheries resources.

The Ethiopian fisheries legislation comes in many articles and sub-articles.

For instance the legislation in its article 5 and sub-articles 10, 11 and 12

stipulates that:

• Anyone who seeks to import or export live fish into or out of the

country should get prior letter of permission from the Minister office

(Sub-article 10).

• Anyone who seeks to introduce an imported or local live fish into the

water bodies of one Region into another Region should get prior

letter of permission from the Minister office (Sub-article 11).

• Anyone who seeks to introduce live fish from a water body of one

Region into another water body of the same Region should get prior

letter of permission from the concerned regional office (Sub-article

12).

Dear student, according to the legislation any violation of these regulations

will lead to penalty in terms of specified amount of money, imprisonment or

both.

Activity:

Dear student, so far we have discussed about the past and present state, and

future prospect of the fisheries sector of the country. We have also mentioned

that the country has endorsed the Fisheries Legislation 315/95. What do you

think are the values of endorsing such fishery legislation?



1. What is the current status or trend of capture fisheries production in the

world?

2. What is the current status or trend of aquaculture production in the

world?

3. What factors could be related to the current status or trend of capture

fisheries production in Ethiopia?

4. What is the capture fisheries production potential of Ethiopia?

5. What is the actual fisheries production potential of Ethiopia?

6. List and discuss the past development history of Ethiopian capture

fisheries

7. Explain the current development status of Ethiopian fisheries.

8. Discuss some important points about the Ethiopian Fisheries Legislation.


Benefits of Fisheries

Chapter Outline

6.1. Nutrition and Socio-economic Benefits of Fish

6.1.1. Subsistence Fishing

6.1.2. Industrial and Recreational Fishing

Chapter Objectives


� Explain the nutritional values of fish diet (proteins, fatty acids,

vitamins and minerals)

� Discuss the socio-economic benefits generated by the fishermen in

general and by the Ethiopian fishermen in particular

� Discuss the contribution of industrial and recreational fishing to

the countries’ economy.


6.1. Nutrition and Socio-economic Benefits

6.1.1. Subsistence or Artisanal Fishing

Subsistence or artisanal fishing primarily provides a nutritious fish diet

particularly to the poor society. Therefore, fishes caught through these forms

of fishing are virtually used for domestic or local consumption. Moreover,

these fishing practices also generate so many socio-economic benefits to the

fishermen. These include income and employment secured through gear

construction, fish processing, transportation and retailing. Therefore, these

fishing activities play a very crucial role in supporting food security in the

face of the nations’ population explosion particularly in developing countries

like Ethiopia. Dear student, let’s now briefly elaborate on the nutritional

values of fish and the associated socio-economic benefits.

Activity:

Dear student, in section 3.1 of this module we have defined the various

types of capture fisheries such as subsistence fishing, artisanal fishing,

small scale commercial fishing, industrial fishing, recreational fishing,

etc. Please go back to the section and try to write down the definition

of each in your note book before you go on reading the following

sections.

Activity:

Dear student,

Q1. When we talk about the importance of subsistence fishing as a

source of food especially to the poor, does it merely mean to fill up the

empty stomachs that couldn’t otherwise afford beef or lamb?

Q2. How does the nutritional value of fish compare with that of beef or

other types of meat?


Fish is highly nutritious. It provides proteins, fats, vitamins and minerals. It is

evident that fish contribute more to people’s diets than just the high quality

protein they are so well known for. Fish should, therefore, be an integral

component of every one’s diet, preventing malnutrition by making these

macro- and micro-nutrients readily available to the body.

Fish proteins are easily digestible and are of high biological values. On a

fresh-weight basis, fish contains a good quantity of protein, about 18-20%,

and contains all the eight essential amino acids including the

sulphur-containing lysine, methionine, and cysteine. Thus, fish can

complement diets such as maize and cassava that are poor in protein.

The fat content of fish varies depending on the species as well as the season

but, in general, fish have less fat than red meats. The fat content ranges from

0.2% to 25%.

Fish fats contain the polyunsaturated fatty acids (PUFAs) namely

eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These are

omega-3 fatty acids which are essential for proper growth of children and are

not associated with the occurrence of cardiovascular diseases such as

coronary heart disease. In other studies, omega 3 fatty acids have also been

associated with reduced risk of pre-term delivery and low birth weight.

Moreover, in pregnant women, the presence of PUFAs in their diets has been

associated with proper brain development among unborn babies. Fish fat

Activity:

Dear student,

Q1. What kind of fatty acids are contained in fish fat?

Q2. What are the biological roles of fish fatty acids? Do they have any side

effects on human body if consumed?


also contributes to energy supplies and assists in the proper absorption of fat

soluble vitamins namely A, D, E, and K.

Fish is a rich source of vitamins, particularly vitamins A, B1 (thiamin), B2

(riboflavin), B3 (niacin), D, as well as C (if eaten fresh). Vitamin A is mainly

found in fatty fish and is more readily available to our body as compared to

that obtained from plant. Vitamin A is required for normal vision and for

bone growth. Studies have shown that mortality is reduced for children less

than five years of age with a good vitamin A status. As sun drying destroys

most of the available vitamin A better processing methods are required to

preserve this vitamin. Vitamin D present in fish liver and oils is crucial for

bone growth since it is essential for the absorption and metabolism of

calcium. Vitamins B1, B2 and B3 are important for energy metabolism.

Vitamin C found in fresh fish is important for proper healing of wounds,

normal health of body tissues and aids in the absorption of iron in the human

body.

The minerals present in fish include iron, calcium, zinc, iodine (from marine

fish), phosphorus, selenium and fluorine. These minerals are highly

Activity:

Q1. Does fish contain vitamins that are important for proper functioning

of human body?

Q2. If yes, what are those vitamins and their biological roles? How do

they compare with the vitamins obtained from plants?

Activity:

Q1. Does fish contain minerals that are important for the proper

functioning of human body?

Q2. If yes, what are the minerals found in fish and their biological roles?


‘bioavailable’ i.e. they are easily absorbed by the body. Iron is important in

the synthesis of hemoglobin in red blood cells which is important for

transporting oxygen to all parts of the body. Iron deficiency is associated

with anemia, impaired brain function and in infants is associated with poor

learning ability and poor behavior. Due to its role in the immune system, its

deficiency may also be associated with increased risk of infection.

Calcium is required in bone formation and making bones strong, and for the

normal functioning of muscles and the nervous system. It is also important in

the blood clotting process. The intake of calcium, phosphorus and fluorine is

higher when small fish are eaten with their bones rather than when the fish

bones are discarded. Deficiency of calcium may be associated with rickets in

young children and osteomalacia (softening of bones) in adults and older

people. Fluorine is also important for strong bones and teeth.

Zinc is required for most body processes as it occurs together with proteins

in essential enzymes required for metabolism. Zinc plays an important role in

growth and development as well in the proper functioning of the immune

system and for a healthy skin. Zinc deficiency is associated with poor growth,

skin problems and loss of hair among other problems. Iodine, present in

seafood, is important for hormones that regulate body metabolism and in

children it is required for growth and normal mental development. A

deficiency of iodine may lead to goiter (enlarged thyroid gland) and mental

retardation in children.


Nowadays, modern gears based on synthetic string and netting are

manufactured in factories, and are readily available in most areas of the

world. However, traditional gears constructed by local fishermen become

important where the modern gears are not available. Therefore, the

fishermen or their families can secure income and employment from such

traditional gear construction in such instances.

As described in chapter 7 of this module, the major activities need to be

accomplished following fish catch, until the fishes are supplied to the final

consumer, include processing, transportation and retailing. All these

processes create job opportunities for larger number of people depending on

the scale of fishery.

Activity:

Dear student, we have discussed above about the nutritional values of

fish. Let’s now come to the discussion of how people generate income

and employment from the various aspects of subsistence or artisanal

fishery.

Q. Before you proceed to reading the following sections on this topic, can

you please think of the various ways people secure income and

employment from subsistence or artisanal fishery?

Activity:

Dear student,

Q1. After the fishes are caught from the water body, what are the

subsequent major activities to be accomplished until the fishes arrive

in the hands of the final consumer?

Q2. How do these activities benefit various stake holders involved in

the process of fishing?


6.1.2. Industrial and Recreational Fishing

The values of industrial or large scale commercial fishing extend beyond the

food, income and employment benefits generated at the subsistence or

artisanal fishing scale. The fishes caught through industrial fishing are

processed into various products (see chapter 7 of this module) for export to

international market in order to generate income that contributes to the

national Gross Domestic Product (GDP). Moreover, the development of

recreational fishing attracts tourists and thus generates a tourism industry

which will also add to the national economy.



1. How are fish proteins nutritionally important to human being as

compared to proteins obtained from other sources?

2. How is fish nutritionally important to human being as compared to fats

obtained from other sources?

3. How are vitamins and minerals in fish diets important to human being?

4. List down the various socioeconomic benefits of fisheries

5. Discuss the role of fisheries in improving food security


Processing and Products of Fish

Chapter Outline

7.1. Fish Processing Methods

7.1.1. Post-mortem Changes and Fish Quality

7.1.2. Processing Methods

7.1.2.1. Preliminary Processing

7.1.2.2. Main Processing Stage

7.2. Types of Fish Products

Chapter Objectives


� Discuss the post-mortem changes that affect fish quality

� Distinguish between the preliminary and main fish processing stages

� List down the preliminary fish processing methods

� List down the various methods of fish preservation

� List down some of the fish products


7.1. Fish Processing Methods

7.1.1. Post-mortem Changes and Fish Quality

Fish and fish products are fast deteriorating or perishable materials. The

easy deterioration in fish quality is because of the post-mortem (after death)

biological changes that take place in the body of dead fish. Therefore, fish and

fish products need appropriate processing procedures in order to prevent

deterioration which would otherwise pose public health risks. Changes in

fish quality often come in the form of unpleasant odour and microbial

spoilage. We shall come to the discussion of fish processing methods in

section 7.1.2.

Post-mortem changes that take place in fish tissue occur in the following phases:

slime secretion on the surface of fish, rigor mortis, autolysis as enzymatic

decomposition of tissues, microbiological spoilage. The duration of each phase

can change or phases can overlap. This depends on storage conditions, especially

the temperature which greatly influences these processes.

Activity:

Dear student, now we are going to discuss how an increased slime

secretion by fish after death makes fish easily perishable. Before that

can you define what slime is?

Activity:

Dear student, can you mention some of the post-mortem biological

changes that take place in fish tissue causing it (the fish) to easily perish?


Slime is a mucous or thick sticky substance secreted by the skin cells of

certain animals including fish. When fish is alive slime production adds to the

survival of the fish by making its body too slippery to be caught by its

predators.

In fish the process of slime secretion becomes very active just after fish death

often producing as much as 2-3% of the fish mass. The post-mortem

production of huge amount of slime by fish creates problems during

processing and generally leads to fish spoilage. Of course, slime secretion

stops with the onset of the second phase of change namely rigor mortis.

Slime contains large amounts of nitrogenous compounds and these provide

good nourishment for micro-organisms originating from the environment.

Therefore, the slime spoils quickly: first giving an unpleasant smell to the

fish, and second opening the way for further and deeper bacterial

penetration into the fish.

Activity:

Dear student, can you guess how the post-mortem slime production

creates problems in fish processing and how it may cause fish

spoilage?

Activity:

Dear student,

Q1. From our above discussion write down two points how post-

mortem slime secretion in fish can cause a quality change in fish and

fish products.

Q2. Now, we are proceeding to the second phase of post-mortem

change in fish body namely rigor mortis. Can you define what rigor

mortis is?


Rigor mortis is a condition in which muscle fibres become short and tight,

causing fish to become rigid. This occurs as a result of complicated

biochemical reactions that take place in fish muscle cells.

Dear student, before trying to mention the effects of rigor mortis on the

quality of fish, let’s briefly talk about a fish muscle.

The edible flesh of fish is largely a skeletal muscle, which accounts for more

than 50 % of the total fish weight. The structure of skeletal muscles of fish is

different from that of other vertebrates (e.g. birds or mammals) in that fish

skeletal muscles are largely composed of short bundles of muscle fibres or

cells called myomeres (Fig 7.1). Myomeres assume more or less a v-shaped

appearance and one myomere is separated from another by various

connective tissues. This unique structure and thin connective tissue cover of

fish muscle give the meat its characteristic soft and easily crumbling texture.

Activity:

Q. What do you think is the effect of rigor mortis on the quality of fish

flesh?

Activity:

Do you think that fish muscle differs from that of other vertebrates in

some way? How?


Fig. 7.1. Myomeres in fish muscle structure.

Dear student, it is easy to recognize this unique fish muscle structure just by

comparing beef meat against fish meat. Please, do it whenever you happen to

get beef and fish fleshes and notice the difference carefully.

Therefore, rigor mortis, because of the shortening of muscle fibers or cells,

makes fish flesh short and thick. If, for example, the bones are removed prior

to rigor mortis the length of the flesh shortens by as much as 30%. Moreover,

because of the tightness of the fish muscle, the connective tissues that hold

the myomeres together (see Fig 7.1) break apart causing the separation of

myomeres from each other, which in turn is a quality defect in fish flesh.

The time rigor mortis begins, after fish death, and its duration depend on the

fish species (e.g. for carp at 0° C it starts after 48 hours, for perch at 0° C it

starts after 24 hours), on the fish catching technique, and on fish

temperature. Moreover, unnecessary and rough handling of the fish can

shorten the time of occurrence and duration of rigor mortis. Fish put to death

just after removal from the water reach a state of rigor mortis later than

those fish which died after a long suffering. For instance, in a carp fish put to

Activity:

Dear student, we have mentioned above the effects of rigor mortis in

fish. How do you think that it is possible to avoid or reduce the effects of

rigor mortis in fish?

Myomere

Skin


death just after capture rigor mortis begins after 48 hours (at 0° C), but if the

carp dies after a long suffer it sets in after 24 hours (at 0° C).

Fish body temperature is a decisive factor in the onset and duration of the

rigor mortis process in fish. The higher the temperature the sooner it begins

and the faster it ceases. This is evidenced by enzymatic reactions whose

speed increases with increased temperature. At high temperatures it results

in greater changes in proteins, this in turn causing higher loss of tissue juices

such as during processing. Usually, the later rigor mortis begins and the

longer it lasts, the longer are the storage life of the fish and its use for

consumption.

Autolysis is the breakdown or decomposition of larger molecules such as

proteins, lipids and carbohydrates under the influence of enzymes up on the

fish death. The quality of fish as a raw material for consumption or for

Activity:

Dear student, in our discussion above, we have mentioned some of the

factors that affect the rate of onset and duration of rigor mortis in fish.

Q1. Please list down the factors that affect the onset and duration of

rigor mortis in fish.

Q2. Can you explain how temperature affects the rate of onset and

duration of rigor mortis in fish?

Activity:

Dear student,

Q1. From our above discussion of the relationship between temperature

and rigor mortis in fish, can you explain how keeping fish in relatively cold

temperature increases the shelf life of fish?

Q2. The next phase of post-mortem changes in fish body is an autolysis.

What is it?


processing depends largely on proteolysis, which is the autolysis of proteins.

This is because the proteolysis process leads to a decrease in the capacity of

tissue to retain tissue juice, resulting in tough texture of the final product.

Moreover, the degradation of proteins creates ideal conditions for the growth

of spoilage bacteria.

Microbial (e.g. bacteria) entry into the fish muscle or flesh is facilitated by the

by structural changes in the tissue caused by rigor mortis and autolysis.

Penetration of bacteria into fish tissue and microbiological decomposition

begins with autolysis and these processes are practically parallel. However,

their rate and intensity strictly depend on the storage temperature. Low

temperature strongly inhibits the activity of microorganisms in which case

the autolysis process dominates.

Microorganisms such as bacteria cause decomposition of proteins and other

compounds containing nitrogen, lipids, aldehydes, ketones and lower

aliphatic acids. However, the decomposition of nitrogenous compounds

occurs much faster than in the case of lipids.

Activity:

Q. Freshwater fish tissues undergo microbiological decomposition

more slowly than the marine fish tissues. Can you explain why?

Activity:

Dear student,

Q1. Please write down the two ways in which proteolysis (i.e. autolysis of

proteins) affect the quality of fish.

Q2. The last stage of post-mortem changes in fish body is microbial

decomposition. What materials do microorganisms decompose in fish

body?


Bacteria decompose proteins into smaller or low-molecule products such as

amino-acids and other low-molecule nitrogenous compounds, which in turn

provide nourishment to the bacteria. Thus, due to the lower content of these

substances, in freshwater fish tissues microbiological decomposition occurs

more slowly in freshwater species than in marine fishes.

Compounds such ammonia, hydrogen sulphide, indole, skatole, etc, are the

final products of microbiological spoilage of fish, which produces an

unpleasant and disgusting flavour. Although both autolysis and microbial

decomposition cause the decomposition or breakdown of fish tissue, the

former is caused as a result of enzymatic action whereas the latter is because

of the action of microorganisms.

7.1.2. Processing Methods

Fish processing refers to the processes associated with fish and fish products

from the time fish are caught or harvested, from capture fisheries and/or

aquaculture, to the time of the delivery of the final product to the end user. Fish is

a highly perishable food which needs proper handling and preservation if it is to

have a long shelf life and retain a desirable quality and nutritional value. The

central concern of fish processing is to prevent fish from deteriorating.

Activity:

Dear student,

Q1. Why do you think that a microbially spoiled fish may have

unpleasant flavor?

Q2. Both microbial decomposition and autolysis processes cause the

breakdown of fish macromolecules such as carbohydrates, proteins and

lipids. So, what is the difference between the two processes?


Fish wastes (offal) generated from fish processing should not be just dumped into

the environment. Fish wastes can be managed by converting them in to by-

products (secondary products) such as fish oil and fishmeal. Fish oil is a healthy

diet for human being because it contains the omega-3 fatty acids including

eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) that reduce

inflammation throughout the body. Fish meal is used as a high-protein

supplement in aquaculture feed to rear carnivorous fishes.

Dear student, the whole fish processing procedures can be divided in to two

major phases: preliminary processing and the main processing stage. We

shall see these two phases or stages one after the other.

7.1.2.1. Preliminary Processing

This stage is also referred to as pre-processing stage in some literature.

Preliminary processing of fish usually consists of grading, removal of slime,

deheading (also called beheading), scaling, washing, cutting of fins, gutting

and evisceration, slicing of whole fish into steaks, skinning, filleting, grinding

of skinned fillets, meat-bone separation, and various combinations of these.

Some of these procedures are shown in Fig. 7.2.

Activity:

What are the activities or procedures to be accomplished in the

preliminary fish processing stage?

Activity:

Dear student, fish processing generates wastes often known as fish offal.

How do you think that fish offal can be managed to prevent

environmental pollution?


Fig. 7.2. Some of the pre-processing procedures in fish processing

Definitions of some of the terminologies which may not be explicit are given

below:

� Grading refers to the sorting of fishes according to certain criteria

such as by species or size.

� Deheading (beheading) refers to the cutting and removal of head

using a sharp knife.

� Scaling refers to the removal of scales. This can be done manually

using knife.

Activity:

Dear student, can you define the terms mentioned above in pre-

processing stage of fish and fish products?


� Skinning refers to the removal of skin from the flesh. This can also be

done manually and easily using a sharp knife.

� Gutting and evisceration refer to the removal of gut content

� Filleting is preparation of fillet, which is a bone free flesh of fish.

7.1.2.2. Main Processing Stage

This phase or stage is mainly concerned with fish preservation processes.

Microorganisms particularly the spoilage bacteria require appropriate

temperature, sufficient water and oxygen, and less acidic environments to

reproduce and multiply. Thus, the various techniques used in fish

preservation are based on the principles of interrupting these conditions as

summarised in Table 7.1. In fish preservation we often use combinations of

these methods.

Table 7.1. Summary of fish preservation methods and parameters controlled

Parameter to be controlled Method used

Temperature Chilling, Refrigeration, Freezing

Water activity Smoking, Freeze-drying

Oxygen Vacuum pumping

Chemical control of microbes (pH) Addition of acids

Physical control of microbes Microwave heating, Ionizing

Dear student, among the various methods of fish preservation summarised

in Table 7.1, we will only briefly discuss below some of them.

Chilling, Freezing and Refrigeration

These are preservation methods that involve temperature control. In all the

three methods temperature is decreased in order to reduce the metabolic

activities that take place in the fish due to autolytic or microbial processes (see

section 7.1.1). Moreover, these methods make water less available for bacterial


growth thereby slowing the growth of microorganisms. In chilling and

refrigeration the temperature drops to about 0 °C whereas freezing drops the

temperature to below -18°C. Therefore, differences among the three methods lie

on the materials used: ice box for chilling, refrigerators for refrigeration and deep

freezers for freezing. Thus, fish should be quickly kept at low temperature

immediately after capture or harvest, during transport, processing and

distribution. Fish and fish products are often transported frozen. However, it

should be accompanied by insulated containers or transport vehicles and

adequate refrigeration to ensure safety.

Drying and Smoking

Drying is a process of removing water from the food in order to inhibit the

growth of microorganisms. Microorganisms such as bacteria and moulds

need the water in the food to grow. Thus, drying effectively prevents them

from surviving in the food. Water is usually removed by evaporation (air

drying, sun drying, smoking or wind drying) but, in the case of freeze-drying

water is removed by sublimation after food is first frozen. Smoking is done

by exposing fish to the smoke from burning or flaming wood (Fig 7.3b). It not

only preserves fish but also helps in flavoring and cooking. Therefore, it

comes in different forms such as cold smoking and hot smoking. In cold

smoking fish is exposed to temperatures below 38 °C. It thus helps mainly in

flavouring; but does not cook foods. In hot smoking fish is exposed to a

temperature range of 74 °C-85 °C. Within this temperature range, foods are

fully cooked, moist, and flavorful.

Salting and Pickling

Salting is the preservation of food with dry edible salt. It is related to

pickling (i.e. preparing food with salty water). Salting is used to inhibit the

growth of microorganisms including most bacteria and fungi by creating a

highly salty or hypertonic condition. Pickling (also known as brining or

corning) is the process of preserving food by anaerobic fermentation in


brine (i.e. a solution of salt in water) to produce lactic acid. It creates a pH

less than 4.6, which is sufficient to kill most bacteria. Pickling can preserve

fish foods for months.

In Canning, processed fish products are sealed or packed in airtight

containers called cans (Fig. 7.3a). In canning a tight packing prevents

microorganisms from getting inside. Canning provides a longer shelf life

ranging from one to five years, although under specific circumstances a

freeze-dried canned product can last as long as 30 years in an edible state.

Fig. 7.3. (a) Canned fish, (b) Smoked fish

In order to prevent fish spoilage before and during canning various methods

are used. These include sterilization using pressure canners, pasteurisation,

boiling, refrigeration, freezing, drying, vacuum treatment, addition of

antimicrobial agents that are natural to the recipe of the foods being

preserved, a sufficient dose of ionizing radiation and submersion (in strong

solutions such as saline, acid, base, sugar etc).


7.2. Types of Fish Products

Some of the fish products are cooked fish, frozen fish, dried fish, smoked fish,

salted fish, canned fish, fermented fish etc.

Cooked fish products are most usually for immediate consumption and

require no sophisticated packaging. The shelf-life can be extended for a few

days by using refrigerated storage and the product should be covered to

prevent recontamination. Frozen fish products have relatively long-term

preservation, but the technique is relatively expensive in terms of equipment

and operating costs. Thus, it is not recommended for the majority of small-

scale fisheries.

Cured fish (dried fish, smoked fish and salted fish) products have reduced

water content and thus prevent the development of spoilage bacteria.

Canned fish products have much longer shelf life. Fermented fish products

are formed by encouraging the development of bacteria that increase the

acidity of the fish so that pH of the fish products is lowered. Low pH

discourages the growth of spoilage microorganisms.

Activity:

Dear student, from our discussion of fish processing methods above in

section 7.1, can you mention or define some of the types of fish

products?



1. List down the post-mortem changes in fish and define each

2. How does each of the post-mortem changes in fishes relate to fish

quality?

3. List down and define each of the preliminary fish processing stages

4. List down the various fish preservation methods and how they work

5. List down some of the fish products


Fisheries Management

Chapter Outline

8.1. Stock Assessment

8.1.1. Definitions and Stock Concept

8.1.2. Survey of Stock Population

8.1.3. Quantitative Estimation of Stock Population

8.2. Sustainable Exploitation of the Fisheries Resources

Chapter Objectives


� Define stock assessment

� Define stock and stock concepts

� List down and discuss the various methods of survey of stock

population

� List down and define the various methods in quantitative

estimation of stock population

� Define the maximum sustainable yield (MSY)?

� List down and discuss the various management measures that can

be implemented for sustainable utilization of fisheries resources


8.1. Stock Assessment

8.1.1. Definitions and Stock Concept

Fish stock assessment may be defined as gathering information and

estimating parameters related to fish biomass and its population dynamics

(e.g. fish growth and age). The information gathered via stock assessment is

important in fisheries management system because management measures

can be implemented based on such information about fish population and its

dynamics.

Fisheries management is a science of sustainable exploitation or utilization of

fisheries resources. Fisheries management puts forward measures that that

need to be implemented in order to sustainably utilize the fisheries

resources.

Activity:

Dear student,

Q1. What is fish stock assessment?

Q2. Why is a stock assessment important in fisheries?

Activity:

Dear student,

Q. What is a fisheries management?


Sustainable utilization refers to the use of resources without compromising

or affecting their future existence. The various fisheries management

objectives include:

� Maximizing sustainable fish yield or production from the existing

exploited fish stock

� Conservation of exploited fish species

� Controlling water quality through biomanipulation of fish

Therefore, the importance of fish stock assessment is to provide information

on how these fisheries management objectives may be achieved to ensure

sustainable utilization. Dear student, please refer to section 8.2 of this

module for better details on the fisheries management and sustainable

exploitation of fisheries resources.

The “stock” concept and methods of stock identification are integral part of

and a preliminary step in sock assessment. Fisheries assessment and thus

fisheries management are based on small groups of fish population known as

a unit stock. There is no unanimous definition of “stock” in fisheries

Activity:

Dear student,

Q1. What is a “stock” in fisheries management?

Q2. How can a “stock” be identified?

Activity:

Dear student,

Q1. What do we mean by sustainable utilization of resources?

Q2.What do you think are the various objectives of fisheries management

that require appropriate stock assessment for their achievement?


management. The concept and definition of fish stock are evolving from the

early time.

Details of the stock concept and its definitions in fisheries assessment and

management are focus of advanced courses. However, as far as this course is

concerned, we may define a unit stock as a discrete or semi-discrete group of

fishes that has the same gene pool, is self-perpetuating, inhabits the same

geographical area, and has the same parameters of population dynamics (e.g.

growth and mortality) over the whole area occupied.

Next to stock identification any stock assessment process should pass

through two major steps to yield complete assessment information for

fisheries management. These are:

� Population survey of the stock population

� Quantitative estimation of the stock population

8.1.2. Survey of Stock Population

Population survey is a stock assessment phase when all the necessary

information or data (e.g. growth, age, mortality, etc) relevant to the stock (s)

under consideration are gathered; whereas in quantitative estimation we

estimate stock abundance and/or its production based on the collected data.

Activity:

Dear student, once we complete the preliminary step in stock

assessment i.e. stock identification, what do you think are the

subsequent major steps in order to produce a complete information for

fisheries management?


As we have already mentioned population survey is the first step in stock

assessment. The survey techniques or methods used in gathering population

information can be catch dependent or catch independent. The catch-

dependent population survey techniques are based on gathering the

population information by catching the fish. This can be accomplished

through a planned survey programme to catch fish or by collecting the

required information or data from the already existing fishery data.

In a planned survey, investigators catch the fish and collect the required

information using various techniques such as netting, trolling, trawling, etc.

Please refer to section 3.3 of this module for the various fishing or catch

techniques. However, in collecting data from the existing catch data,

investigators collect information from the record of the already existing

capture fishery data. In this case the source of information can be the

artisanal or commercial fishery.

Activity:

Dear student, we have mentioned above that there are two major

phases or steps in stock assessment. Can you give some examples for

each phase?

Activity:

Dear student,

Q1. What is the difference between the planned survey and collecting

information from the existing fishery data in stock assessment?

Q2. Can you give examples for each of these two types of catch

dependent population survey methods?


Catch independent method is the second and another technique of

population survey in stock assessment in fishery. This method does not

involve the catch of fish to gather the necessary information.

Catch independent methods of population survey include electronic fish

counter, observation and hydroacoustic. However, hydroacoustic survey is

the main catch-independent method that is often applied to freshwater

bodies particularly lakes and reservoirs. Hydroacoustic, also known as

remote sensing, involves the use of an instrument called sonar with an echo

sounder and echo-integrator to make an estimation of fish biomass.

Stock assessment survey is relatively easier in small streams and rivers, and

becomes progressively difficult in large water bodies such as deep lakes due

to an increase in depth and area that limit the efficiency of the methods being

used. Stock assessment methods in temperate and tropical regions tend to be

similar except that collecting data appears to be more difficult in the tropics,

and poses difficulty in stock assessments methods. For instance, in tropics

ageing (i.e. estimating an age) of fish from the hard body parts such as otolith

and scales is often difficult because fish growth in tropics is not always

sufficiently seasonal.

Activity:

Dear student, do you think that there is any difference in stock

assessment:

(a) in streams and large water bodies?

(b) between tropical and temperate regions?

Activity:

Dear student, can you give one example of such catch independent

population survey method in stock assessment?


Fish age can be estimated from hard body parts of fish such as scales, otolith

and bones (e.g. vertebrae or back bones). The age estimation is often based

on counting of growth rings from the cross sections of these hard parts that

are formed seasonally as fish grows. This is more or less similar to age

estimation of large trees from growth rings of tree trunk. Another approach

for estimating fish age is by length frequency method. In this method the

length measurement of large number of fish is taken or recorded and then

converted into age using length distribution curves or length-age keys.

8.1.3. Quantitative Estimation of Stock Population

Various methods and models exist for the quantitative appraisal or

assessment of fish stock abundance and its yield or production.

Some of the methods used in fish stock quantitative assessment include:

• Empirical models

• Surplus production models

• Analytical methods (e.g. virtual population analysis and catch curve

analysis)

• Mark-recapture method

• Mass removal or depletion methods

Activity:

Dear student, fish ageing is one of the activities in fish population survey

to estimate the rate of growth. Can you mention the different methods

used to age fish?

Activity:

Dear student, can you mention some of the methods or models used in

quantitative estimation of fish population abundance and its production?


• Swept area method

• Catch per unit effort (CPUE) method.

Empirical and surplus production models help to make an appraisal of fish

productions or yields; whereas the rest of the methods enable to give an

estimation of stock abundance.

Catch per unit effort (CPUE) measures the relative change in population

abundance; whereas the rest of the methods measure the absolute fish

abundance.

Dear student, it is, however, important to note that these quantitative

methods of estimating fish production and abundance are associated with

certain limitations. For instance,

• They all are based on assumptions that may not be always achieved.

Therefore, the reliability of any single method becomes questioned.

• Some of the methods such as surplus production and analytical

models are costly because they need long time series data.

Thus, when we do quantitative appraisal of fish stock population, it is

advisable to use combinations of methods to increase reliability of the

results.

Activity:

Dear student, which of the methods of fish population quantitative

assessment mentioned above are used to estimate:

(a) potential yield or production of fish population?

(b) fish population abundance?

Activity:

Dear student, which of the methods used in estimating fish stock

abundance give absolute and relative estimation of stock abundance?


The results of stock assessments are used for planning appropriate

management measures to ensure sustainability of fish resources or for

biodiversity conservation. Thus, if the results of stock assessment are

unreliable, wrong management decisions will be made.

8.2. Sustainable Exploitation of Fisheries Resources

Ensuring sustainable fisheries exploitation is a sole job of fisheries

management. As discussed in Chapter 5 of this module most of the natural or

capture fisheries from the major water bodies have already leveled off due to

overfishing.

Overfishing, i.e. fishing beyond the maximum sustainable yield, is the major

threat that has caused the natural or capture fisheries to peak off in most of

the natural water bodies.

Activity:

Dear student, why do you think that the result of quantitative estimation of

fish population should be as genuine as possible?

Activity:

Dear student, what do we mean by overfishing?


Maximum sustainable yield in fisheries may be defined as the level at which

fisheries resources can be exploited without exhausting them. This is

demonstrated in a simple fishery bioeconomic model given in Fig. 8.1. Any

fishing effort exerted up to the point of the maximum sustainable yield (MSY)

is biologically sustainable. In contrast, any fishing effort that goes beyond the

MSY is unsustainable as can be seen from Fig. 8.1. Dear student, please

carefully look at Fig. 8.1. As fishing effort is increased beyond the MSY,

production starts to decline indicating that any fishing activity beyond the

MSY causes the fishery resources to exhaust.

Fig. 8.1. A Simple Bioeconomic Fishery Model. After Karagiannagos (1995).

Activity:

Dear student, what do we mean by the maximum sustainable yield

(MSY)?


The various management measures that can be implemented in order to

prevent overfishing and thus to ensure sustainability of fishery resources

include:

• Limiting the efficiency and types of fishing gear

• Closures

• Size limits

• Rejection of spawning females

• Catch quotas

A. Limiting the Efficiency and Types of Fishing Gear

Dear student, at this point you should be able to refresh your memory on the

various types of fishing gears discussed in chapter 3 of this module.

In fisheries management, the use of fishing gears can be limited in number,

type and size, and some highly efficient or destructive fishing gears and

methods can be banned altogether. For instance, it is possible to restrict the

minimum mesh size of gillnet in order to limit the smallest possible size of

fish to be caught. Setting or limiting the smallest allowable mesh size

Activity:

Dear student, we have seen above that overfishing is the major threat

to sustainable utilization of fisheries resources. What different

management measures do you think need to be implemented in order

to ensure sustainability of the fisheries resources?

Activity:

Dear student, can you explain how each of the above mentioned

management measures can be used to ensure sustainability of the

fisheries resources by preventing overfishing?


prevents the catch of undersize or young fishes and allows the catch of larger

size fishes.

Fishing gear restriction usually leads to fishing inefficiency and thus raises

the cost of catching fish particularly in large scale or industrial fishing. Thus,

gear restriction as a management strategy can be more applicable in small

scale or artisanal fishing. It, however, puts the large scale commercial fishers

at disadvantage.

B. Closures

In fisheries management closures refer to restrictions of either fishing season

or fishing areas. Restriction of fishing season is called temporal closure,

which is a particular time period or season when fishing is prohibited.

Restriction of fishing area is known as spatial closure, which is avoiding

fishing of a particular part or area of a water body.

Dear student, please refer to chapter 2 of this module and refresh your

memory on recruitment and spawning before you read the following

paragraph.

Activity:

Dear student, although gear restriction is a useful management strategy,

it often is associated with some constraints. Can you mention any

limitation associated with gear restriction in fisheries management?

Activity:

Dear student, how does the temporal closure help in fisheries

management?


If the period of recruitment of a particular species of fish is short and well

defined, enforcing temporal closure during the time of recruitment allows

small or young individuals to grow to marketable size. Similarly closed

seasons during well defined spawning period can allow adults to breed

without interference.

Spatial closure can be imposed in nursery and spawning grounds. Identifying

nursery areas, usually the shore areas where the juvenile or young fishes are

fed and grown up, and enforcing no fishing in such areas, can help protect the

juveniles from early fishing. Similarly, if spawning areas are properly

identified and fishing activities are banned or prohibited, the reproducing

adults are protected. Temporal and spatial closures can be implemented

separately or in combination; the overall effect of closures being an increase

in the total fish catch depending on other factors.

C. Size Limits

Limiting the size of individual retained in a catch during fishing is enforced

through regulations. The regulation involves returning captured individual

smaller than a prescribed minimum size to water body.

D. Rejection of Spawning Females

This method involves returning females or egg bearing females to the water

body but it may not be always applicable especially to in fast growing species

with high fecundity.

Activity:

Dear student, how does the spatial closure help in fisheries

management?



1. What is a fish stock?

2. What is a fish stock assessment?

3. Why is stock assessment important in fisheries?

4. What are the various methods used to survey stock populations?

5. What are the various methods used in quantitative estimation of stock

population?

6. What is the maximum sustainable yield (MSY)?

7. What are the various methods or strategies used to ensure sustainable

exploitation of fisheries resources?


Fish Parasites and Diseases

Chapter Outline

9.1. Fish Immunity

9.2. Major Fish Parasites

9.2.1. Protozoan Parasites of Fishes

9.2.2. Helminthic Parasites of Fishes

9.2.2.1. Platyhelminthic Parasites of Fishes

9.2.2.2. Nematode Parasites of Fishes

9.2.2.3. Annelid Parasites of Fishes

9.2.2.4. Acanthocephalan Parasites of Fishes

9.2.3. Copepod Parasites of Fishes

9.3. Major Fish Diseases

9.3.1. Bacterial Diseases of Fishes

9.3.2. Viral Disease of Fishes

9.3.3. Fungal Diseases of Fishes

Chapter Objectives


� Distinguish between the specific and non-specific fish immune

system

� List down the major groups of fish parasites (protozoan,

helminthic, copepod or arthropod, bacterial, viral and fungal

parasites)

� Give examples of each major group of fish parasite

� Describe the life cycle and infective stage of some of the important

fish parasites

� Give examples and effects fish bacterial, viral and fungal diseases


9.1. Fish Immunity

The body defense of fishes has a variety of both non-specific and specific

immune system. The non-specific defense system comprises the skin and

scales, as well as the mucus or slime layer secreted by their skin epidermis.

Non-specific immunity is comprised of defenses that combat a variety of

pathogens at one time rather than a single microbe. Skin and scale cover the

fish body externally like our (human) body is covered with skin. Therefore,

fish skin and scale give primary protection by preventing direct entry of

disease causing organisms into the fish body. Mucus or slime is important in

trapping microorganisms and inhibiting their growth.

Unlike the non-specific defense system, specific defense involves specialised

responses to particular pathogens recognised by the fish's body. For

instance, fish can develop an inflammatory response that increases the flow

of blood to the infected region and delivers white blood cells (WBCs) that will

attempt to destroy the pathogens. In recent years, vaccines have become

widely used in aquaculture and also with ornamental fish.

Activity:

Dear student,

Q1. How are the fish skin, scale and mucus secretion important in fish

immune system?

Q2. Why is a fish defense comprising of these structures called “non-

specific”?

Activity:

Dear student, next we are going to discuss about the non-specific

immune system of fish. How do you think that this is different from the

specific immune system?


9.2. Major Fish Parasites

A parasite is an organism that lives in or on another larger organism of a

different species (the host), upon which it depends for food and shelter. In

host-parasite relationship, the parasite benefits from the association whereas

the host is harmed.

Fish parasites and diseases are more usually common in aquaculture or fish

farming ponds than in the natural fish environments. This is because of the

increased stress to the fish due to their confinement to small area in the

former whereas fishes can disperse over wide area in the latter reducing

stress. Therefore, as we shall see later in the subsequent sections, treatment

of fish parasites and diseases often apply to the aquaculture settings.

Depending on the species, the host-parasite relationship may be temporary

or permanent. Parasites generally do not kill their hosts, but some can

Activity:

Dear student, now we will proceed to the discussion on fish parasites.

Q1. What is a parasite?

Q2. Can you list down some of the major groups of fish parasites?

Activity:

Dear student,

Q1. Do you expect fish parasites and diseases to me a main problem in

aquaculture or in natural environments of fish? Why?

Q2. What do you think are the various effects of fish parasites to their

fish hosts?


severely stress fish populations to the point of becoming biological and

economical concerns. The effects caused by fish parasites include destroying

fish tissue, removing fish blood and cellular fluids, diverting part of fish

nutrient supply and allowing secondary infections to develop in fish body.

The transfer of parasites from animals such as fish to human is known as

zoonosis. Almost all fishes have parasites but only very few fish parasites

can be transferred to humans. Even when a fish exhibits obvious signs of

disease or parasites, most likely the fish is still edible when cooked, hot

smoked, or frozen. People, however, could be infected with tapeworms

(Diphyllobothrium latum) after consuming marinated, uncooked fish.

Infection is caused by parasites that get into the host (fish) body (e.g. fish

tape worm) whereas infestation is caused by parasites that inhabit the

external or superficial body parts (e.g. fish lice).

Dear student, fish parasites come from all the known parasite groups as

summarized in Table 9.1.

Activity:

Q1. Do you think that fish parasites are communicable to human being?

Q2. If yes, what do we call such a transfer of parasites from animals such

as fish to humans?

Q3. What is the difference between the terms parasite infection and

infestation?


Table 9.1. The Major Groups of fish Parasites

Major Groups Subgroups Genus/Common name examples

Protozoa Ciliates Ichthyophthirius, Chilodonella,

Tetrahymena, Trichodina,

Ambiphyra, Apiosoma, Epistylis,

Capriniana

Flagellates Ichthyobodo, Hexamita, Spironucleus,

Piscinoodinium, Cryptobia

Myxozoa

Microsporida

Coccidian

Helmithes

(Worms)

Platyhelminthes

(Flatworms)

Monogenic flukes (Dactylogyrus,

Gyrodactylus and Benedeniella);

Digenic flukes (Clinostomum and

Uvulifer); Cestodes (Protcephalus,

Diphyllobothrium and Ligula )

Nematodes

(Roundworms)

Camalanus, Capillaria, Contracaecum

and Eustrongyloides

Annelids (True worms) Leech

Acanthocephalans (Spiny

headed worms)

Arthropods Copepods (Crustaceans) Argulus, Ergasilus and Lernaea

Viruses

Bacteria

Fungi

Dear student, owing to the time restriction to discuss all the various

parasites, we will restrict ourselves to one or a few typical examples.

Mulugeta Wakjira, Department of Biology, Jimma University, June 2011,

9.2.1. Protozoan P

Among the ciliates

fish parasite. It is a

freshwater fish including both cold water and tropical species. It

called known as

gross appearance of fish infected with this parasite

Fig. 9.1. A fish infes

The small white spots are fish body parts that contain

beneath and under severe infections several spots may coalesce to form irregular

Activity:

Dear student, carefully observe the various fish body parts I. multifilis

infects.

Q1. What are the white spots in

Q2. Which body

Mulugeta Wakjira, Department of Biology, Jimma University, June 2011,

Protozoan Parasites of Fishes (e.g. Ciliates)

Among the ciliates Ichthyophthirius multifiliis is the most commonly encountered

. It is a single-celled protozoan parasite that can infes

freshwater fish including both cold water and tropical species. It

led known as white spot disease (Ich). The term “white spot” refers to

gross appearance of fish infected with this parasite as in Fig 9.1.

. A fish infested with a ciliated I. multifilis

The small white spots are fish body parts that contain the parasite (



Q1. What are the white spots in I. mulifilis infection of fish?

Which body parts of fish does the parasite I. multifilis infect?


the most commonly encountered

rotozoan parasite that can infest virtually all

freshwater fish including both cold water and tropical species. It causes a disease

(Ich). The term “white spot” refers to the

the parasite (I. multifilis)



parts of fish does the parasite I. multifilis infect?


white patches. The parasite infests fish body parts such as skin, fins and gills

though the white spots may not be seen on gills. The gills rather appear swollen

and become covered with thick mucus.

The effects of I. multifilis infection include:

� Parasites feed on body fluids and cells

� Heavily parasitized fish may have many white spots and often

scratch against rocks and gravel, may show increased gill

movements.

� Development of secondary bacterial infections since the slime coat

and epithelium of the fish are compromised.

The life cycle of I. multifilis is direct involving only one species of host. The

parasite consists of both the free-living and parasitic stages. These include

the mature parasite stage, cysts stage and swarmmer stage. The mature

parasites feed on the host tissues (beneath the white spots), break through

the skin and fall away from the fish into water. In water they form the next

and free living stage known as cyst. Cyst is parasite’s the reproductive stage

that divides many times, eventually producing many hundreds of the

swarmmers. Eventually, swarmmers get out of the cysts and swim off to find

a fish host to infect. If they find a host fish, they enter into the fish bodies

such as skin, scale, fin and gill and gradually develop into the mature parasite

Activity:

Dear student, what are the effects of I. multifilis infestation to fish?

Activity:

From our discussions above about the fish parasite namely I. multifilis,

Q1. What is the name of the disease it causes to the infected fish?

Q2. What are the effects of I. multifilis infection to fish?

Q3. What kind of life cycle does it have (direct or indirect)?


stage that causes the development of white spot on the fish body. If they do

not find a host within several days, they die. Thus, a swarmmer is the

parasite’s infective stage.

The mode of transfer of the parasite’s infective stage (swarmmers) to the

host fish is a direct entry through penetration. The time required by the

parasite to complete its life cycle depends on the water temperature: the

higher the water temperature, the faster the life cycle. At low temperatures,

the parasite may become dormant for considerable lengths of time.

Most treatments of the parasite are ineffective due to occurrence of the

parasite within the fish skin. The various treatment methods of the parasite

include immersing the infested fish in chemicals or solutions of differing

concentrations such as sodium chloride (salt), formalin (formaldehyde),

Activity:

Dear student, from our above discussion about the life cycle of I.

multifilis,

Q1. What does direct life cycle mean?

Q2. What are the three stages of development of the parasite I.

multifilis?

Q3. Which of the parasite’s stages are free living? Parasitic?

Q4. What is the infective stage of the parasite?

Q5. What is the mode of transmission of the infective stage of the

parasite to the host fish?

Q6. What factors do you think may influence the parasite to complete

its life cycle?

Activity:

Dear student, what do you think is the possible treatment of the parasite?


malachite green, victoria green, acriflavin and copper. In aquaculture

systems, the parasite can be introduced with fish, live plants or live food

brought from another water body. Thus, prevention methods such as proper

quarantine of the infested fish are more important.

9.2.2. Helminthic Parasites of Fishes

As summarised in Table 9.1 the helminthic (worm) parasites of fish belong to

four major subgroups. We will discuss only a few representatives from each

subgroup.

9.2.2.1. Platyhelminthic Parasites of Fishes

These are commonly known as flat worms due to their dorso-ventrally flat

appearance. The parasitic flat worms of fish belong to two major

subcategories namely flukes (trematodes) and cestodes (tape worms). Dear

student, we will discuss briefly about the fish flukes (A) and cestodes (B)

below.

A. Flukes (Trematodes)

These are also known as trematodes and are categorized into two subgroups

as monogenea and digenea.

Activity:

Dear student, what are the major differences that exist between these

two subgroups of flukes: monogenean and digenean?


Table 9.2. Comparison and contrast between monogenic and digeneic flukes

Character Monogenea Digenea

Mode of

parasitism

Mainly ectoparasites Mainly endoparasites

Attachment

organ

Series of hooks (see Fig. 9.2) Two suckers: oral and

ventral suckers (see Fig.

9.3)

Life cycle Direct (only one species of

host is required)

Indirect (≥ 2 species of

hosts are required)

Mode of

transmission

Direct transfer from fish to

fish

Penetration and ingestion

Treatment Bathing fish in solutions of

praziquantal, Trichlorofon

or salt

Bathing in praziquantal

solution

Genera

Examples

Dactylogyrus, Gyrodactlylus,

Benedeniela

Clinostomum and

Marginatum

Dear student, hereunder we will briefly discuss about the monogenic and

digenic flukes. We will begin our discussion with monogeneans and then

proceed to that of the digenic flukes.

Activity:

Q1. Which of the three genera examples of monogenetic flukes given in

Table 9.2 are parasitic to (a) freshwater fishes, (b) marine fishes?

Q2. What is the difference among the three parasites in terms of their

development and developmental stages?


Fig. 9.2. The three genera examples of monogenic flukes

Dactylogyrus and Gyrodactylus are mainly parasitic to freshwater fishes while

Benedeniella is a parasite of marine fish species. The parasites are

hermaphrodites having both male and female reproductive organs.

Dactylogyrus is an oviparous (i.e. egg layer) and thus has three

developmental stages consisting of egg, larva and adults. Whereas

Gyrodactylus is viviparous (i.e. gives birth to live young or larva) and thus has

two developmental stages: larva and adults.

Freshwater fish infested with skin-inhabiting flukes become lethargic or

sluggish, swim near the surface, seek the sides of the pond and their appetite

dwindles. They may be seen rubbing the bottom or sides of the holding

facility (flashing). The skin, where the flukes are attached, shows areas of

scale loss and may ooze a pinkish fluid. Heavy gill infestations result in

respiratory disease. Gills may be swollen and become pale, respiration rate

may be increased, and fish will be less tolerant of low oxygen conditions.

Hooks for attachment

Activity:

Dear student, what do you think are the clinical signs and effects of fishes

infected or infested by these monogenic flukes?


Gulping air at the water surface may be observed in fishes with severe respiratory

distress. Large numbers of monogeneans on either the skin or gills may result in

significant damage and mortality. Secondary infection by bacteria and fungus is

common on tissue that has been damaged by monogeneans.

The larval stage of the monogenic flukes is transmitted from fish to fish via

body penetration. The larval stages invade the superficial fish tissues such as

skin, fin and gills and develop into the adult stage. Then, adults of

Dactylogyrus shed eggs to be discharged into the water and hatch into larva;

whereas adults of Gyrodactylus give birth to live larva that will also enter

water in search of another fish host as we have already discussed above. The

possibility of the parasite being acquired by humans is rare since these

parasites occupy fish tissues such as skin that can be easily avoided through

proper skinning. Thus, the public health concern is less important but the

parasites may cause economic loss in aquaculture systems by causing

damage to fish health.

Dear student, we will now discuss some important points about the digenean

flukes. The suckers used by the parasite for attachment during feeding on the

host tissue are shown in Fig. 9.3.

Activity:

Dear student,

Q1. What is the infective stage of monogenan flukes to (a) fish, (b)

human?

Q2. What is the mode of transmission of the parasites from (a) fish to

fish, (b) from fish to humans?

Q3. What are the public and socio-economic impacts of fish infested by

these parasites?


Fig. 9.3. An adult digenean fluke

The two common examples of the digenean flukes are Clinostomum

marginatum (commonly known as yellow grub) and Uvulifer ambloplitis

(commonly known as black grup or spot). Both parasites have indirect life

cycle as shown in Fig. 9.4 for the C. marginatum.

Ventral sucker

Oral sucker

Activity:

Dear student,

Q1. Can you give some examples of digeneic flukes?

Q2. How many species of hosts are required by each of these digenic

parasites to complete their life cycles?

Q3. What is the difference between the definitive host and

intermediate host?

Q4. What are the stages of development for each example of digenic

parasite?


Fig.9.4. Life cycle of Clinostomum marginatum (yellow grub)

Three species of hosts are required by both parasites to complete their life

cycles. These are fish, birds (e.g. heron) and mollusk (snail). Fish is a

definitive host whereas bird and snail are the intermediate hosts. The

developmental stages of both parasites consist of adult, egg and three larval

stages known as miracidium (1st larval stage), cercaria (2nd larval stage)

and metacercaria (3rd larval stage). The larval stages may be encysted i.e.

found in the cyst form.

Adult digeneans are usually found in the gastro-intestinal tract (GIT) such as

mouth, throat, esophagus and intestine of their hosts; whereas the larval

stages can be found in various host tissues and cause problems if they invade

such organs as the eyes or heart in high numbers.

Snail, Intermediate host

Fish, Definitive host Heron, Intermediate host


The larval stages of the parasite are found as whitish or yellowish cysts in the

flesh and near or just beneath the skin of fish, especially at the base of the

fins and tail.

• The adult C. marginatum are found in the mouth, under tongue, in the

throat and esophagus of fish eating birds such as heron. Heron serves as

the first intermediate host to the parasite.

• The adult parasites thus shed their eggs into water when the birds feed in

the water.

• Very soon the eggs hatch into the actively swimming first larval stage

known as miracidium.

• Miracidium continues to swim actively in the water until it will find a

second intermediate host, which is a mollusk known as snail. When

miracidium encounters a snail, it enters the snail by penetrating the body.

Activity:

Q1. Which of the fish tissues do you think are invaded by the larval stages

of the fluke parasite C. marginatum?

Activity:

By referring to the life cycle of C. marginatum diagrammatically

represented in Fig. 9.4.,

Q1. Can you identify the definitive and intermediate hosts?

Q2. Can you identify the various stages of the parasite?

Q3. Can you identify the modes of transmission of the parasite to the

various hosts?


The mode of transfer of miracidium to the second intermediate host (snail) is

through penetration. Thus, miracidium is an infective stage of the parasite to

the second intermediate host (snail).

• Within the body of the snail the first larval stage i.e. miracidium develops

into the second larval stage called cercaria. Cercaria leaves or gets out of

the snail’s body again through penetration. After leaving the snail’s body,

the cercaria then swims freely in the water until it will find a fish host,

which is a definitive host of the parasite.

• The second larval stage of the parasite, cercaria, is infective to the

definitive host (fish). It gets into the definitive host (fish) by penetrating

the body. After penetrating into the body of fish, cercaria develops into

the third larval stage known as metacercaria that encysts in fish internal

organs (such as liver and muscle) or in external tissues (such as fin, skin

and gill).

Activity:

Dear student,

Q1. What is the infective stage of the parasite to the definitive host

(fish)?

Q2. What is the mode of transfer of the parasite to the definitive host

(fish)?

Activity:

Dear student,

Q1. From the above discussions, what is the mode of transfer of

miracidium to the second intermediate host (snail)?

Q2. What is the infective stage of the parasite C. marginatum to the

intermediate host (snail)?


• When the infected fish is eaten by a bird such as heron, metacercaria is

transmitted to a bird to become adult, and the cycle continues. Thus, the

mode of transmission of the parasite to the first intermediate host is

through ingestion.

• Once ingested by the bird, metacercaria develops into the adult stage that

stays in the bird’s GIT such as mouth and esophagus. The parasite

ultimately gets back to the water when adults discharge their eggs as

described above.

The parasite (metacercaria stage) is transmitted to human being when fish is

ingested or consumed. However, zoonotic infection cases are generally rare

because the parasite can be avoided via proper skinning if the parasite

(metacercaria stage) occupies the external tissues such as skin; and if the

parasite (metacercaria stage) occupies the internal tissues such as muscle it

can be prevented by eating well cooked fish. Thus, there is no serious public

health concern; economic concern is rather more important. That is because

the infection makes fish unmarketable for aesthetic reasons i.e. due to the

bad appearance of fish infected by the parasite.

Activity:

From our above discussions about the life cycle of C. marginatum, what can

you infer about the public health and socio-economic impacts of fish

infected by this parasite?


The best control of C. marginatum, and all other digenean trematodes, is to

break the life cycle of the parasite. For instance, the parasite’s life cycle can

be broken by elimination of the first intermediate host, the snail especially in

aquaculture systems. In this regard, addition of copper sulfate solution in

ponds has been used with limited success and is most effective against snails

when applied at night, due to their nocturnal feeding activity. Also refer to

Table 9.2.

B. Cestodes (Tapeworms)

Unlike the fish flukes, cestodes are almost entirely endoparasites whose adults

live in the digestive tract (GIT) of their hosts. Tapeworms are characterized by a

long, flattened and segmented body which often tapers towards one end. A

distinct head (Scolex) is present which has several small suckers followed by long

chain of segments (Proglottides) as in Fig. 9.5.

Activity:

Dear student, so far we have discussed about the life cycle of the

digenean fluke parasite known as C. marginatum. From our

discussions,

Q1. What is the stage of the parasite that enters water from the

mouth of bird (heron) during the heron’s feeding in the water?

Q2. What is the infective stage of the parasite to the (a) snail, (b) fish,

(c) bird (heron)?

Q3. What is the mode of transmission of the parasite to (a) snail, (b)

fish, (c) bird, (d) human?

Q4. What is the stage of the parasite found in the body of (a) snail, (b)

fish, (c) bird?

Q5. What do you think are the best control methods the parasite?


Fig. 9.5. A fish tapeworm

The Scolex (head) consists of suckers or hooks for attachment to the host tissue;

whereas the segments (proglottides) consist of reproductive structures including

eggs. As the segments become sexually mature a portion of chain of segments

(proglottides) is broken off the main chain and passes into the environment via

fish excreta.

Those cestodan parasites known to infect fishes are Proteocephalus ambloplitis

(commonly known as bass tapeworm), Ligula intestinalis (commonly known

as ribbon tapeworm), Diphyllobothrium latum (commonly known as fish tape

worm) and Bothriocephalus acheilognathi. Among these four common fish

tape worms, the major characters of only P. amblopitis and D. latum are given

in Table 9.3.

Scolex

SegmentsSuckers

Activity:

Q1. Can you give some examples of cestodan fish parasites?

Q2. What are the similarities and differences among these cestodan fish

parasites?


Table 9.3. Some major characters of two representative fish tape worms

Character P. ambiloplitus D. latum

Mode of

parasitism

Endoparasites Endoparasites

Attachment

organ

Hooks or suckers (see Fig. 9.5) Hooks or suckers (see Fig.

9.5)

Life cycle Indirect ( 2 species of hosts

are required): Fish & Copepod

Indirect ( 2 species of hosts

are required):Fish & Copepod

Definitive host Fish Copepod

Interm. host Fish Copepod

Developmental

stages

Egg, Procercoid larva,

Plerocercoid larva and Adult

Egg, Coracidium larva,

Procercoid larva,

Plerocercoid larva and Adult

Infective stage

to def. host

(fish)

Procercoid larva Procercoid larva

Infective stage

to interm. Host

(copepod)

Eggs Coracidium larva

Infective stage

to human

Plerocecoid larva Plerocercoid larva

Mode of

transmission

to def. host

Ingestion of the parasite with

intermediate host (copepod)

Ingestion of the parasite with

intermediate host (copepod)

Mode of

transmission

to interm. host

Ingestion of eggs in the water Penetration of coracidium

larva into the copepod body

Mode of

transmission

to human

Ingestion of plerocercoid in

the fish muscle

Ingestion of plerocercoid in

the fish muscle

Treatment of

infected fish


solution


solution

Prevention of

human or

zoonotic

infection

Serving well cooked fish Serving well cooked fish

Dear student, in Table 9.3 we have compared the two representatives of tape

worms in terms of various characters most of which revolve around their life

cycle. Let’s now briefly describe the life cycle of these parasites. The


diagrammatic representation of P. ambloplitus (bass tape worm) is shown in

Fig. 9.6.

• The adult stage lives in the intestine of the fish and thus eggs pass into

the water with fish droppings.

• The eggs are eaten by the intermediate host, copepods, in which they

hatch into a larval stage, procercoid.

• When copepod is eaten by fish the larval stage gets access in to the fish

where it undergoes further development to become plerocercoid that

ultimately becomes adult in the intestine of the host. The cycle repeats.

Fig. 9.6. Life cycle of Bass tapeworm, Proteocephalus ambloplitis

Activity:

Dear student, by comparing Table 9.3 with Fig. 9.6, what difference do

you observe between the life cycle of P. ambloplitus (bass tape worm)

and D. latum (fish tape worm)?


In case of P. ambloplitus (bass tape worm) eggs of the parasite shed into the

water are directly ingested by the intermediate host (copepods). However, in

case of D. latum (fish tape worm) the eggs passed into water via the fish

droppings hatch into a larval stage known as coracidium that will

subsequently penetrate into the body of copepods.

The plerocercoid larvae are the most damaging parasites to freshwater fish. They

decrease carcass value if present in muscle, and impair reproduction when they

infect gonadal tissue. Problems also occur when they damage vital organs such as

the brain, eye or heart and symptoms of tapeworm infections may be a swollen

abdomen, swimming difficulty and wasting or killing. Therefore, economic

impact from fish death can be significant in aquaculture systems. There is also a

possibility of zoonotic infection when the plerocercoid exists in fish muscle.

However, it is possible to avoid the infection by avoiding uncooked or

undercooked fish.

Activity:

Dear student, I hope that you have got a picture of the life cycle,

developmental stages and modes of transmissions of the

representative fish tape worms from our discussion so far. From your

knowledge of the topic,

Q. what do you think could be the public and socio-economic impacts

of these fish tape worms?

Activity:

Dear student, from the description of the life cycle of D. latum using only

words and arrows sketch the life cycle of D. latum.


9.2.2.2. Nematode Parasites of Fishes

These are commonly known as roundworms and have elongated non-

segmented body tapered at both ends lacking suckers as shown in Fig. 9.7.

Examples include Camalanus, Capillaria, Contracaecum and Eustrongyloides.

The developmental stages of nematodes comprise egg, four larval stages

and adults. These are endoparasites with adult roundworms living in fish

intestine whereas the larval stages are found in almost the entire internal

organs and muscle tissue.

The fish nematodes have complex indirect life cycles involving various hosts

including copepods, insect nymph, birds, mammals; and fish can serve as both

intermediate and final hosts. Nematodes can cause problems to fish. In severe

infections, they may be seen protruding from the vent. Zoonotic infections are

possible if raw or poorly cooked fish is consumed. The parasites can be prevented

by breaking the life cycle and infected fishes can be treated using appropriate

antihelmintic drugs such as fenbendazole.

Activity:

Q1. Do you think that the fish nematodes have direct or indirect life

cycle?

Q2. How do you think that these fish parasites can be prevented or

treated?


Fig. 9.7. A fish roundworm

9.2.2.3. Annelid Parasites of Fishes

These are commonly known as true or ringed worms. Annelid parasites of

fish include leeches that serve as vector to transmit blood parasites

including Trypanosoma and Trypanoplasma. Leeches could attach to the host

internally in the throat region or externally to a skin as shown in Fig. 9.8.

Fig. 9.8. Leeches externally attached to fish body


9.2.2.4. Acanthocephalan Parasites of Fishes

Acanthocephalan parasites are commonly known as spiny or thorny headed

worms due to their possession of spines like structures as in Fig. 9. 9.

Fig. 9.9. Acanthocephalan fish parasite

They are internal or endoparasites highly specialized for life in the digestive

tract of their fish host. They have indirect life cycle with crustaceans serving

as the intermediate host. Acanthocephalans rarely cause problems in captive

or aquaculture fish.

9.2.3. Copepod Parasites of Fishes

These are crustacean arthropods that are parasitic to fish. Copepod parasites

of fish are basically ectoparasites that exist on the external body surface of

fish. Examples include Argulus (commonly called fish louse) and Ergasilus

(commonly called gill maggot or gill louse). Fig. 9.10 demonstrates some

structural features of fish louse and gill maggot.

Proboscis with spiny structures


Fig. 9.10. Crustacean fish parasites (a) Argulus (fish louse), (b) Ergasilus (gill

maggot)

Argulus (fish lice) attach themselves to the skin and fins and feeds on blood

of their fish host. Consequently,

• They may cause intense irritation to the fish that will in turn cause the

fish to scratch against rocks

• Heavily infested fish may be seen jumping out of the water because of

the irritation.

• Red lesions occur where the lice have attached, and this opens the

skin up to secondary bacterial and fungal infections.

Ergasilus (Gill maggots or lice) are usually found attached to the gills, gill

covers and inside the mouth.

(a) (b)

Activity:

Dear student,

Q1. What do you think are the effects of fish lice and gill maggots to fish?

Q2.What are the possible treatments of fishes infested by these

ectoparasites?


• Heavy infestation by gill maggot can result in severe gill damage,

emaciation, anemia and death.

Both Argulus and Ergasilus parasites, because they suck blood, can transmit

certain infections between fish. Treatment of these ectoparasites requires

use of an organophosphate insecticide and baths of infested fishes

Trichlorfon.

9.3. Major Fish Diseases

The known major fish diseases are infectious being caused by the pathogenic

parasites such as bacteria, viruses and fungi. Many of the fish diseases have

no known therapy (cure), and thus prophylaxis (prevention) is the only

control measure. As we have already discussed in our introduction to the

chapter, fish diseases are more common and cause major problems in fish

farming or aquaculture settings than in natural environments. Dear student,

in sections 9.3.1 through 9.3.3 we will briefly discuss about the fish diseases

caused by bacteria, viruses and fungi.

9.3.1. Bacterial Diseases of Fishes

Some of the major bacterial fish diseases are summarized in Table 9.4.

Activity:

Dear student, with our discussion so far we are coming to a conclusion

on some of the major parasites of fish.

Q1. As part of our conclusion, from your knowledge on fish diversity,

can you give examples of fishes that are parasitic to other fish?


Table 9.4. Some of the major bacterial fish diseases.

Disease Causative agent Effect Fish infected

Furunculosis Gram negative

bacteria (Aeromonas

salmonicida)

Development of

furuncles (small

swellings) on skin

Mainly

Salmonids

Columnaris

(saddleback disease)

Flavobacterium

columnare

Gill & cutaneous

infection

Mainly eels,

salmonids,

cyprinids, etc

Enteric red mouth

disease (Yersiniosis)

Gram negative rod

bacteria (Yersinia

ruckeri)

Reddening of the

throat and mouth

and haemorrhages

along the gumline of

the mouth and the

tongue

Mainly

Salmonids

Edwardsiellosis Gram negative

bacteria (Edwardsiella

tarda)

Muscle lesion Cyprinids

and others

Vibrosis Gram negative

bacteria (Vibrio)

Primarily skin lesion Variety of

fishes

Pasteurellosis Photobacterium

damselae

Spleen and kidney

infection

Rainbow trout fry

syndrome (Bacterial

cold water disease )

Flavobacterium

psychrophilum

Acute septicemia

with spleen

hypertrophy

Salmonids

Marine flexibacteriosis Flexibacter maritimus Eroded and

hemorrhagic mouth,

ulcerative skin

lesions, frayed fins,

and tail rot.

Marine fish


Pseudomonaiasis (red

spot disease)

Pseudomonas species Septicemia with

abdominal

distention and

hemorrhage of skin

and internal organs

Streptococcosis Cocci bacteria such as

Lactococcus garvieae

Central nervous

damage involving

exophthalmia ("pop-

eye") and

meningoencephalitis

Bacterial kidney

disease

Gram-positive

bacteria

(Renibacterium

salmoninarum)

Kidney infection Salmonids

Mycobacteriosis (fish

tuberculosis)

Mycobacterium

species

Tuberculosis

development

Many fishes

Piscirickettsiosis Piscirickettsia

salmonis

Dear student, let’s know have a brief look at a few of the fish bacterial

diseases summarized in Table 9. 4.

A. Furunculosis

Furunculosis is a bacterial disease that results in the development of blisters

or small swellings known as furuncles on the skin of the infected fish. This is

a bacterial disease caused by a gram negative Aeromonas salmonicida sub sp.

Salmonicida.


The formation of furuncles cannot be a sure sign of the disease because

furuncles could also occur in other types of infections plus in acute cases of

Furunculosis, furuncles may not be formed.

Furunculosis causes economically devastating losses in cultivated salmonids

in fresh and marine waters. It also affects a variety of non-salmonid fish and

shows a widespread distribution.

B. Vibriosis

Virbriosis is a bacterial disease caused by various species of a genus Vibrio,

which is a Gram-negative bacterium possessing a curved rod-shape. Vibrio

usually enters fish through the surface wounds and acts mostly on the skin,

where lesions are formed. The ulcers can extend deep in to the muscles and

internal haemorrhage, kidney damage and a swollen spleen are sometimes

found in dying fish. It infects a great variety of warm and cold water fish

species of economic importance. The disease is known to be accelerated at

higher temperature.

Activity:

Dear student, do you think that the development of furuncles on the

skin of infected fishes is a confirmation of infection by Aeromonas

salmonicida sub sp. Salmonicida?

Activity:

Dear student, which fish groups do you think are mainly affected by this

disease?


C. Mycobacteriosis

Mycobacteriosis is also known as fish tuberculosis, piscine tuberculosis, acid-

fast disease, granuloma disease, etc. It is a wasting disease known to affect

near 200 freshwater and saltwater species.

It is caused by a large number of Mycobacterium species such as M. marinum

M. fortuitum, M. chelonae, M. smegmatis, M. abscessus, M. neonarum, M. simiae,

M. scrofulaceum and M. poriferae

Fish infected with tuberculosis may become lethargic, hollow bellied, pale,

show skin ulcers and frayed fins, have fin and scale loss, and loss of appetite.

Yellowish or darker nodules may appear on the eyes or body and may

deform the fish.

9.3.2. Viral Diseases of Fishes

Some of the fish diseases caused by viruses include infectious pancreatic

necrosis (IPN), infectious haemtopoietic necrosis (IHN), Infectious dropsy of

carp (IDC), viral haemorrhagic septicaemia (VHS), channel catfish virus

disease (CCVD), carp pox (CP) and lymphocystis. We will briefly discuss only

a few of these viral fish diseases below.

Activity:

Dear student, what do you think are the effects of the disease

Mycobacteriosis to the infected fishes?

Activity:

Dear student, we now proceed to the discussion of some of the fish

diseases caused by viruses. Can you mention some of the viral fish

diseases?


A. Infectious Pancreatic Necrosis (IPN)

This viral disease causes necrosis in pancreatic tissue and is also

characterized by the occurrence of clear or milky mucus in the intestine. At

extreme situations the infected fishes undergo a characteristic “whirling

behaviour” in which the fishes swim in a rotating manner about their axis,

and death occurs in an hour or so.

Prior to this stage, however, the affected fishes may remain on the bottom,

showing weak respiration and conclusive movements. It is common in

salmonids and is transmitted from fish to fish and from parent to progeny

through seminal fluids or infected eggs.

Since there is no effective treatment for IPN, the only means to control the

disease is through preventive measures, which include the incubation of

virus free eggs and propagation of IPN-free fishes in uncontaminated water

in aquaculture systems.

Activity:

Dear student, can you mention the effects and symptoms of fishes

affected by this disease?

Activity:

Dear student, how do you think that this viral disease can be treated or

prevented especially in farmed or aquaculture systems?


B. Viral haemorrhagic septicaemia (VHS)

This is an acute to chronic viral disease characterized by the clinical signs of

dark appearance of fish, lethargic fish, and haemorrhages in the fin sockets.

With the advance of the disease, the fish becomes nearly black and develops

acute anemia. The gills become pale in colour and bleeding occurs in the gills

and muscle. Prevention is more important as in the case of other viral

infections there is no known cure for VHS.

C. Lymphocystis

This is a viral disease that occurs in several species of freshwater, brackish

water and marine species of fish. It occurs as abnormally large white lumps

or nodules on the fins or other parts of the body. The disease is highly

contagious and especially under culture conditions it can spread rapidly.

Like other viral diseases lymphocyctis has no known cure and prophylaxis

(prevention) is the only means of control. In culture systems the infected fish

should be destroyed to prevent the spread of virus and the rearing facilities

should be thoroughly disinfected in aquaculture systems.

9.3. 3. Fungal (Mycotic) Diseases of Fishes

Dear student, lastly in fish disease we are going to discuss about the fungi

caused diseases. Two of the common fungal or mycotic fish diseases are

saprolegniasis and branchiomycosis.

A. Saprolegniasis

Saprolegniasis is a fish disease caused by a fungus of Saprolegnia species. The

disease affects skin and gills of freshwater fish and crustaceans. It is also


known that the disease is also caused by non-saprolegniaceous fungi such as

Pythium and Leptomitus.

The causative agents can be identified by the characteristic profusely

branched, non-separate, cotton-wool like tufts of mycelium. In Saprolegnia

infection lesion may appear as grey white patches on the skin, fins, eyes,

mouth and gill. The colour may change to dark grey or brown as the

mycelium tangle and trap debris. Under culture systems some chemical

treatments with potassium permanganate, common salt, copper sulphate and

malachite green may help.

B. Branchiomycosis

This fungal disease that causes a gill rot and caused mainly by two species of

Branchiomyces: B. sanguinis and B. demigrans. To prevent the infection dense

stocking should be avoided in culture systems, high concentrations of organic

matter should be avoided and clean fresh water should be provided.

Activity:

Dear student,

Q1. Can you mention any identification characteristics of fungi such as

Saprolegnia in infected fishes?

Q2. What does Saprolegnia infection cause to the fishes?

Q3. How can the Saprolegnia infection be treated in aquaculture

systems?


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Fisheries Resources Development Plan Study (October, 2004). Grant

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Food and Agriculture Organization of the United Nations (2002). The State of

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Gulland, J.A. & Rosenberg, A.A. (1992). A review of length-based approaches

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Aquaculture in the Ecosystem. Springer Science and Business Media

B.V.

Karagiannagos, A. (1995). Fisheries Management in the European Union.

Ashgate Publishing Limited, England.


King, M. (1995). Fisheries Biology, Assessment and Management. Fishing

News Books, Blackwell, Oxford

Ministry of Agriculture and Rural Development (April 2009). National

Aquaculture Development Strategy of Ethiopia. Funded and supported

by Food and Agriculture Organization Sub-Regional Office for East Africa.

Noga, E. J. (2010). Fish Disease: Diagnosis and Treatment. 2nd ed. Wiley-

Blackwell, USA.

Reyntjens, D. & Tesfaye Wudneh (1998). Fisheries Management_A review of

the current status and research needs in Ethiopia. SINET: Ethiop.J.Sci.,

21 (2): 231-266.

Sidorkewicj, N.S., Canzorla, A.C.L., Sabbatini, M.R., Fernandez, O.A.,

Domaniwaski, J.C.J. (1998). Interaction of common carp with aquatic

weeds in Argentin drainage channels. Aquatic Management, 36:5-10.

Welcomme, R. L. (2001). Inland Fisheries Ecology and Management.

Blackwell Science, London.


Recommended Additional Reading Materials

Allan, J.D. (1995). Stream Ecology. Chapman & Hall. London.

Begon M., Colin R. T. and John L. H. (2006). Ecology: From Individuals to

Ecosystems, 4th ed.,Blackwell Publishing Ltd, Oxford.

Cole, G.A. (1983). Textbook of Limnology. (3rd ed.) The C.V. Mosby Company,

St. Louis.

FAO Training Series No.21/1, 21/2 (1998). Management for Freshwater Fish

Culture and Farm Management.

Fuiman, L. A. and Werner, R. G. (2002). Fishery Science: The Contribution of

Early Life Stages. Blackwell Science Ltd, UK.

Goldman, C.R. & Horne, A.J. (1983). Limnology. McGraw-Hill Book Company,

New York.

Horne, A.J. and Goldman, C.R. (1994). Limnology. MacGraw Hill, New York.

Moss, B. (1998). Ecology of Fresh Waters, Man and Medium. Blackwell

Scientific Publications, London.

Pillay, T. V. R. (1993). Aquaculture: Principles and Practices. Fishing News

Books, UK.

Scheffer, M. (1998). Ecology of Shallow Lakes. Chapman and Hall, London.

Swift, D. R. (1993). Aquaculture Training Manual. Fishing News Books, UK.

Wetzel, R. and Likens, G. (2001). Limnology. Academic Press.


Appendix 1: Data on the status of world fisheries production by FAO (2000-

Table1), FAO (2008-Table 2) and FAO (2010-Table3).

Table 1: FAO (2000) World fisheries production and utilization


Appendix 2: Assignment Questions

Instruction

The following questions will constitute not less than 25 % of your total

marks. You should answer all the questions. You should be self dependent

while working out all the questions. Copying from others or any form of

plagiarism in relation to the answers for the questions will lead to

disqualification of your total marks when discovered by the instructor in

charge of the course.

1. Give some examples of fishes that are exceptions to:

(a) Gill breathing

(b) Possession of paired fns

2. What are the characteristics of fishes that make them diverse? List at

least five examples

3. What is the basic difference between agnathan and gnathostoman

fishes?

4. What kind of aquatic environments does each of the following fish

groups inhabit?

(a) Cyclostomes

(b) Chondrichthyes

(c) Osteichthyes

5. What are the various types of aquatic habitats occupied by various

groups of fishes?

6. Define heterosexuality, bisexuality and unisexuality in fishes

7. What are the differences among the simultaneous hermaphroditism,

sequential hermaphroditism and serial hermaphroditism in fishes?

8. How is sperm transferred from male to female in fishes undergoing

internal fertilization?

9. Distinguish between spawning and recruitment in fishes?


10. List down the various developmental stages in fishes

11. What are the two major forms of fisheries? Define each.

12. Give four examples of fishing gears and methods, and show their

similarities and differences.

13. Give examples of commercially important fishes in the:

(a) World

(b) Ethiopia

14. What are the differences among pond culture, cage culture, pen

culture and tank culture?

15. What are the differences among extensive culture, intensive culture

and semi-intensive culture?

16. Distinguish among integrated culture and polyculture

17. What is the current status or trend of capture fisheries and

aquaculture production in the world?

18. Discuss the past and present development history of Ethiopian

capture fisheries

19. Discuss the nutritional content and values of fish diet

20. Discuss the socio-economic benefits of fisheries and the role of

fisheries in improving food security

21. List down the post-mortem changes in fish and relate each to fish

quality.

22. List down and define each of the preliminary fish processing stages

23. List down the various fish preservation methods and how they work

24. What is a fish stock, and a fish stock assessment?

25. What are the various methods or strategies used to ensure

sustainable exploitation of fisheries resources?

26. List down the various major groups of fish parasites and give some

specific examples for each major group.

27. List down some examples of fish diseases caused by bacteria, virus

and fungi with their species or genus level causative agents.

fisheries and aquaculture

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