a critic to visual census method - 1982

8/2/2019 A Critic to Visual Census Method - 1982

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BULLETIN OP MARINE SCIENCE, 32(1): 269-276, 1982 CORAL REEP PAPER

A CRITIQUE OF THE VISUAL CENSUS METHOD FOR

ASSESSING CORAL REEF FISH POPULATIONS

Richard E. Brock

ABSTRACT

Many investigators have noted that estimates of coral reef fish populations by visual census

are biased but its precision has never been quantitatively determined. It is still used, how-

ever, because this technique is usually assumed to be the best non-destructive method of

population assessment. This study compares the results of visual censuses conducted on an

isolated 1,500 m2 patch reef to the collection of all fishes made subsequently with rotenone

on that reef. The visual censuses missed the presence or underestimated abundance of cryptic

fish species. Diurnally active species were reasonably well censused, but the most common

were often underestimated. Thus comparisons between fish communities based on visualcensus data should be restricted to the diurnally exposed species.

Visual recognition and tallying of species and numbers underwater is one ofthe few non-destructive methods presently available to assess coral reef fish com-munities. Since the first serious attempt to estimate reef fish populations usingthe visual census technique (Brock, 1954), several studies of coral reef fishes

have used the procedure. In these studies, authors have acknowledged the pitfallsand shortcomings of the method. Obvious errors include problems in species

identification, and in counting and estimating size of fishes seen. Additionally,the observer cannot accurately assess the species composition and abundance ofcryptic fishes particularly in areas of high topographical relief. This study attemptsto specify these biases by comparison of visual census data with that from aquantitative collection of fish from the same area.

MATERIALS AND METHODS

This study was conducted on an isolated patch reef in Kaneohe Bay, Oahu, Hawaii. Kaneohe Bayis fronted by a barrier reef behind which is a lagoon with small patch reefs. The patch reef selectedfor this investigation is cone-shaped with a flattened top which lies about 30 em below the surface at

low tide. The reef top is roughly circular with a diameter of approximately 25 m and is located in ]4m of water surrounded by a mud substratum. The reef is relatively isolated; the nearest other reef is

over 130 m away. Substratum on both the middle of the top and lower sides of the patch reef is

rubble; live corals are restricted to the upper 5 m of the slope and crest of the reef. The lower slopes

provide little shelter for larger fishes. Most of the fishes resident on this reef are usually seen on the

upper slopes adjacent to available cover.

As part of a study on colonization patterns of marine fishes, this patch reef was depopulated of its

fishes in October ]977 (Brock et aI., ]979). On the afternoon prior to the removal of the fishes, a

number of visual fish transects were conducted using SCUBA in the vicinity of the upper reef slope

and crest to inventory the fish species and obtain an estimate of numbers of each species present. All

fish encountered on three 20 x 4 m transects were identified and tallied; this was followed by a careful

search encompassing the entire reef in compiling a list of all species.

On the following morning the reef was quickly encircled by a large] 1 x ]50 m net (stretched mesh2 cm) that extended from the surface to the 9 m contour. The net enclosed a planar area of 1,513 m2It prevented the escape of larger fishes, After emplacement, 54.5 kg of 5% rotenone powder was

mixed to a paste and spread over the entire enclosed area. As they died, fishes were picked up, takento the laboratory, identified, weighed, and preserved, The entire field operation took 20 collectors

about 6 h.

RESULTS AND DISCUSSION

In total, 53 species representing 21 families of fishes were visually censused onthe study reef. On the following morning, 81 species from 29 families of fishes

2 6 9


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270 BULLETIN OF MARINE SCIENCE, VOL. 32, NO. I. 1982

Table I. Comparative list of fish species visually censused (indicated by "x") to those removed byrotenone from the experimental reef a day later (October, 1977). Numbers of individuals are given

for the rotenone collection. On the far right each species is described as either a wanderer (W) or

resident (R) and as being either diurnal]y cryptic (C) or exposed (E)

2

7

x ],0903

x 123

x I]

x 12

x 9

x 2

x 2

Family and Species

Dussumieriidae

Etrumeus micropus

Synodontidae

Saurida gracilisSynodus variegatus

Muraenidae

Uropterygius tigrinusU. fuscogutta/us

Anarchias /eucurus

Echidna zebra

E. po/yzonaGymnothorax eurostus

G. steindachneri

G. flavimarginatus

G. javanicus

G. meleagris

G. undula/us

G. gracilicaudus

G. buroensisGymno/horax sp.

Congridae

Conger cinereus

Aulostomidae

Aulostomus chinensis

Syngnathidae

Doryrhamphus melanop/eura

Holocentridae

Adioryx lacteoguttatus A. diadema

Bothidae

Bo/hus mancus

Pleuronectidae

Samariscus /riocella/us

Priacanthidae

Priacanthus cruen/a/us

Apogonidae

Foa brachygramma Apogon erythrinus

A. snyderi

Carangidae

Caranx me/ampygus

Mullidae

Mulloidich/hys flavolineatus

M. vanico/ensis

Parupeneus porphyreus

P. multifascia/us

Visual Census

x

x

x

x

x

x

x

x

Rotenone

Collection No.

8

2 3

I

24

7

6

I

19

I

3]

I

] 6 4

I

11

8

2 4

2

23

Species Habits

WE

RC

RC

RCRC

RC

RC

RC

RC

RC

RC

RC

RC

RC

RC

RCRC

RC

RE

RC

RCRC

RC

RC

RC

RCRC

RC

WE

RE

RE

RE

RE


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BROCK: ASSESSING CORAL REEF FISH POPULATIONS 2 71

Table I. Continued

Rotenone

Family and Species Visual Census Collection No. Species Habits

Chaetodontidae

Chaetodon auriga x 5 RE

C. /unu/a x REC. ornatissimus x I RE

C. miliaris x 42 1 RE Heniochus acuminatus x I RE

Pomacentridae

Dascyl/us a/bisel/a x 99 RE

Abudefduf abdominalis x 666 RE

Chromis hanui 2 RE

C. ova/is x 7 REP/ectrog/yphidodon johnstonianus x I RE Eupomacentrus fascio/atus x 44 RE

Labridae

Bodianus bi/unu/mus x 5 RECheilinus rhodochrous x 4 RE

Tha/assoma duperryi x 67 RE

T. ba/lieui x REGomphosus varius x 25 RE

Stethojulis ba/teata x 6 RE

Macropharyngodon geoffroyi x 3 RE

Scaridae

Scarus perspici/latus x 19 RE

S. sordidus x 142 RE

S. taeniurus x 40 RE

Scarus spp. (juveniles) x 344 RE

Ca/otomus sandvicensis x RE

Scarops rubrovio/aceus x WE

Zanclidae

Zane/us canescens x 3 RE

Acanthuridae

Acanthurus nigrofuscus x 2 RE A. triostegus sandvicensis x 7 RE

A. xanthopterus x 4 RE

Ctenochaetus strigosus x 15 RE

Zebrasoma fiavescens x 54 RE

Z. ve/iferum x 5 RE

Naso unicorn is I RE

Eleotridae

Asterropteryx semipunctatus x 988 RE

Gobiidae

Bathygobius cotticeps 6 RC

B. fuscus 5 RC

Gnatho/epis sp. x 7 RC

Gobiidae sp. A 4 RC

Gobiidae sp. B I RC

Blennidae

Cirripectus vari%sus x 2 RE

/stib/ennius zebra 4 RE

Brotulidae

Brotu/a mu/tibarbata 4 RC


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2 7 2

Table I, Continued

BULLETIN OF MARINE SCIENCE, VOL. 32, NO. I, 1982

Family and Species

Scorpaenidae

Dendrochirus brachypterus

Scorpaenopsis gibbosa

S, cacopsis

Scorpaena coniorta

Monacanthidae

Pervagor spilosorna

Ostraciontidae

Ostracion rne/eagris

CanthigasteridaeCanthigaster jactator

Diodontidae

Diodon hystr;xD. holocanthus

Antennariidae

Antennarius drornbus

A. rno/uccensis

Total no, of species

Total no, of families

Visual Census

x

x

x

x

x

53

21

Rotenone

Collection No.

II

15

3

2

14

15

3

3

81

2 9

Species Habits

RE

RC

RC

RC

RE

RE

RE

WE

WE

RC

RC

were taken in the collection with rotenone. These data are presented in Table 1.

Sixty-five percent of the species collected had been seen in the visual census onthe preceding day. In comparing the total number of species censused to those

collected in the rotenone station the visual census provided a poor assessmentof the species composition of the fish community. However, at the family levelthe method appears to be more accurate.

In Table 1 is a subjective classification of how cryptic or exposed each fishspecies usually is (based on coloration and habits) as well as an estimate of theduration of residency on the patch reef. Species were considered to be eitherresidents or wanderers; wandering species are defined as having no particularties with any single reef area or having a home range whose boundaries encom-

passed an area much larger than the patch reef. Criteria for these judgments are

based on the literature where information exists and on more than 20 years of

observing Hawaiian fishes. Five species (Etrumeus micropus, Carnax melam-

pygus, Scarops rubroviolaceus, Diodon hystrix and D. holocanthus) are consid-

ered to be wanderers; three of these species were seen in the visual census.

Thirty-four species (42%) of the total collection are classed as being cryptic (Table1). Only 9 (26%) of these fish species were seen in the visual assessment.

In Table I, 43 species are classified as diurnally exposed residents. These

species are listed in Table 2. Ninety-one percent (or 39 species) were enumeratedin the visual survey. The number of individuals of each species counted in the

three 20 x 4 m visual transects were summed and used to estimate total abun-dance for the entire 1,500 m2 patch reef area. This estimate and the actual numberof each species removed on the following day are presented in Table 2 and plottedin Figure 1. The power function,

Y=

0.74X1.15


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BROCK: ASSESSING CORAL REEF FISH POPULATIONS 273

Table 2. List of those species either censused (estimated number) and/or collected in the completeremoval of all fishes (actual number) on the 1,500 m2 Kaneohe Bay experimental reef that have beenclassed as diurnally exposed resident species in Table I

Species

Aulostomus chinensis MuJloidichthys jlavo/ineatus

M. vanicolensis

Parupeneus porphyreusP. multifasciatus

Chaetodon auriga

C.lunulaC. ornatissimus

C. miliaris

Heniochus acuminatus

Dascyllus albisella Abudefduf abdominaUs

Chromis hanuiC. ova/isPlectroglyphidodon johnstonianus

Eupomacentrus fasciolatus

Bodianus bilunulatus

Cheilinus rhodochrous

Thalassoma duperryi

T. ballieui

Gomphosus varius

Stethojulis balteata

Macropharyngodon geoffroyiScarus perspiciJlatus

S. sordidus

S. taeniurus

Scarus spp. (juveniles)

Calotomus sandvicensis

Zane/us canescens Acanthurus nigrofuscus

A. triostegus sandvicensis

A. xanthopterusCtenochaetus strigosus

Zebrasoma jlavescens

Z. veliferum

Naso unicornis

Asterropteryx semipunctatus

Cirripectus variolosus

Istiblennius zebra

Dendrochirus brachypterus

Pervagor spilosoma

Ostracion meleagrisCanthigaster jactator

Estimated No.

9

21

6

3

3

3

3

3

196

3

3 1

193

o

6

6

3 1

93

45

3

37

3

3

12

163

40

347

6

3

61 5

3

4071

3

o

77

3

o6

6

o9

Actual No.

2 4

1 2

9

22

5

oI

421

I

99

666

2

7

1

44

5

4

67

o2 5

6

3

1 9

1 42

40

344

o3

27

4

15

545

1

9 8 8

2

4

II

14

9

15

best describes the relationship between the number of individuals visually as-

sessed (X) to the number removed (Y). The agreement between the number

observed to the number collected (r2) is 82%.The results indicate that the technique provides a reasonable assessment of

those fish species that are diurnally active and thus are exposed to the censustaker. The power function relationship (Fig. 1) suggests that at higher abundances

of a species within a transect area, the error associated with the visual estimatemay be large. If true, the accuracy in the counts of individuals may possibly be

increased by conducting a greater number of short (20-25 m) rather than a few


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27 4

1000

100

10

BULLETIN OF MARINE SCIENCE, VOL. 32, NO. I, 1982

10 100 1000

ESTIMATED

Figure I. A logarithmic plot of the number of individuals of diurnally exposed reef fish species

visually estimated (X) present on the study reef against the actual number removed (Y) using rotenone,

The line of best fit is given by the equation, Y =Q,74X1.15 (r2 =Q,82), Data from Table 2.

very long (100-200 m) transects. Short transects will census fewer individuals

thus keeping errors on abundant species to a minimum. Usually long transectson coral reefs will sample a number of habitats (i.e., areas of rubble, coral, hard

substratum, sand, etc.), whereas short lines may be kept within a particular hab-itat. However, criteria for transect length should also be dependent on the ques-

tions to be answered.The visual estimate of population size of the eleotrid Asterropteryx semipunc-

tatus was 77 individuals, but 988 were taken in the rotenone collection (Table 2).This species provided the greatest disparity between estimated and actual num-

bers collected in Figure 1. It is probable that at any given time, large numbers ofthis small ubiquitous species are hidden, thus the visual estimates would be low.

Cryptic species mayor may not be seen and counts of individuals of these specieswill be of little quantitative significance. Areas of high substratum relief provide

greater shelter for cryptic fishes making enumeration still more difficult.

Many of the cryptic and smaller species have more restricted home ranges or

territories and often remain near a transect line than will or do larger species(Russell et al., 1978). Larger home ranging or territorial species as well as wan-

derers may leave the area of the transect or alternatively be attracted to the

SCUBA diver (Chapman et al., 1974). These biases should be taken into accountin any comparative study of reef fish abundance and community structure. Thus,

the accuracy of the census results are, to a large degree, dependent on the pro-


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BROCK: ASSESSING CORAL REEF FISH POPULATIONS 2 7 5

ficiency of the investigator as well as his being knowledgeable about the behaviorof the species present during censusing.

The use of rotenone in conjunction with a net enclosing a study area is not

always practical or desirable, a major objection being the elimination of the res-ident fish community. Rotenone is a non-selective agent causing asphyxiation ofreef fishes. Many cryptic fishes, when poisoned by it, will leave their holes fa-cilitating their collection (Randall, 1963a). The collection of fishes by rotenonehas been discussed by Randall (1963b, c), Smith (1973), Goldman and Talbot(1976), and Russell et al. (1978).

The destructive characteristics of the few more quantitative methods of coralreef fish assessment have led many investigators to use the visual census tech-nique (e.g., Brock, 1954; Odum and Odum, 1955; Bardach, 1959; Clarke et aL,1968; McVey, 1970; Risk, 1972; Key, 1973; McCain and Peck, 1973; Smith and

Tyler, 1973; Hobson, 1974; Chave and Eckert, 1974; Itzkowitz, 1974; Jones andChase, 1975; Nolan et aI., 1975; Gundermann and Popper, 1975; Grovhoug andHenderson, 1976; Stone et al., 1979; Brock et al., 1979). These studies have all,in one way or another, pointed out the problems and shortcomings of the visualcensus technique, but have accepted the method. A recently developed variationin the technique is to use the species-time procedure. This method replaces timefor area and is based on the rate at which fish species are encountered. It hasbeen successfully used by Thompson and Schmidt (1977) and Jones and Thomp-son (1978) in the Caribbean. The technique provides an index of relative abun-dance of fishes seen; it is, however, beset with some of the same problems facingthe species-area method, particularly the identification of more secretive fishes.

The visual census method underestimates both the most cryptic as well as themost abundant fish species. It reasonably samples most other resident coral reef

fishes. These results are generalities probably applicable to fishes on most coraJlreefs of the world. They have been substantiated on a Hawaiian reef (presentstudy) as well as on an artificial reef in the Caribbean Sea (Stone et al., 1979).Thus, in summary, only diurnally exposed fish species are censused with anyaccuracy using the visual census technique; the degree to which the more crypticspecies are enumerated is related to the shelter present, to chance, and profi-

ciency of the census-taker.

ACKNOWLEDGMENTS

The author wishes to thank all of the individuals who helped in the field collection of fishes and to

Drs. J. Brock, T. Clarke and J. Grovhoug for commenting on the manuscript. This research was

supported by U.S. Environmental Protection Agency Grant R80398301O. Hawaii Institute of Marine

Biology Contribution No. 593.

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DATE ACCEPTED: July 29, 1980.

ADDRESS: Hawaii Institute of Marine Biology, P.O. Box /344, Kaneohe, Hawaii 96744.
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