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Fleeting images dynamics of North Sea ray populations

Walker, P.A.

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Citation for published version (APA):Walker, P. A. (1999). Fleeting images dynamics of North Sea ray populations.

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Download date: 22 Sep 2020

Chapter 1 General Introduction

Prologue

Rays are one of the key attractions in many public aquaria. Their graceful movements and elegance delight us and their apparent curiosity is appealing. The young hatch as touchingly small replica's of the adults and are eminently charismatic, thorns and all. However, admiring rays in captivity is far removed from understanding the dynamics of their populations in sea. In the latter case, we distil our knowledge from that part of the population we can catch, count and analyse and are often dependent on the commercial fishing fleet for information. Even though the analyses are carried out within a robust theoretical framework, the information received is patchily distributed in space and time. Our perception of ray ecology is based on these 'fleeting' images, hence the thesis title.

Background

Rays {Raja spp.) are cartilaginous fish, belonging to the class Chondrichthyes. The species are sometimes referred to under the collective term of rays and skates, although the distinction is not a taxonomie one, but based on morphology. The species are characterised by their large size and high age at maturity, low fecundity and the production of well-developed eggs, encased in hard capsules. All ray species have the same general reproductive cycle (Holden et al. 1971; Ellis and Shackley, 1995), which is shown schematically in Figure 1. Mating can take place many weeks in advance of the spawning season, sperm is stored and adult females lay fertilised eggs in capsules throughout a large part of the year with the young being born as small replica's of the adults (Holden et al., 1971; Du Buit, 1976b; Holden, 1977; Ellis and Shackley, 1995). Timing and length of the spawning season, annual egg production, incubation time and age at maturity are dependent on species and ambient conditions.

Eleven ray species are caught in the North Sea, of which five (common skate - Raja batis, thornback ray - R. clavata, spotted ray - R. montagui, cuckoo ray - R. naevus and starry ray - R. radiata) will be treated in detail in this thesis. Of the other species, only the blonde ray (R. brachyurd) is also likely to be resident, the rest (sandy ray - R. circularis, shagreen ray - R. fullonica, small-eyed ray - R. microocellata, long-nose skate - R. oxyrinchus and undulate ray - R. undulata) are probably migrants from surrounding areas. An overview of reproductive parameters (Du Buit, 1976a; 1976b; Holden et al, 1971) and stock

Chapter 1

biomass estimated for the period 1977-1986 (Daan et al, 1990) for each of the five species dealt with in this thesis is given in the boxes below.

Figure 1 . Schematic representation of the reproductive cycle of Raja species. Figure drawn by Henk Hobbelink.

Research objectives

Anecdotal evidence suggests that at the end of the last century and the beginning of this century rays were common at fish markets around the North Sea. Qualitative information on the relative scarcity of these previously common ray species in the North Sea during recent years caused reason for concern about the status of the different stocks and their sustamability under the pressure of fishing.

Raja batis (Linneaus, 1758). GB: common skate; NL vleet. Commercial: value; stock biomass: 2.2 thousand tonnes. Max. length: 254 cm; length at maturity: 140-180 cm; age at maturity: 11 years; fecundity: 40 egg capsules/year; size of young at hatching: 21 cm.

However, the first consideration, is how hard is the evidence for changes in the ray community? And if there have indeed been marked changes is it possible to attribute these to fishing or do they represent natural fluctuations? These questions formed the background for the research presented in this thesis. Although the

General Introduction

questions are straightforward, they are not so easily answered. A total of eleven species have been recorded from the North Sea, many of which are rare or not easy to distinguish. There are limited targeted hook and line fisheries for rays and skates in the North Sea, but most of the catch is taken as a by-catch in demersal fisheries. The larger specimens are valued on the market, and the price is not so much dependent on species as size. Official catch statistics, therefore, recognise only one collective category for the species ('Skates and Rays'). Although these may be used to deduce general trends, they cannot be used to identify changes in the ray community. Insight in this respect is completely dependent on research vessel catch data, where rays have commonly been identified to species level.

Raja clavata (Linneaus, 1758). GB: thornback ray; NL: stekelrog. Commercial value; stock biomass: 11.6 thousand tonnes. Max length: 107 cm; length at maturity: 72 cm; age at maturity: 9-12 years; fecundity 140-150 egg capsules/year; size of young at hatching 13 cm.

The relatively low economic value of the species, as compared to plaice or cod, means that the species complex has been largely ignored by international research, with the exception of the work of the late Mike Holden in the 1970s (Holden, 1963; 1972; 1973; 1974a; 1974b; 1975; 1977; Holden and Vince, 1973; Holden and Tucker, 1974; Holden et al., 1971). Nevertheless, valuable data have been archived from research vessel data from the early part of the century, although the correct species identification is sometimes questionable in these archival data.

Raja montagui (Fowler, 1910). GB: spotted ray; NL: gladde rog. Commercial value; stock biomass: 16.1 thousand tonnes. Max. length: 73 cm; length at maturity: 65 cm; age at maturity: 8-11 years; fecundity: 60 egg capsules/year; size of young at hatching: 12 cm.

These surveys were often limited in time and space and it is only since the early 1970s that systematic trawl surveys have been carried out over the entire North Sea. Although these surveys are aimed at a quantitative description of the total fish fauna and not at rays in particular, the data collected may be expected to reflect changes in the ray species composition. Apart from ad hoc ageing and maturity studies, there has been no systematic study of population dynamics within the North Sea and the small number of studies published on growth and maturation are largely concerned with populations in other areas. This means that the background of biological data was very limited.

Chapter 1

The main objective of the work presented was to intensify ecological research on skates and rays at species level in order to create a background for the evaluation of possible anthropogenic effects on these important components of the North Sea ecosystem. The aim was to describe long-term trends in catch statistics, abundance and distribution of all Raja species and to gain insight into the growth, maturation and mortality of the present populations and, where possible, compare these to previous estimates. The approach chosen was two-fold: analysis of archived long-term survey data and tagging experiments and the collection of new data on growth, maturation and mortality.

Raja naevus (Müller & Henle, 1841). GB: cuckoo ray; NL: grootoog rog. Commercial value; stock biomass: 45.5 thousand tonnes. Max. length: 73 cm; length at maturity: 59 cm; age at maturity: 8-9 years; fecundity: 90 egg capsules/year; size of young at hatching: 12 cm.

A theoretical framework was explored within which the scale and nature of the observed changes could be placed to identify the possible causes. The constraints imposed and possibilities offered by the life history strategies of the different species were explored. Two aspects were important in this respect, species vulnerability to enhanced mortality and shifts in maturation due to altered growth conditions. Rays show a reasonably uniform survival of juveniles and adults which makes it possible to use deterministic demographic models to predict population growth rates and species can be ranked according to their sensitivity to enhanced mortality, for example by fishing (Holden, 1977; Brander, 1981).

Raja radiata (Donovan, 1808). GB: starry ray; NL: sterrog. No commercial value; stock biomass: 308.4 thousand tonnes. Max. length: 67 cm; length at maturity: 43 cm; age at maturity: 4-6 years; fecundity: 13-21 egg capsules/year; size of young at hatching: 9-11 cm.

Ray species show a large variation in their life-history characteristics, as shown, and the analysis of effects of fishing from survey data can be done by comparing changes in abundance of species with different life histories over time, according to their specific sensitivity to enhanced mortality. Changes in food availability as a result of changes in abundance can lead to shifts in growth and maturation (Reznick, 1993; Berrigan and Charnov, 1994; Perrin, 1995; Atkinson and Sibly, 1997). Analysis of the species differences in plasticity of growth and maturation can provide insight into the species response to changes in the environment.

Although historical survey data were not collected to address the questions raised in retrospect and the scarcity of some ray species in recent years has meant 4

General Introduction

that the present analyses are sometimes based on limited numbers of individuals, the comparative approach chosen was expected to help in drawing general conclusions.

Overview of the chapters

An overview of the information available in the literature is given in Chapter 2, as an introductory chapter to the ecology of the species in the Northeast Atlantic, with emphasis on the North Sea.

Once concerted action was taken to collect existing information, quite a lot of archival information came to light and in Chapters 3 and 4 long-term trends in species abundance are shown. During many surveys information on rays was collected. In the past the data were usually not assessed, and often not even computerised. Information from surveys carried out in Dutch coastal waters from the 1950s onwards have been collated from logbooks. This information, together with the (computerised) data from the International Bottom Trawl Survey has been analysed to identify long-term changes in ray abundance and distribution for Chapter 3. Scottish survey data from the north-western and central North Sea starting in the 1930s were also kindly made available and form the basis for the analysis of demographics presented in Chapter 4. The accent in this chapter lies on determining the sensitivity to fishing mortality using the life history characteristics of the species concerned in comparison to the changes observed in species composition and abundance. The different approach in the two chapters is due to the fact that the Scottish survey data showed clear changes in species abundance whereas the International Bottom Trawl Survey data covered a period of time in which species abundance did not change much, except for the starry ray.

Not only archival survey data came to light, but also a series of tagging experiments on rays in the North Sea and eastern English Channel, covering the period 1959-1976, were hidden in the archives of the Fisheries Laboratory in Lowestoft (UK) and had never been analysed. These experiments had been initiated by the late Mike Holden, whose prolific work and far-reaching insights have formed the framework for recent elasmobranch research. In Chapter 5 these historic tagging data are presented to identify the patterns of movement and migration of unit stocks.

During the course of surveys carried out across the North Sea individuals were collected for the analysis of reproduction and growth. Rays do not have otoliths and various techniques have been applied for age determination in the past based on vertebrae. After preliminary tests, one of the methods has been applied and validated. Age and length at maturity are not independent traits under selection but are linked in a trajectory which is itself subjected to selection

Chapter 1

according to the individual conditions of growth and survival experienced. The individual variation in age and length at maturity four ray species has been analysed in relation to growth in order to identify the plasticity of these traits. These results are presented in Chapter 6. Chapter 7 describes the species specific differences in reproductive cycle, effort and potential. Together these two chapters shed some light on the way in which (fisheries-induced) changes in abundance can affect the reproductive potential of the population.

In Chapter 8 the results presented in each of the chapters is discussed within a common framework and eventual discrepancies and new insights are discussed. Conclusions pertaining to fisheries effects and potential management measures are presented, as well as general conclusions. The results of the thesis are summarised in Dutch in Chapter 9.