Regional variability in the dynamics of reproduction and growth of Irish Sea plaice, Pleuronectes platessa L

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Abstract

The differences in size-specific fecundity in relation to size/age at maturity, and in reproductive and somatic investment were analysed for female plaice caught in four regions of the Irish Sea (Cumbrian coast, Liverpool Bay, Cardigan Bay and western Irish Sea), each of which contains a spawning focus. Both the reproductive investment (gonad weight as a function of body size) and fecundity–size relationship of plaice in the western Irish Sea were significantly different from those in the other regions. Rates of annual somatic growth appeared to fall into three distinct groups (Cumbrian coast; Liverpool Bay and Cardigan Bay; western Irish Sea) and, in all cases, the rate of somatic growth fell rapidly after maturity. The data suggest that the highest surplus production (as spawned eggs) occurs in the sub-populations of plaice on the Cumbrian coast and in Liverpool Bay, and is linked to reduced intra-specific competition for food.

Introduction

Variability in fecundity with respect to maturity-at-age or fish size is of considerable importance because it influences a population's ability to sustain additional mortality. Plaice (Pleuronectes platessa L.) in the Irish Sea (ICES Division VIIa) are heavily exploited, with up to 36% of the total spawning biomass being taken by fishing in some years (Anonymous, 2000). Plaice has a determinate spawning strategy, in which the annual fecundity (total number of eggs shed per spawning season) of an individual female is determined at the onset of the spawning season (Urban, 1991). During spawning, eggs are released in batches which are recruited into final maturation at intervals of two to five days over a period of four to six weeks (Rijnsdorp, 1989).

Previous studies on plaice fecundity and growth have centred on the North Sea, where individual fecundity is highly variable and is a function of size (Simpson, 1951, Bagenal, 1966, Rijnsdorp, 1994). Size at maturity in North Sea plaice depends on growth rate prior to first maturity, which occurs around 2–3 years of age (Rijnsdorp, 1993). Nutritional state may also contribute to variability in fecundity (Horwood et al., 1989). There are often geographic variations in fecundity. Bagenal (1966) demonstrated that fecundity of individual plaice at the centre of the species' distribution was approximately half that seen at the extremes of its geographical range, i.e. from 100 to 300 thousand eggs per female of 37 cm. However, ovary weight at a standard fish length was relatively constant, leading Rijnsdorp (1991) to suggest that there may be a trade-off between egg number and egg size. Rijnsdorp (1994) went on to suggest that plaice responded to elevated population abundance with a reduced growth rate, though there was a constant reproductive investment in terms of weight of ovary in relation to body size. It is possible that parts of a stock may attain different reproductive or somatic allocation through variations in growth rate, age or length at attainment of sexual maturity, condition factor etc.

Simpson (1959) showed that the plaice spawning in Cardigan Bay matured at a smaller size than those on other spawning grounds in the Irish Sea. Horwood (1990) found no significant difference in the fecundity per unit length of plaice caught in 1953 (from Simpson's data) and in 1988/89 both in Cardigan Bay and in the north-eastern Irish Sea. In view of this lack of variation in plaice fecundity, Horwood (1990) argued that, since the population had been depleted prior to 1953, Irish Sea plaice are near their maximum fecundity, and that the main source of these differences in fecundity as compared with the North Sea was population density.

Sampling of female plaice in the Irish Sea in 1995, however, indicated regional differences in both overall production and the allocation of production to growth, condition or reproduction. We examine regional variability in fecundity with respect to the parameters of size and age at maturity, and reproductive and somatic investment, using data collected between 1992 and 1996 to provide a fisheries independent estimate of spawning stock biomass using the egg production method (Lockwood et al., 1981, Armstrong et al., 2000). The objective of this paper is to test the null hypothesis that there are no regional variations in the dynamics of growth and reproduction in the Irish Sea. The results are discussed in relation to regional differences in population density.

Section snippets

Study area

Samples of plaice were obtained in September from five annual trawl surveys over the years 1992–1996 and from a survey in February 1995. A 4-m beam trawl fitted with a chain mat was towed for 30 min in all cases. For the purposes of spatial comparisons, the Irish Sea was sub-divided into the four main spawning areas of plaice (St. Bees Head, Great Ormes Head, western Irish Sea and Cardigan Bay) (Simpson, 1959, Fox et al., 1997), with a grid of sampling stations which could also be stratified

Population parameters

Population density (measured as weight of plaice per 30 min tow) was estimated in each region both as a mean value for the surveys in September 1992–1996, and in February 1995 (Table 1). In September, the females would be developing gonads, and the density of mature fish was consistently highest in the western Irish Sea and lowest in Cardigan Bay. Densities off the Cumbrian Coast and in Liverpool Bay were similar. Although the same pattern was seen in the February 1995 data, which are more

Discussion

Plaice spawn between late January and early May in four major locations in the Irish Sea (ICES Division VIIa): off the Cumbrian coast, in Liverpool Bay, off the east coast of Ireland and in Cardigan Bay (Armstrong et al., 2000). The peak of egg production occurs approximately one week earlier in the western Irish Sea, possibly due to higher winter temperatures there. Plaice tagging studies by Daniel and Flemming (1933) in the southern Irish Sea, Griffith (1971) in the western Irish Sea and

Acknowledgements

This work was part-funded under an EU grant AIR3-CT94-2263. The authors are grateful to the personnel from the NW & NWSFC for procurement of samples from Fleetwood, to Sara Taylor for additional fecundity sample analyses, and to Shirley Swaine of the CEFAS Weymouth Laboratory for assistance with histological preparations. The authors wish to thank the General Secretary of ICES for permission to cite the report of the 1999 Northern Shelf Demersal Stock Assessment Working Group.

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