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Abstract: The use of egg production methods to estimate the spawning stock biomass (SSB) of fish depends critically on establishing the biological basis underpinning the methods. In this paper we review recent developments in knowledge of several key aspects of reproductive biology relevant to the estimation of annual egg production and daily specific fecundity at the individual and population level, as required in particular for application of the Annual and Daily Egg Production Methods respectively. The parameters to be estimated include spawning frequency, potential annual fecundity or batch fecundity, and rates of atresia prior to and during the spawning season. The initial development of egg production survey methods led to a significant growth in understanding of the reproductive biology of the target fish species. Drivers for further developments have come from the extension of the surveys to more species and areas, an increasing interest in evaluating time-series of reproductive potential in stocks, and the needs for research and development in support of marine aquaculture. A growing number of studies on field-caught and captive fish since the 1990s have led to important advances in our understanding of the triggering of maturation and gametogenesis, the subsequent processes of oocyte (follicle) growth and vitellogenesis, regulation of fecundity through atresia, batch spawning dynamics, and the influence of extrinsic factors such as day length, temperature and feeding. We conclude that the biological basis for egg production survey methods is now extremely well established for the main species being surveyed. However, the extension of the methods to a wider range of species has highlighted the practical difficulties in obtaining sufficiently precise and unbiased estimates of key parameters such as spawning fraction or sex ratio that are influenced by spatial distribution or catchability of several population components (e.g. mature, immature active spawning or spent). The definition and validation of spawning markers also remains a challenge, particularly for new species being surveyed. Given the often high cost of egg production surveys, particularly the annual method, there is a need for simulation studies to determine the relative performance of simpler and less expensive approaches to tracking SSB from egg surveys for use in stock assessment. Developments that expand the quantity and utility of the information yielded by the surveys, including integration of the surveys into broader ecosystem studies, could also be considered a way to improve cost-effectiveness. [Copyright &y& Elsevier]

Rijnsdorp, A. D., van Damme, C. J. G., and Witthames, P. R. 2010. Implications of fisheries-induced changes in stock structure and reproductive potential for stock recovery of a sex-dimorphic species, North Sea plaice. – ICES Journal of Marine Science, 67: 1931–1938.A key assumption in stock assessment and stock forecasts often is that spawning-stock biomass (SSB) and egg production are proportional and that the reproductive potential is independent of stock structure (age composition and sex ratio). Based on a 60-year time-series of total egg production (TEP) of North Sea plaice, we demonstrate that this assumption could result in a biased perception of the temporal trend in reproductive potential. The time-series incorporates: (i) annual observations on maturity, growth, and condition, (ii) a predictive model for interannual variations in fecundity caused by variations in body condition and by the probability of being a recruit spawner, and (iii) a cohort analysis of sex-specific landings-at-age since 1948. Following an increase in fishing mortality rate, TEP declined by a factor of 7–8 from a peak in the 1970s to a minimum in 1999–2000. Concurrent with this decline, the contribution of recruit spawners and the size difference between spawning males and females decreased. The implications of phenotypic plasticity and fisheries-induced evolutionary changes in growth and maturation for the recovery potential of the plaice stock are discussed. [ABSTRACT FROM PUBLISHER]

Abstract: Some fish species have wide distribution areas that span very different habitats. In this investigation we have studied Atlantic cod (Gadus morhua), which is an example of such a species, to demonstrate how this may have caused adaptations to key features such as fecundity, growth and age and size at first spawning. We have studied cod from the Barents Sea, Icelandic waters, North Sea and Irish Sea. The ovary sampling was undertaken over several years, however, not always sequentially, in order to assess whether the relationships between fecundity and other key features were constant or variable. Also, we compared historical maturity ogives and growth from the different regions. There was a clear pattern with fish maturing at a greater age and size in the north compared to the south. For three of the four cod stocks we demonstrated a significant reduction in relative potential fecundity as maturity progressed towards spawning, i.e., as the mean diameter of vitellogenic follicles increased. To be able to compare potential fecundity in a standardised way both in time and space, we constructed models that included mean diameter as one of the independent variables. Our potential fecundity comparisons clearly indicated a north-south gradient with increasing size-specific fecundity towards the south. The higher fecundity of the fish in the south could only partly be explained by the higher condition and temperature that was observed in these waters. [Copyright &y& Elsevier]

Abstract: In this paper we report on the fate of vitellogenic follicles (VF) as either alpha atretic follicles (αF) or post-ovulatory follicles (POFs) using histology and captive Atlantic cod (Gadus morhua) in three experiments. In Experiment 1 the production and persistence of αF was determined by taking repeated biopsy samples from tagged females held in temperature regimes (mean±SD) controlled at 4.5 (0.3) and 8.1 (0.3)°C. The αF lasted (mean±2 SE, n) 5.3 days (2.5, 8) and 9.7 days (4.9, 8) in the warmer and cooler water respectively and the combined average was 7.5 days (2.9, 16). In Experiment 2 we took biopsy samples at intervals and monitored egg production from individual females accompanied by a male and used the stage of egg development to age POFs found in the biopsy samples. The females, some immature, were killed at intervals, up to 45 days post-spawning, and then the biopsy and ovary samples were stained by periodic acid Schiffs’ reagent to prepare descriptions of POFs aged from 11h to 45 days old. Spent female ovaries contained POFs, and a thicker ovarian wall (tunica) exceeding 0.34mm whilst immature fish lacked POFs and their ovary tunica was thinner (less than 0.15mm). In Experiment 3 the persistence of POFs was monitored in a simulated North Sea (10–16.1°C) and Barents Sea (7.5–11.2°C) regime using ovary sections stained by periodic acid Schiffs’ reagent. In both regimes the POFs regressed at a temperature sensitive rate during the experiment lasting 104 days. Some αF from large VF persisted longer than expected (more than 4 months after spawning) and were called cysts based on their appearance and greater expected lifetime. These histological characteristics were successfully applied to assess maturity of wild cod caught on surveys in the North and Barents Seas after an assumed 150 and 310 days, respectively, after the spawning season. Taken together this article presents reliable figures on the lifetime of atretic and post-ovulatory follicles as well as variation in ovarian thickness with spawning experience, which will be most useful input in the further work to assess reproductive potential. [Copyright &y& Elsevier]

van Damme, C. J. G., Bolle, L. J., Fox, C. J., Fossum, P., Kraus, G., Munk, P., Rohlf, N., Witthames, P. R., and Dickey-Collas, M. 2009. A reanalysis of North Sea plaice spawning-stock biomass using the annual egg production method. – ICES Journal of Marine Science, 66: 1999–2011.Uncertainty about the quality of current virtual population analysis-based stock assessment for North Sea plaice (Pleuronectes platessa) has led to various abundance indices. We compared biomass estimates from the annual egg production (AEP) method with current stock assessments based on catch-at-age to validate the current and historical perception of exploitation. The AEP method was also used to investigate the dynamics of the spatial components of plaice in the North Sea. We corrected for fecundity down-regulation and changes in sex ratio. Estimates from both methods were similar in trend and absolute biomass. On the Dogger Bank, there was a dramatic decline in biomass from 1948 and 1950 to 2004, and in the Southern Bight, the stock appeared to increase from 1987 and 1988 to 2004, although not reaching the historically high levels of 1948 or 1950. The timing of spawning of North Sea plaice does not appear to have changed throughout the period of high exploitation. We conclude that the AEP method is a useful way to hindcast the spatial dynamics of heavily exploited flatfish stocks. [ABSTRACT FROM PUBLISHER]