Developments in understanding of fecundity of fish stocks in relation to egg production methods for estimating spawning stock biomass

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.