The application of tank experiments to the study of reproductive potential in teleosts using Gadus morhua as a test model

Age-based population assessments (VPA) have often failed to show a clear relationship between spawning stock biomass and either recruitment or biomass estimated from annual egg production. Recent work has shown that the inclusion of reproductive potential (a measurement of both the quality and quantity of eggs produced in relation to female size and age) helps to explain some of the variation in the stock and recruitment curve. Tank experiments will make it possible to develop tools to measure reproductive potential either directly from fecundity or indirectly from maternal reserves. This paper describes such experiments involving first year maturing cod (Gadus morhua), in which egg production has been studied in relation to maternal reserves. Ovary biopsy samples were taken from the start until the end of spawning when the fish were killed in order to study the persistence of postovulatory and atretic follicles (the latter are oocytes that are resorbed during the vitellogenic phase) using histological methods. These results were examined to suggest ways to improve the detection of post spawning females (which is essential when making field assessments of maturity). Using this data it was also possible to quantify atresia in relation to the egg production cycle