Why do cod females become eggbound? Studies on possible causes and mechanisms

High mortality amoong female cod that are unable to release their eggs is a significant welfare, as well as economic, problem in the cod farming industry, both among broodstock and in ongrowth systems. The underlying causes for this mortality are not full y understood, and it is not clear if the egg-bound individuals suffer from an inhibition of final maturation, ovulatory failure or if oviposition is blocked. The high fecundity obtained in farmed cod, as well as nutritional imbalance, stress, bacterial in fections and male:female ratio have all been suggested to cause, or at least contribute to the mortality. Based on previous experiments, we propose a project where the impact of stress in combination with high and low fecundity on female spawning failure and mortality will be studied. Fecundity will be controlled by feeding the fish high and low energy diets. In order to achieve a better understanind of the mechansims behind final maturation, ovulation and oviposition, females with signs of becoming egg b ound will be treated with hormones acting at different levels of the brain-pitutitary-gonad axis. An immunoassay fro cod luteinising hormone (LH) will be developed as this is an important tool for understanding the regulation of final maturation and ovula tion.