Factors Associated with the Seasonal Variation in the Incidence of Pituitary Cysts in Nine-Spined Sticklebacks,Pungitius pungitius L.

Cysts among the prolactin cells in the pituitary gland of the nine-spined stickleback, Pungitius pungitius L., are most common during and shortly after periods of high mortality that mark the end of the reproductive season. Fish are oldest at this time of year. Nevertheless, cysts may occur in breeding as well as spent fish, and their presence does not relate to changes in oocyte development. They can be induced by starving animals in February, but not in September—October. Starved fish always have small prolactin cells and nuclei. It is suggested that cysts form in older fish in response to stress, be this natural or induced. No correlation was noticed between the condition factor of fish caught in the wild, and the incidence of pituitary cysts.