Changes in blood cell composition of the brown trout, Salmo trutta L., during the spawning season

Differential blood cell counts were monitored in three different strains of brown trout through three separate spawning seasons. A consistent elevation in the number of circulating erythrocytes was observed in sexually mature male fish (compared with immature fish of either sex) during October-December of each season. A marked lymphocytopenia also occurred during October in mature fish of both sexes with lymphocyte numbers still being significantly depressed as late as March the following year. It is concluded that the use of haematology as a technique for assessing the state of health of salmonid fish must take into account the sex and state of maturation of the fish. The possible consequences of changes in haematology during sexual maturation are discussed in terms of respiratory function and defence against pathogenic challenge.