Changes in the Immune System of Coho Salmon (Oncorhynchus kisutch) during the Parr-to-Smolt Transformation and after Implantation of Cortisol

The primary immune response of coho salmon (Oncorhynchus kisutch), as assessed by the production of splenic antibody-secreting cells (plaque-forming cells, PFC) after an injection of Vibrio anguillarum O-antigen, decreased during smoltification. This period was marked by increases in gill Na+-K+-adenosine triphosphatase (ATPase) activity and plasma thyroxine and cortisol titers. Numbers of leucocytes relative to erythrocytes in peripheral blood and splenic lymphocytes relative to fish body weight were also reduced. Fish reared at normal hatchery density (approximately 2 fish∙L−1) appeared to have reduced rates of development and higher numbers of PFC than fish reared at one-third normal density. Moreover, in fish changed from normal density to low density 2 wk before sampling, ATPase activity and plasma thyroxine levels were equal to those in fish reared continuously at normal density, but plasma cortisol levels and PFC were equal to those in fish reared at low density. Fish with cortisol implants had higher plasma cortisol titers, reduced numbers of splenic PFC, splenic lymphocytes, and circulating leucocytes, and greater mortality when fish were exposed to V. anguillarum.