THE HARMFUL PHYTOPLANKTER CHAETOCEROS-CONCAVICORNIS CAUSES HIGH MORTALITIES AND LEUKOPENIA IN CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA) ANDCOHO SALMON (ONCORHYNCUS-KISUTCH)

Chaetoceros concavicornis is a harmful phytoplankter that occurs in ma ny temperate coastal seawaters and can cause fin fish mortalities when present at concentrations as low as 5 cells . mL(-1). At even lower c oncentrations, this diatom can stress salmonids to such an extent that they may express a disease to which they are most prone at the time o f C. concavicornis exposure. We report mortality rates of coho salmon (Oncorhynchus kisutch) and chinook salmon (O. tshawytscha) exposed to different concentrations of C. concavicornis. Our data indicate that i n the presence of harmful concentrations of C. concavicornis, blood he matocrit and erythrocyte, glucose, and lactate concentrations of yearl ing chinook salmon (O. tshawytscha) increase. The microridges of the p rimary lamellae decreased in prominence in the presence of harmful con centrations of this phytoplankter, while the goblet cells became more prominent and more numerous. Neutrophil, lymphocyte, and thrombocyte c oncentrations in the blood became depleted. These data suggest that su ppression of a portion of the chinook's immune system is occurring whi ch may partially explain the earlier observation that salmonids cultur ed in the presence of harmful C. concavicornis phytoplankton became mo re susceptible to disease, including Vibrio infections.