INCREASED SUSCEPTIBILITY OF JUVENILE CHINOOK SALMON FROM A CONTAMINATED ESTUARY TO VIBRIO-ANGUILLARUM

Previous studies have shown that juvenile chinook salmon Oncorhynchus tshawytscha from a contaminated estuary of Puget Sound, Washington, ar e immunosuppressed. Immunosuppressed fish may be more susceptible to d isease and ultimately experience an increase in mortality. To evaluate this possibility, differences in susceptibility to a marine pathogen in outmigrating juvenile chinook salmon from an urban estuary and a no nurban estuary in Puget Sound were assessed. Juvenile chinook salmon w ere sampled from hatcheries before their release and subsequently from their respective estuaries as the population outmigrated from freshwa ter to the saltwater environment during the springs of 1993 and 1994. The study was repeated during a 3-month period to assess the duration of the effect after the fish were removed from the source of contamina nts and was replicated during a 2-year period to examine interannual v ariation. Bile, liver, and stomach contents were collected from fish a fter capture to determine exposure to organic chemical pollutants. Exa mination of these tissues demonstrated that juvenile salmon from the u rban estuary were exposed to higher concentrations of polycyclic aroma tic hydrocarbons and polychlorinated biphenyls than juveniles from the nonurban estuary or hatcheries. Juvenile salmon were challenged with serial doses of a marine pathogen, Vibrio anguillarum (serotype 1575), and mortality was measured daily for 7 d. In both years, salmon from the urban estuary challenged with V. anguillarum exhibited a higher cu mulative mortality after exposure to the pathogen than salmon from the hatcheries or the nonurban estuary. Our results together with our pre vious findings support the hypothesis that contaminant-associated immu nodysfunction in juvenile chinook salmon may lead to increased suscept ibility to infection by a virulent marine bacterium.