Increased Susceptibility of Juvenile Chinook Salmon from a Contaminated Estuary toVibrio anguillarum

Abstract

Previous studies have shown that juvenile chinook salmon Oncorhynchus tshawytscha from a contaminated estuary of Puget Sound, Washington, are immunosuppressed. Immunosuppressed fish may be more susceptible to disease 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 nonurban estuary in Puget Sound were assessed. Juvenile chinook salmon were sampled from hatcheries before their release and subsequently from their respective estuaries as the population outmigrated from freshwater 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 contaminants and was replicated during a 2-year period to examine interannual variation. Bile, liver, and stomach contents were collected from fish after capture to determine exposure to organic chemical pollutants. Examination of these tissues demonstrated that juvenile salmon from the urban estuary were exposed to higher concentrations of polycyclic aromatic 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 cumulative mortality after exposure to the pathogen than salmon from the hatcheries or the nonurban estuary. Our results together with our previous findings support the hypothesisthat contaminant-associated immunodysfunction in juvenile chinook salmon may lead to increased susceptibility to infection by a virulent marine bacterium.