This study determined baseline health and condition values of Pacific herring Clupea pallasi in 1994 in Prince William Sound (PWS), Alaska. In April 1994, 134 herring were collected from 3 spawning sites in PWS, including a sequential sampling from 1 site. For each herring, morphometric characteristics, sex and presence of gross external and internal lesions were documented, and samples were processed for aging, virological, bacteriological and histological analysis. The study did not reveal trends in herring health and condition in 1994 that could reasonably be attributed to the 'Exxon Valdez' oil spill in 1989. No viruses or pathogenic bacteria were detected, but herring worms, a coccidian parasite and a systemic and virulent fungal infection were found in the herring. The degree of vacuolation in liver cells, previously thought to indicate exposure of fish to oil, varied significantly with the stage of reproductive development of the herring. Similarly, the liver melanomacrophage index, also believed to be correlated with environmental toxicant exposures, varied significantly among sample locations and with collection date. In this study, significant differences in age distribution of spawning herring populations occur in close geographic proximity and collection time intervals in PWS. Based on our results, the use of condition factor, disease and indices of liver function to indicate pollutant exposure are likely to be invalid unless other factors unrelated to pollutant exposure are taken into account, such as reproductive stage of the herring, spawning behavior and location, age of herring and collection date. Thus, we conclude that various hypotheses advanced regarding impacts of the 'Exxon Valdez' oil spill as well as other cases of environmental contamination cannot be supported without rigorous statistical evaluation of natural variations in indices of fish health and condition.
