Dieldrin. Effects of Chronic Sublethal Exposure on Adaptation to Thermal Stress in Freshwater Fish

The freshwater fish, Etheostoma nigrum, Were pretreated with 2.3 ppb dieldrin for 30 days and then exposed to thermal stress, applied by heating the water at a rate of 1°C/hr to a maximum of 7-9°C above ambient temperature. Pretreated fish had significantly higher mortality than fish not pretreated with dieldrin but also exposed to the same thermal stress. Measurements of blood glucose, feeding, growth, oxygen consumption, opercular movment, whole body lipid content, and liver condition were made throughout the experiment. Pretreated fish demonstrated significant changes in all of the above parameters except opercular movement during the first 15 days of exposure to dieldrin. After 15 days, pretreated fish were found to have adapted to dieldrin exposure, as indicated by significant reversals of the above changes (except for liver damage, which was not reversed but progressed over time). In the adapted state, the pretreated fish remained different from untreated fish, in terms of these parameters. These differences provide grounds for predicting differential response to sublethal thermal stress for the pretreated and untreated fish. Such response is discussed as it may relate to the effects of chronic sublethal exposure to dieldrin.