| Summary: | Adult Atlantic salmon (Salmo salar L.) returning to Exploits River to spawn encounter low concentrations (< 1%) of effluent as they migrate upstream. Laboratory experiments were designed to assess various endpoints of sublethal stress in adult Atlantic salmon acutely exposed to TMP mill effluent. Critical swimming speed (U[sub crit]) and hepatic EROD induction were determined following 12-hour exposure to 0, 6, 12, and 25% (v/v) TMP effluent. Additionally, the effects of 6-hour exposure to 0, 6, 12, and 25% (v/v) TMP effluent on cardiac output (Q), U[sub crit], hematocrit, blood glucose, and plasma Cortisol, lactate, and osmolality were examined. The same physiological endpoints were examined in another experiment in which fish were exposed to a gradually increasing concentration gradient of effluent (0-25% v/v) while swimming at a steady velocity. Relative to other treatment groups, Q during routine and low-level activity was 7-10% higher (p < 0.05) in fish exposed to 12%, 25%, and an increasing concentration gradient of effluent. Effluent concentrations up to 12% had no effect on U[sub crit], Q[sub scope], and Q[sub max]. However, at the 25% exposure concentration, U[sub crit] following 12-hour exposure was significantly depressed relative to control fish. Fish exposed to 25% effluent for 6 hours had a distinctly lower U[sub crit], Q[sub scope], and Q[sub max] than fish in other treatment groups, although these changes only approached statistical significance. These results suggest that fish responded to TMP effluent by reducing metabolically costly activities such as swimming to allow for increased maintenance and repair costs associated with effluent exposure. There were few noteworthy changes in any of the other endpoints examined. The findings of this study suggest that free-swimming adult Atlantic salmon in the Exploits River probably do not suffer from sublethal physiological stress as a result of exposure to TMP effluent in the river. Riverine effluent concentrations are approximately 1% and effluent levels as high as 12% and 25% were needed to cause physiological changes in the present study. This is the first study to illustrate that Q is a sensitive indicator of sublethal xenobiotic-induced stress and has potential for use as a biological monitoring tool. Land and Food Systems, Faculty of Graduate
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