Electrofishing mark-recapture and depletion methodologies evoke behavioral and physiological changes in cutthroat trout (Oncorhynchus clarki) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/t722hd09b

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  • We examined the behavioral and physiological responses of wild and hatchery-reared cutthroat trout Oncorhynchus clarki subjected to a single electroshock, electroshock plus marking, and multiple electroshocking in natural and artificial streams. In a natural stream, trout released after capture by electrofishing and marking showed distinct behavioral changes: fish immediately sought cover, remained relatively inactive, did not feed, and were easily approached by a diver. An average of 3 to 4 h were required for 50% of the fish to return to a seemingly normal mode of behavior, although responses varied widely among collection sites. Using the depletion method, we observed little change in normal behavior of fish remaining in the stream section (i.e. uncaptured fish) after successive passes with electrofishing gear. In an artificial stream, hatchery-reared and wild cutthroat trout immediately decreased their rates of feeding and aggression after electroshocking and marking. Hatchery trout generally recovered in 2 to 3 h whereas wild trout required at least 24 h. Analysis of feeding and aggression data by hierarchical rank revealed no distinct recovery trends among hatchery trout of different ranks; in wild trout, however, recovery seemed to be faster in social dominants than in intermediates and subordinates. Physiological indicators of stress (plasma cortisol and blood lactate) increased significantly in cutthroat trout subjected to electroshock plus marking, or single or multiple electroshocking. As judged by the magnitude of the greatest change in cortisol and lactate, multiple electroshocking elicited the most severe stress response; however, concentrations had returned to unstressed control levels by 6 h after treatment. It was evident that electrofishing and the procedures involved with estimating fish population size elicited a general stress response that manifested itself not only physiologically but behaviorally as well. We believe these responses could affect the accuracy of estimating population size by violating key assumptions of the methods.
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