Electrophysiological Studies of Morpholine-Imprinted Coho Salmon (Oncorhynchus kisutch) and Rainbow Trout (Salmo gairdneri)

Abstract

Two groups of coho salmon (Oncorhynchus kisutch) were exposed to a 5 × 10−5 mg/liter concentration of a synthetic chemical, morpholine, for 4 wk during the smolting period, while two corresponding groups were left unexposed (controls). All groups were marked by finclipping and stocked into Lake Michigan near the mouth of Oak Creek (South Milwaukee, Wisconsin). A second series of exposed and unexposed salmon was released 13 km north of Oak Creek. Rainbow trout (Salmo gairdneri) were tested in a similar experiment.Homing salmonids were censused and then used in physiological experiments. Electroencephalographic studies showed significantly greater responses by the experimental (exposed) compared to control fish to 1% morpholine (P ≤ 0.001 − 0.05) and by one paired group to a stream sample scented with morpholine (about 10−3 mg/liter morpholine P ≤ 0.01). Responses to morpholine were specific in that another chemical similar to it did not elicit responses. These data support the olfactory hypothesis that salmon may imprint to naturally occurring homestream odors in the same way as they do to morpholine.