Juvenile coho salmon were exposed to morpholine or phenethyl alcohol (p-alcohol) for 1 1/2 months and then released in Lake Michigan. During the spawning migration 18 months later, morpholine and p-alcohol were metered into separate streams, and the number of morpholine- and p-alcohol-exposed fish returning to each stream was determined. Seventeen other locations were also monitored. The majority of the fish exposed to morpholine were captured in the stream scented with morpholine and most fish exposed to p-alcohol were captured at the p-alcohol-treated stream. This field study demonstrates that coho salmon imprint to and utilize chemical cues for homing.
To test the olfactory hypothesis of salmon homing, fingerling coho salmon (Oncorhynchus kisutch) were exposed to low concentrations of an odorous synthetic chemical, morpholine during presmolt and smolting periods. Equal numbers of these fish were not exposed (controls). Both groups were stocked directly into Lake Michigan near Oak Creek, South Milwaukee Wisconsin. Another paired group of exposed and unexposed fish was released 13 km north of Oak Creek. During the adult spawning migration 18 mo later, morpholine was dripped into Oak Creek and the returning salmon were censused.For four experiments over 2 consecutive yr, the number of exposed and unexposed fish captured at the artificially scented stream were, 216 vs. 28, 437 vs. 49, 647 vs. 65, and 439 vs. 55. These differences were highly significant (P <.001).During the 3rd yr, morpholine was not added to the stream during the spawning migration and exposed and nonexposed fish returned in equal, low numbers (51 vs. 55, P >.05). These results confirm the existence of odor imprinting and long-term olfactory memory in coho salmon, It is possible that this mechanism is also used in the natural imprinting situation.
We compared the homing ability of three groups of' rainbow trout (Salmo gairdneri) stocked in Lake Michigan by different procedures. One group of juvenile rainbow trout was imprinted to a synthetic chemical, morpholine, during the presmolt and smolt stages, and a second group was not imprinted. Both groups were stocked directly into Lake Michigan, 1 km north of the Little Manitowoe River. A third group of trout was retained in a pond on the Little Manitowoe River during the presmolt and smolt stages and then released into Lake Michigan at the same location as the other two groups. During the adult spawning migration, morpholine was metered into the Little Manitowoe River. This river and 16 other locations were monitored for returning fish. The morpholine-imprinted fish returned to the Little Manitowoe River in greater numbers and strayed less than did fish from the other two treatment groups. This result is a consequence of exposure to a unique odor cue at the critical period for imprinting. 4 We do not mean to imply that smolting ponds are not useful for managing salmon or trout. For example, in Wisconsin, in the spring coho salmon (Oncorhynchus kisutch) are held in smolting ponds with no difficulty (i.e., water temperatures are cool and dissolved •xygen remains high). However, in the summer, problems with temperature and oxygen are encountered in the same ponds. SCHOLZ ET AL.•HOMING OF RAINBOW TROUT 443 Wisconsin Department of Natural Resources) and Foreman Don Czekalb and his crew at Wild Rose Fish Hatchery for technical and logistical support. We also acknowledge Peter Hirsch and Peter Johnsen at our laboratory for advice in all aspects of this study.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.