QUINN ET AL.: IMPRINTING AND HOMING IN COHO SALMON 



(Quinn et al. 1983). The return of Group 2 to 

 Seward Park can be explained only by the fact 

 that the salmon had been reared there. Iss 

 hatchery produced about 12 times as many coho 

 salmon as the UW hatchery in 1987, but no fish 

 from Group 2 entered Iss, indicating that little 

 wandering took place. 



The patterns of freshwater residence and sea- 

 ward migration vary greatly among and within 

 salmonid species, yet homing to the natal site 

 prevails throughout the family. There seems to 

 be a flexible system by which site-specific odors 

 are learned prior to and during seaward migra- 

 tion. Hasler and Scholz (1983) demonstrated a 

 link between the thyroid hormones associated 

 with smolt transformation (Dickhoff et al. 1978; 

 Dickhoff and Sullivan 1987) and olfactory im- 

 printing. However, the ability of salmon to learn 

 odors on more than one occasion is not fully com- 

 patible with a single peak of thyroid hormones in 

 spring. The solution to this problem may lie in 

 the discovery by Dickhoff et al. (1982) that ex- 

 posure of coho salmon to novel water sources at 

 the time of year when they would migrate to sea 

 induces transient peaks in thyroid hormone 

 levels. Thus, if thyroid hormones are linked to 

 olfactory learning, there may be feedback from 

 migi'ation to hormones, resulting in additional 

 learning during migration. Exposure to novel 

 waters (e.g., at the confluence of rivers) might 

 induce elevated hormone levels and trigger 

 learning of the water source as an olfactory 

 way-point to be used during upstream migration 

 years later. 



ACKNOWLEDGMENTS 



This research was supported by National Sci- 

 ence Foundation Grant BNS 8515202. We thank 

 the Washington Department of Fisheries for 

 providing the coho salmon from Issaquah Creek 

 hatchery and the many individuals who partici- 

 pated in the marking and recovery efforts, 

 especially Ann Setter, John Konecki, Michael 

 Miller, and Chuck Peven. 



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