NOTE Olson and Quinn: Vertical and horizontal movements of adult Oncorhynchus tshawytscha 



177 



that fish were generally surface-oriented in mixed wa- 

 ters and remained at or below the thermocline in strati- 

 fied waters. In contrast, steelhead trout spent up to 

 96% of their tracked time within lm of the surface, 

 but made occasional dives through the thermocline/ 

 halocline located 5-7 m below the surface (Ruggerone 

 et al. 1990). Ichihara & Nakamura (1982) reported 

 that chum salmon 0. keta in coastal waters off Japan 

 spent 44% of their time within 5 m of the surface, and 

 seldom dove through the thermocline. On the other 

 hand, Soeda et al. ( 1987) reported that a chum salmon 

 tracked for 57 h off the north Hokkaido coast spent 

 much of its time swimming within the thermocline. 



The vertical salinity and temperature profiles in this 

 study showed that water structure ranged from uni- 

 form to highly stratified during a single tracking pe- 

 riod. Chinook spent most of the tracked time either 

 close to the bottom or within the salinity gradient, i.e., 

 within the water layers predicted by Westerberg's 

 ( 1984) hypothesis. Tracking studies of Atlantic and Pa- 

 cific salmon suggest that the vertical movements of 

 salmonids may be influenced by haloclines or thermo- 

 clines, but do not always demonstrate a consistent pat- 

 tern of vertical movements relative to the water struc- 

 ture. These differences suggest that salmonids may 

 have multiple mechanisms for orienting during their 

 homing migrations which may change according to level 

 of maturity, proximity to the home river, and the verti- 

 cal water structure. 



Moreover, if salmon derive directional information 

 from current shears at the halocline, they may be 

 able to maintain directed swimming using other guid- 

 ance mechanisms (e.g., sun or magnetic compass). If 

 so, only occasional excursions through the halocline 

 may be sufficient for orientation, and other factors may 

 affect their position in the water column. Thus, varia- 

 tion in vertical movement patterns among species and 

 study sites does not contradict Westerberg's (1984) 

 hypothesis. 



In summary, tracked chinook salmon demonstrated 

 no substantial net upstream movements while under 

 observation, and tidal currents were a major factor 

 influencing their horizontal movements. Two patterns 

 of vertical movements were observed: Fish tended to 

 swim in surface waters where salinity and tempera- 

 ture gradients were greatest, or they swam near the 

 bottom. Vertical movements may have been influenced 

 by temperature and salinity preferences related to stock 

 origin and individual physiological requirements, or 

 movements may have been used as a searching strat- 

 egy for clues to orientation. Despite the lack of net 

 upstream horizontal movements, the vertical move- 

 ments of tracked chinook tended to support predic- 

 tions that the vertical distribution of homing salmo- 

 nids are influenced by water column structure which 



may be utilized for orientation towards natal river sys- 

 tems (Westerberg 1984). 



Acknowledgments 



We thank Andrew Ditman for his help during the field 

 work, and Bruce Crookshank for the charter of his 

 gillnet boat. Funding for this project was provided by 

 the Washington Department of Fisheries and the Uni- 

 versity of Washington's College of Ocean and Fisheries 

 Sciences. 



Citations 



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