154 



Fishery Bulletin 104(1) 



gyres over the extremely cold waters of the cold inter- 

 mediate layer (CIL), thus creating a thermally complex 

 habitat. Strong winds periodically force cold waters 

 from the CIL to the surface (Ouellet, 1997). Postsmolts 

 appear to be opportunistic feeders in the ocean, and 

 despite their predominant use of surface waters, they 

 have been known to use benthic habitats and water 

 column features as well (Levings, 1994; Sturlaugsson, 

 1994). These behaviors would distribute fish in a man- 

 ner independent of surface temperatures because forage 

 behaviors are not necessarily controlled by temperature 

 preferences. Growth in postsmolts is also affected by 

 other physical parameters, such as photoperiod (Fors- 

 berg, 1995), and possibly by variation in sea surface 

 salinity as shown for chum salmon (Oncorhynchus keta) 

 (Morita et al., 2001). We can predict migration timing 

 and initial migration trajectories for postsmolts. but we 

 are hard pressed to predict the physical nature of their 

 habitats over time and the physiological effect of these 

 habitats on postsmolt growth. 



Acknowledgments 



We thank T. Sadusky for help with the initial phases of 

 this study and two anonymous reviewers for productive 

 comments. 



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