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Fishery Bulletin 93(2), 1995 



caught in the ocean in May and June (0.93 mm-d -1 , 

 rc=142; Fisher and Pearcy, 1988). Mean growth rates 

 of CWT juvenile coho salmon caught in the ocean in 

 the July-September period were higher (1.43 mm-d -1 , 

 n=69) than growth rates in early summer. Unfortu- 

 nately, we caught too few CWT age- 1.0 chinook 

 salmon in late summer to compare their growth rates 

 with those of coho salmon in late summer. Our esti- 

 mates of growth rates of CWT age-1.0 Columbia River 

 chinook salmon based on mean size offish in the CWT 

 group at time of release or during downstream migra- 

 tion may be biased by size-related differences in mor- 

 tality rates or migration rates out of the sampling area. 



The ratio of age-1.0 to age-0.1 chinook salmon in 

 our ocean purse-seine samples was disproportion- 

 ately high compared with the relative numbers of 

 these two age classes entering the ocean. Although 

 subyearling chinook salmon were much more numer- 

 ous than yearling chinook salmon among down- 

 stream migrating smolts in the Columbia River 

 (Dawley et al., 1985a), and many more subyearling 

 than yearling chinook salmon were released from 

 hatcheries (Table 1), yearling fish predominated in 

 our catches in the ocean. The high catches in the 

 ocean in May and June of age-1.0 fish coincided with 

 the period of peak downstream migration and ocean 

 entry of these yearling fish. However, no similarly 

 large catches of the more numerous age-0.0 fish oc- 

 curred in the June, July, and August ocean samples, 

 following their peak period of downstream migration 

 in the Columbia River. In fact, CPUE of age-0.0 fish 

 during July and August in Areas A and B, was not 

 nearly as great as the CPUE of age-1.0 fish earlier 

 in May and June (Fig. 2). 



The relatively low numbers of small age-0.0 

 chinook salmon caught in water >37 m bottom depth 

 support the hypothesis of Miller et al. (1983) that 

 offshore movement of subyearling chinook salmon is 

 size dependent; few fish move offshore until they 

 reach a size of around 130 mm FL. Many of the small 

 age-0.0 fish caught over deep water in July and Au- 

 gust appeared to have been carried offshore in the 

 Columbia River plume, since they were found in 

 waters of high temperature and low salinity (Fig. 3). 

 During June, July, and August large numbers of 

 small age-0.0 chinook salmon may have been present 

 in shallow nearshore waters inshore of our sampling. 

 Subyearling smolts of those stocks caught in ocean 

 fisheries far to the north as maturing fish (e.g. Co- 

 lumbia River Upriver Bright Fall chinook, Howell et 

 al. 7 ) may migrate to the north while they are still in 

 shallow waters near the surf zone and thus may not 

 be available to sampling over deeper water. Even 

 large age-0.0 fish may prefer shallow water habitats. 

 In late summer, when many age-0.0 fish should be 



quite large, this age class was rare in our samples. 

 Our only large catches of age-0.0 fish (in September 

 1983) were inshore of 4 km (Fig. 4). 



The apparent difference in inshore-offshore distri- 

 bution between age-0.0 and age-1.0 chinook salmon 

 suggests that these two age groups are exposed to 

 different environmental conditions in the ocean and 

 that different factors may be critical in determining 

 their survival in the ocean. Small subyearling chi- 

 nook salmon may be more susceptible than yearling 

 chinook salmon to processes affecting the nearshore 

 environment, such as storms that cause heavy surf 

 conditions, concentrations of nearshore predators, or 

 nearshore dredging and other habitat modifications. 

 On the other hand, by staying in shallow nearshore 

 waters, where southward currents in the summer tend 

 to be slower than 15-20 km farther offshore (Kundu 

 and Allen, 1976; Huyer, 1983), the northward move- 

 ment of the small subyearling fish may be facilitated. 



In contrast to stream-type age-1.0 chinook salmon 

 from the Columbia River, some ocean-type fish may 

 overwinter in areas near where they enter the ocean. 

 We found four age-0.1 CWT fish of the Columbia 

 River URB fall chinook salmon stock, which is har- 

 vested mainly in Alaska and British Columbia ocean 

 fisheries (Howell et al. 7 ), near or south of the Co- 

 lumbia River many months following their release 

 from hatcheries. Apparently, some individuals of this 

 highly migratory ocean-type stock may delay their 

 northward migration for a long period of time. Alter- 

 natively, the extent of the migrations of individuals 

 of this stock may vary. Those that mature at younger 

 ages (jacks for example) may undertake less exten- 

 sive migrations than those that spawn at older age. 5 



Acknowledgments 



We thank D. Larden and his crew of the FV Pacific 

 Warwind and the many people who helped during 

 the cruises or in the laboratory, especially A. Chung, 

 R. Brodeur, J. Shenker, W. Wakefield, D. Gushee, C. 

 Banner, J. Long, K. Krefft, and C. Wilson. Helpful 

 comments by two anonymous reviewers are appreci- 

 ated. This research was supported by NOAA, Na- 

 tional Marine Fisheries Service (Grants NA 

 27FE0162 and NA 37FE0186) and by Oregon State 

 University Sea Grant (Grant No. NA89AA-D-SG108, 

 Project No. R/OPF-33). 



Literature cited 



Chapman, D. W. 



1986. Salmon and steelhead abundance in the Columbia 

 River in the nineteenth century. Trans. Am. Fish. Soc. 

 115:662-670. 



