MIGRATIONS OF COHO SALMON, ONCORHYNCHUS KISUTCH , 

 DURING THEIR FIRST SUMMER IN THE OCEAN 



William G. Pearcy and Joseph P. Fisher' 



ABSTRACT 



Marked juvenile coho salmon caught in fine-meshed purse seines during the summers of 1981-84 off 

 Oregon and Washington generally demonstrated northward migrations from their rivers of ocean 

 entrance. Northward movements in summer were preceded by southerly movements during spring, 

 probably caused by southerly advection. Catch rates and sizes offish caught in different months and 

 regions of the coast also indicated northerly movements of both yearling and subyearling coho 

 salmon. Despite this movement, the average catch of juvenile coho salmon per purse seine set along 

 the coasts of Washington and Oregon in late summer, including marked fish from the Columbia 

 River, was still a substantial proportion of that in May and June soon after ocean entrance, suggest- 

 ing that many coho did not migrate great distances. Additionally, recoveries of marked juvenile coho 

 salmon by sports and commercial fishermen from Alaska to California and by scientists in Alaska 

 were generally in the region of release. These data indicate that migrations of juvenile coho are of 

 limited extent during their first summer in the ocean and are not strong support for an earlier 

 conclusion that juvenile coho salmon from the Columbia River, Oregon, and California may form a 

 large proportion of the stocks of this species that migrate northward along the coastal belt in Cana- 

 dian and Alaskan waters each summer. 



Although there has been Httle research on juve- 

 nile salmon during their first summer at sea, this 

 phase of the life history may be critical to survival 

 and recruitment to fisheries (Hartt 1980). High- 

 est ocean mortality is thought to occur early in 

 marine life (Foerster 1968; Parker 1968; Ricker 

 1976). Production (catch and escapement) of adult 

 coho salmon, Oncorhynchus kisutch, in the Ore- 

 gon Production Index (OPI) Area (from Leadbet- 

 ter Point, WA, to Monterey Bay, CA) is usually 

 accurately predicted in one year by the number of 

 precocious males (jacks) returning to index 

 streams in the previous year (Gunsolus 1978; Or- 

 egon Department of Fish and Wildlife 1982; 

 Pacific Fishery Management Council 1986). 

 Hence survival from jacks to adults is fairly con- 

 stant from year to year. Because survival rates 

 from smolt to adult are variable (Nickelson 1986), 

 however, variable year-class survival must occur 

 before the time that jacks return, after only a few 

 months in the ocean. This relationship, and the 

 positive correlation between coastal upwelling 

 and survival of OPI coho salmon (Gunsolus 1978; 

 Scarnecchia 1981; Nickelson 1986), strongly sug- 

 gest that the first few months in the ocean consti- 

 tute the "critical period" in determining subse- 

 quent adult survival. 



iCollege of Oceanography, Oregon State University, Corval- 

 lis, OR 97331. 



Between 1976 and 1985 the production of coho 

 salmon in the OPI area drastically declined, de- 

 spite large increases in the number of public and 

 private smolt releases (Oregon Department of 

 Fish and Wildlife 1982; Nickelson 1986). Reduced 

 upwelling and ocean productivity, perhaps cou- 

 pled with density-dependent mortality, is one of 

 the hypothesized causes for this decrease in sur- 

 vival (Scarnecchia 1981; Peterman and Rout- 

 ledge 1981; McCarl and Rettig 1983; McGie 1984; 

 Nickelson 1986). To understand the mechanisms 

 affecting survival of juvenile salmonids at sea, we 

 must first know where salmon reside at the time 

 of their high and variable mortality. Are the 

 smolts highly migratory, immediately leaving 

 local coastal waters and migrating into waters of 

 the Gulf of Alaska (Hartt and Dell 1986), or are 

 they nonmigratory, spending their early ocean 

 life in local coastal waters? 



This paper summarizes research on the move- 

 ments and migrations of coho salmon during their 

 first summer in the ocean in the northeastern 

 Pacific Ocean based on purse seine catches made 

 mainly in coastal waters off Oregon and Wash- 

 ington. A few records of migrations of tagged ju- 

 venile (age .0)^ coho salmon were given by God- 



Manuscript accepted November 1987. 

 FISHERY BULLETIN: VOL. 86, NO. 2, 1988. 



2The numeral preceding and following the decimal indicate 

 the number of winters spent in fresh water and in the ocean, 

 respectively. 



173 



