LIFE HISTORY OF COHO SALMON, ONCORHYNCHUS KISUTCH, 

 IN SASHIN CREEK, SOUTHEASTERN ALASKA 



Richard A. Crone' and Carl E. Bond^ 



ABSTRACT 



The freshwater life of coho salmon, Oncorhynchus kisutch, in Sashin Creek, southeastern Alaska, was 

 studied from the fall of 1963 through the summer of 1968. Additional information on age composition 

 and fecundity of adults returning to Sashin Creek and a nearby stream was collected through the fall of 

 1972. Some pre-1963 data on coho salmon entering and leaving Sashin Creek were used. Weir counts and 

 estimates of numbers of adult salmon determined from spawning ground counts and mean redd life 

 were poor measures of the total escapement of coho salmon in Sashin Creek; an estimate made from 

 tagging a jwrtion of the escapement and subsequently determining tagged-to-untagged ratios of 

 spawners on the riffles proved to be a more reliable measure. The number of spawning coho salmon 

 varied for the years 1963 through 1967 from 162 to 916; the dominant age group was 43. The salinity of 

 the surface water of the estuary of Sashin Creek usually is less than 10-15"/oo; bioassays of salinity 

 tolerance indicated that coho salmon fry can survive in these salinities. In 1964, 44,000 coho salmon fry 

 migrated to the estuary soon after emergence, although none of the. scales collected from returning 

 spawners in subsequent years showed less than 1 yr of freshwater residence. Survival curves 

 constructed from periodic estimates of the stream populations of juvenile coho salmon for the years 

 1964-67 showed that mortality was highest in midsummer of the first year of life, when 62% to 78% of 

 the juveniles were lost in a 1-mo period. Most coho salmon smolts migrated from Sashin Creek in late 

 May or early June. In the spring of 1968, 1,440 smolts left Sashin Creek-37% were yearlings, 59% were 

 2-yr-olds, and 4% were 3-yr-olds. The average fork lengths were 83 mm for yearlings, 105 mm for 

 2-yr-olds, and 104 mm for 3-yr-olds. 



Coho salmon, Oncorhynchus kisutch (Walbaum), 

 occur over a broad geographic range in the North 

 Pacific Ocean and Bering Sea. They spawn in 

 coastal streams from northern California to 

 northwestern Alaska and from northern Hok- 

 kaido, Japan, to the Anadyr River, USSR (Figure 

 1). The young usually remain in fresh water for 1 

 to 3 yr before migrating to sea as smolts; they are 

 sexually mature after about 14 to 18 mo in the sea. 

 In some systems some fry emigrate to salt water 

 in their first spring or summer of life, but they 

 apparently do not contribute significantly to the 

 adult return (Chamberlain 1907; Gilbert 1913; 

 Pritchard 1940; Wickett 1951; Foerster 1955). 



Among the numerous populations of coho salm- 

 on, there are differences in freshwater life history 

 that appear to be related to latitude. In the 

 southern one-third of their range, coho salmon 

 typically remain in fresh water about 1 yr before 



'Northwest Fisheries Center Auke Bay Fisheries Laboratory, 

 National Marine Fisheries Service, NOAA, Auke Bay, AK 99821. 

 Permanent address: 1211 Snipes Street West, The Dalles, OR 

 97058. 



-Department of Fisheries and Wildlife, Oregon State Univer- 

 sity, Corvallis, OR 97331. 



they migrate to sea in their second year of life-15 

 to 18 mo from egg deposition (Pritchard 1940; 

 Briggs 1953; Smoker 1953). Farther north, in 

 Alaska, coho salmon remain 1, 2, or 3 yr 

 (occasionally 4) in fresh water after they emerge 

 from the gravel (International North Pacific 

 Fisheries Commission 1962; Godfrey 1965; Drucker 

 1972). In some of the Alaska streams and in 

 Kamchatka, USSR, coho salmon that remain in 

 fresh water for 2 yr may represent a larger 

 percentage of the population than those that 

 remain for 1 yr (Gilbert 1922; Semko 1954; An- 

 drews 1962; Logan 1963; Engel 1966; Kubik 1967; 

 Redick 1968; Armstrong 1970; Drucker 1972). 



Most studies of coho salmon behavior and sur- 

 vival in fresh water have been conducted in the 

 southern and central parts of the range: Califor- 

 nia, Oregon, Washington, and British Columbia in 

 the eastern Pacific (Neave 1948; Wickett 1951; 

 Briggs 1953; Smoker 1953; Shapovalov and Taft 

 1954; Foerster 1955; Salo and Bayliff 1958; Chap- 

 man 1962, 1965; Koski 1966); and Kamchatka in the 

 western Pacific (Kuznetsov 1928; Gribanov 1948; 

 Semko 1954). Information on more northerly 

 stocks is much less detailed. 



Manuscript accepted: April 1976. 



FISHERY BULLETIN: VOL. 74, NO. 4. 1976. 



897 



