FISHERY BULLETIN: VOL. 70, NO. I 



temperature range than sockeye salmon, whose 

 migration generally ends when water tempera- 

 tures reach about 10° C (see footnote 2, Table 1) . 



At Karluk Lake, for each year from 1961 to 

 1968 (excluding 1964) the date by which 50' > 

 of the coho salmon smolts had migrated was 

 later than the comparable date for sockeye salm- 

 on smolts (Table 5). The difference in time of 

 the two migrations ran from 6 to 19 days (aver- 

 age 11 days). Not only did more of the coho 

 salmon smolts migrate later than the sockeye 

 salmon smolts, but the coho salmon smolts usu- 

 ally migrated during a period of relatively warm- 

 er water, when the abundance of migrating 

 sockeye salmon smolts had greatly diminished. 

 Similarly, Foerster and Ricker (1953) found 

 that the coho salmon smolt migration in Cultus 

 Lake and Sweltzer Creek, British Columbia, al- 

 ways followed the sockeye salmon smolt migra- 

 tion by about 10 days. 



Although the seasonal timing of the outmigra- 

 tion of coho salmon smolts may vary from system 

 to system, it is relatively consistent within a 

 particular system. When time of migration is 

 plotted against latitude, a definite south to north 

 cline in time of migration becomes evident (Fig- 

 ure 8). Coho salmon smolts migrate later in 

 the season in northerly systems than in more 

 southerly ones. More than a month separates 

 the midpoint of smolt migration from the central 

 coast of California (lat 37° N) to the Gulf of 

 Alaska (lat 60° N). This relation also applies 

 for the Asiatic side of the Pacific Ocean. 



DIEL PATTERN OF MIGRATION 



The transformation of juvenile coho salmon 

 from either lake- or stream-type residents to 



Table 5. — Dates by which 50% of the coho and sockeye 

 salmon smolts migrated from Karluk Lake, 1961-68. 



Year 



Sampling period 



50% migration date 



Coho salmon Sockeye salmon 



60' 



55' 



HOOD BAY CREEK, ALASKA* 

 SASHIN CREEK, ALASKA* 



LAKELSE LAKE, 

 BRITISH COLUMBIA* 



50* 



o 



z 



UJ 



^< 



(E 

 (J 



40" 



35° 

 0' 



BEAR CREEK, ALASKA* 



*LAKE EVA, ALASKA 



•karluk LAKE, 

 ALASKA 



*BOLSHAYA RIVER, 

 KAMCHATKA, USSR 



*CULTUS LAKE, BRITISH COLUMBIA 



*MINTER CREEK, WASHINGTON 



*WADOELL CREEK. CALIFORNIA 



10 



15 20 

 MAY 



25 



31 

 /I 



10 15 20 25 

 JUNE 



Figure 8. — Average date when 50% of the coho salmon 

 smolts had migrated from river and lake systems along 

 the Pacific coast of North America and Asia. 



smolts is associated with avoidance of light and 

 increasing nocturnal activity (Hoar, Keenley- 

 side, and Goodall, 1957; Hoar, 1958; Smirnov, 

 1960). 



Although most of the migration of smolts to 

 salt water occurs during the darkest hours of 

 the night, some occurs during the daytime. At 

 Karluk Lake, for instance, during some years 

 almost 40% of the coho salmon smolts migrated 

 in the daytime — between 0600 and 1800 hr (Fig- 

 ure 9). In other coho salmon rivers, the per- 

 centage of smolts that migrate seaward during 

 daylight is quite variable. In the Bolshaya 

 River in Kamchatka, during the years 1944-47, 

 6.3 to 50.0% of the age 1 smolts"^ and 8.8 to 73.2% 



Fish that go to sea in their second year. 



88 



