DISTRIBUTION, SEASONAL ABUNDANCE, AND SOME 



BIOLOGICAL FEATURES OF STEELHEAD TROUT, 

 SALMO GAIRDNERI, IN THE NORTH PACIFIC OCEAN 



Doyle F. Sutherland' 



ABSTRACT 



Cruise and catch data of Canadian, Japanese, and United States research vessels for 1953 and 

 1935-67, together with information from published literature, were used to investigate the 

 distribution, age, and size of steelhead trout, Sulnio gairdneri, in the North Pacific Ocean. 



Steelhead trout were distributed virtually throughout the entire North Pacific Ocean, north 

 of about lat. 42° N. Their abundance was greatest in the Gulf of Alaska and eastern North 

 Pacific, decreased to the westward, and was lowest in the western North Pacific and the 

 western Aleutian Islands area. The relative abundance in all areas sampled was far less than 

 that of Pacific salmon, Oncorhynchus spp. The distribution of steelhead trout at sea appears 

 to be influenced by surface water temperatures, apparently conforming closely to the 5°C 

 isotherm on the north and the 15 °C isotherm on the south. Definite seasonal shifts of 

 abundance were associated with changes in water temperature. Northward and westward 

 movement began in late winter and early spring and became most extensive in summer; it 

 shifted to a southward and eastward movement in late summer, fall, and early winter. 



Ages of steelhead trout caught at sea ranged from 2 to 8 yr. The majority of fish were 3, 4, 

 or 5 yr old and belonged to either the 2.1, 3.1, or 3.2 age groups. 



The length of time spent in salt water had a marked effect on size of fish. In their first 

 summer in the ocean, they grew in average size from about 15 to 36 cm in length and to 0.8 

 kg in weight. At the end of the first year at sea (age .0 to .1), their average size was 57 cm 

 and 2 kg; in the second year (age .1 to .2), it was 70 cm and 3 kg. 



This paper summarizes the considerable infor- 

 mation available from field data obtained by 

 research vessels of Canada, Japan, and the 

 United States on the distribution, relative 

 abundance, age, and size of steelhead trout, 

 SaUno gainhieri, in the North Pacific Ocean. 

 The steelhead, or sea-run rainbow trout, is a 

 highly prized sport fish on the Pacific Coast of 

 North America. Since World War II the sport 

 fishery for steelhead trout has increased many- 

 fold. Because of its value as a spoil fish, and to 

 a lesser extent as a commercial fish, the fishery 

 agencies of the Pacific Coast states and British 

 Columbia have research and development pro- 

 grams for managing the stocks (Pautzke and 

 Meigs, 1940; Larson and Ward, 1955). Manage- 

 ment of the fisheries, however, is based almost 



' Northwest Fisheries Center, National Marine Fisheries 

 Service, NOAA, Seattle, WA 98112; present address; Gulf 

 Coast Fisheries Center, National Marine Fisheries 

 Service, NOAA, Panama City, FL 32401. 



entirely on the knowledge of the freshwater life 

 history of the fish, which has been described by a 

 number of authors (Pautzke and Meigs, 1940; 

 Shapovalov and Taft, 1954; Larson and Ward, 

 1955; Maher and Larkin, 1955; Bali, 1958; 

 Chapman. 1958; Hailman. 1965; Withler, 

 1966). 



Spawning stocks of steelhead trout are widely 

 distributed along the Pacific Coast of North 

 America. Carl, Clemens, and Lindsey (1959) 

 described their distribution as ranging from 

 southern California to Bristol Bay, Alaska. The 

 true limit of their range may extend no farther 

 than central or northern California to the Alaska 

 Peninsula. They are virtually extinct in streams 

 south of San Louis Obispo County, Calif., and 

 research vessels fishing in Bristol Bay over a 

 period of several years have not confirmed their 

 l)resence in that area. Also, scales from many adult 

 S. (/(fi)-(liieri taken from Bristol Bay streams and 

 examined by personnel of the Fisheries Research 

 Institute. UniversityofWashington, have without 



Manuscript accepted January 1973. 



FISHERY BULLETIN: VOL. 71, NO. .^, W7.^. 



787 



