SUTHERLAND: DISTRIBUTION OF STEELHEAD TROUT 



Table 7. — Ocean surface temperatures and catches of 

 steelhead trout by research vessel of the United States, 

 Japan, and Canada. 



' Combined catches of National Marine Fisheries Service and 

 of Fisheries Research Institute. 



surface water was warmer than 13.9° C (0.2% 

 of the total catch) or colder than 6°C (1.7%). 

 Seasonal shifts of the isotherms undoubtedly 

 result in shifts of the steelhead trout popula- 

 tions. Neave and Hanavan (1960), reviewing 

 catch data for 1956-57 for a number of fish 

 species (including steelhead trout), concluded 

 that distribution patterns in the Gulf of Alaska 

 conformed with the general pattern of near- 

 surface isotherms. On the basis of 2 years' data, 

 they believe that the total area of surface distri- 

 bution of steelhead trout shrank by some 

 770,000 km2 (300,000 square miles) in the Gulf 

 of Alaska between early and late summer. 

 Analysis of steelhead trout catch records of 

 Canada, Japan, and the United States shows 

 that the two steelhead trout reported from all 

 fishing in the Bering Sea were caught in the 

 summer, the period of highest surface tempera- 

 ture. Analysis of temperature charts of Eber, 

 Saur. and Sette (1968) for part of the years 

 (1953-62) included in this report shows surface 

 temperatures in the Bering Sea generally were 

 higher than 5°C from July to October and were 

 5°C or less from November to June. One steel- 

 head trout was caught by NMFS in July 1958 

 (lat. 53°00'N, long. 179°48'E) where the 

 Bering Sea surface temperature was 7.2 °C. 



The other steelhead trout from the Bering Sea 

 was taken by the Japanese in August 1967 

 (lat. 56°29'N, long. 176°00'E), but the tempera- 

 ture at the location was not reported. 



The true horizontal distribution of steelhead 

 trout may not be indicated by surface or near- 

 surface catches because, as Neave and Hanavan 

 (1960) noted, the fish may be present at dejiths 

 below the fishing range of surface gill nets. 

 Neave and Hanavan observed that when the 

 surface temper'atui-es in the Gulf of Alaska 

 exceeded 15 °C, water of that temper-atur-e was 

 usually present within 25 m of the surface. 

 Significant numbers of steelhead trout have 

 been caught in gill nets in summer at depths of 

 15 to 23 m (French et al., 1970), although over 

 85% of the catch had been gilled in the upper 7 

 m of the nets. It is not unlikely that some steel- 

 head trout react to excessive cooling of surface 

 water in the fall and winter by seeking warmer 

 strata at depths below the surface. Similarly, 

 they may seek cooler waters at depths in sum- 

 mer to avoid warmer surface waters. The catch 

 and efforf data are inadequate to demonstr'ate 

 the occurrence or absence of steelhead tr'out 

 below the surface isothermal layer in either 

 case. 



Although the limits of distribution of steel- 

 head trout in r-elation to surface temper-ature 

 ai^pear to be shown by the catch data, ocean 

 distribution of steelhead trout should not be 

 assumed to be related to temperatur-e alone. As 

 pointed out by Favorite and Hanavan (1963) 

 in a study of salmon distribution, "Until we 

 know mor-e of the reaction of salmon to changes 

 or extr-emes of temper-ature and salinity, we 

 must consider the environment as a whole." 

 Similar limitations should be applied to data on 

 steelhead trout distribution. 



MIGRATIONS OF TAGGED 

 STEELHEAD TROUT 



Tagging experiments furnish some informa- 

 tion about the migr-ations of steelhead trout. 

 Although it is limited, the information is of 

 value in describing routes, r-ates, and timing of 

 ocean migr-ations. It also confirms some of the 

 infer-ences on oceanic movement of steelhead 

 trout dr-awn from catch data. 



801 



