SUTHERLAND: DISTRIBUTION OF STEELHEAD TROUT 



research vessels averaged 0.007 steelhead trout 

 per longline set. Purse seine catches by FRI 

 averaged 0.07 fish per set. 



5. The relative abundance of steelhead trout 

 in all areas sampled was far less than that of 

 salmon. In NMFS gill net catches, salmon 

 exceeded steelhead trout by the average ratio of 

 105:1. The average ratio of salmon to steelhead 

 trout in Japanese gill net catches was 13,743:1. 

 The longline catch ratios averaged 30:1 in 

 NMFS catches. 54:1 in Canadian catches, 68:1 

 in FRI catches, and 49,632:1 in Japanese 

 catches. In purse seine catches by the FRI, the 

 ratio was 1,012: 1. 



6. Ages of steelhead trout caught at sea 

 ranged from 2 to 8 yr. Twenty different age 

 groups, including various combinations of 

 freshwater ages of 1 to 5 yr and saltwater ages 

 of to 6 yr, were represented. The majority 

 belonged to three age categories — 2.1 (22% ), 3.1 

 (30% ), and 3.2 (12% ) — and were 3, 4, or 5 yr 

 old. 



7. Age, length, and weight data reinforce 

 previous knowledge that the number of years 

 .steelhead trout spend in fresh water has little or 

 no effect on their ultimate length and weight. 

 The period in salt water has a profound effect on 

 growth, which is particularly rapid in the first 

 and second years. The average length of all 

 freshwater age groups differed by no more 

 than 4 cm after the second summer of feeding in 

 salt water. Maximum growth is achieved by 

 prolonged residence in salt water. In the first 

 summer in the ocean, the steelhead trout grew 

 from an average length of about 14-17 cm as 

 seaward migrants to an average size of about 

 36 cm in length and 0.8 kg in weight. The 

 average size increased to 57 cm and 1.8 kg for 

 males and to 2.4 kg for females in the second 

 summer. Growth continued at a substantial, but 

 less rai)id, rate in succeeding years. One female 

 in its seventh summer (six winters at sea) 

 measured 90 cm and weighed 6.7 kg. 



8. The catch and effort data showed a 

 definite seasonal shift of steelhead trout in the 

 ocean. A northward and westward movement 

 began in late winter and early spring. The most 

 extensive movement northward occurred in 

 summer. Data on fall distribution are limited. 

 Larger catches south of lat. 54 °N and the return 



of si)awning adults to North American .streams 

 in the fall, however, indicated a general shift of 

 the populations southward and eastward in late 

 summer, fall, and early winter. 



9. Catch records showed a relation between 

 ocean distribution of steelhead trout and surface 

 water temperatures. Catches were made in sur- 

 face water that ranged from 5° to 14.9° C The 

 majority of catches (61%) were in areas with 

 surface water of 8° to 11.4°C. Very few steel- 

 head trout were caught in water warmer than 

 13.9 °C or colder than 6°C. The data suggest the 

 limits of distribution in the ocean conform 

 closely to the 5°C isotherm on the north and the 

 15 °C isotherm on the south. 



10. The vertical distribution of steelhead 

 trout has not been adequately investigated. 

 Over 85% of the gill net catch by NMFS in 

 experimental deep nets was in the upper 7 m of 

 net. Significant numbers have been caught in 

 summer at depths of 15 to 23 m. If steelhead 

 trout in significant numbers range to depths 

 greater than 7 m, the true horizontal distribu- 

 tion may differ from that described from gill net 

 and longline catches. 



11. Tagging studies showed that some steel- 

 head trout make extensive oceanic migrations. 

 Individuals tagged in the eastern North Pacific 

 Ocean, including the Gulf of Alaska and south 

 of the central Aleutian Islands, were recovered 

 in streams of Washington and Oregon 1 to 2 yr 

 after release. A steelhead trout marked by a fin 

 clip in Oregon was caught and tagged 5 mo 

 later south of Kodiak, Alaska, after traveling a 

 minimum distance of 2,963 km (1.600 miles) at 

 a minimum speed of 19.8 km (10.7 miles) per 

 day. Seventeen months after tagging, the fish 

 was recaptured in the home stream in Oregon. 

 Another fish, tagged and released in Washing- 

 ton, in April 1968 had migrated some 4,213 

 km (2,275 miles) northwestward when recap- 

 tured south of the central Aleutian Islands in 

 August 1969. 



12. Time of return of tagged fish to coastal 

 streams and time spent at sea varied by geo- 

 grai)hic area. The majority of tagged steelhead 

 trout returning to British Columbia were 

 recovered in July-Sei)tember the majority of 

 returns to Washington, Oregon, and California 

 were recovered in Decmember-March. Steel- 



805 



