FISHERY BULLETIN: VOL. 71, NO. 3 



assumed that, like salmon, they remain in 

 coastal waters in their first summer at sea. Of 

 323 steelhead trout caught in several years of 

 fishing on the high seas and whose age was 

 determined, only 21 (6%) were ocean age .0. 

 Presumably, this age group made up a sub- 

 stantial part of the total oceanic steelhead trout 

 population. After having advanced to ocean 

 age .1 and having increased in size from 36 cm 

 at ocean age .0 to an average length of 57 cm 

 (Table 6), they were readily caught by the 

 sampling gear. In the same of 323 steelhead 

 trout caught over several years, 65% were age 

 .1, 24% age .2, and 4% age .3. The older age 

 groups, .4, .5, and .6, contributed less than 1% 

 to the population sampled. 



Data on the age composition of returning 

 adults, together with tag and recovery informa- 

 tion, indicate that most steelhead trout spent 

 two summers at sea before returning to fresh 

 water. 



Length and Weight 



The average lengths and weights of steelhead 

 trout of various age groups caught at sea by 

 NMFS research vessels are shown in Table 6. 

 The bulk of the samples from which length 

 and weight data were taken were in ocean ages 

 .1 and .2. Although the number of samples in 

 the other ocean ages was small, particularly for 

 weight data, some observations on length and 

 weight can be made. 



On entering salt water, juvenile steelhead 

 trout vary in size among the freshwater age 

 groups represented, by season of migration, 

 and by geographical area. For the freshwater 

 groups 0. to 4., Shapovalov and Taft (1954) 

 found that the average lengths of these sea- 

 ward migrants from Waddell Creek, Calif., 

 ranged from 6.0 to 27.0 cm. Sumner (1953), 

 in a study at Sand Creek, Oreg., found average 

 fork lengths of 16.0, 14.2, and 15.2 cm, with 

 ranges from 3.0 to 23.4 cm, for migrants in 

 three seasons. In British Columbia, studies by 

 Maher and Larkin (1955) in the Chilliwack 

 River showed the average fork length of 

 migrants in five seasons (1948-53) for ages 1., 

 2., 3., and 4. were 11.1 cm, 16.5 cm, 20.0 cm. 



and 22.9 cm, respectively. The average length 

 for all ages combined was 17.6 cm. 



The average length of steelhead trout taken 

 at sea in gill nets ranged from 34 to 90 cm 

 among the various age groups (Table 6). 

 Those fish caught after a few months in salt 

 water (age .0) had an average length of 36 

 cm. The increase in length amounted to a gain 

 of about 100-150% from the average of 14-17 

 cm observed by various authors (Sumner, 

 1953; Shapovalov and Taft, 1954; Maher and 

 Larkin, 1955) for seaward migrants of all age 

 groups. The greatest difference in the average 

 length of all freshwater age groups of ocean age 

 .0 caught offshore was only 3 cm. During the 

 first year at sea (age .0 to .1) the average length 

 of those steelhead trout caught had increased 

 by 58% to 57 cm, in the second year (age .1 to 

 .2) by 23% to 70 cm, and in the third year (age 

 .2 to .3) by 11% to 78 cm. The length range of 

 the various age groups increased with ocean age 

 but not progressively from the youngest to the 

 oldest. With the added variability due to age, 

 the maximum differences averaged only 4 cm 

 within age group .1 and 5 cm within age group 

 .2. The samples of older age categories were 

 too small to yield realistic average lengths, 

 although growth apparently continues at a sub- 

 stantial rate in succeeding years. One steelhead 

 trout caught after 4 yr at sea had attained a 

 length of 91 cm and another after 6 yr at sea 

 was 90 cm. 



The number of steelhead trout weighed was 

 inadequate for a detailed analysis of this growth 

 parameter for all ocean ages except .1 and .2. 

 Females weighed more than males in all age 

 categories. At age .1 the females averaged 2.4 

 kg compared with 1.8 kg for males. At age .2 

 the average weight of females increased to 3.4 

 kg compared with 2.9 for males. The oldest 

 fish, an age .6 female, weighed 6.7 kg when 

 caught. 



OCEANIC DISTRIBUTION AND 



SEASONAL ABUNDANCE OF 



STEELHEAD TROUT 



The research vessel catch and effort data 

 described above are used here to describe the 

 oceanic distribution and seasonal abundance 



796 



