USE OF OTOLITHS TO SEPARATE JUVENILE STEELHEAD 

 TROUT FROM JUVENILE RAINBOW TROUT^ 



J. T. Rybock^ H. F. H0RT0N^ AND J. L. Fessler' 



ABSTRACT 



Otolith nuclei were investigated as a means of separating juvenile steelhead trout, Salmo gairdneri, 

 from juvenile rainbrow trout, S. gairdneri, in the lower Deschutes River, Oreg. An intensive recrea- 

 tional fishery necessitated development of a technique for differentiation so that impact of the fishery 

 on each race could be assessed independently. 



Investigations of adults and hatchery-reared young of S. gairdneri revealed that otolith nuclei of 

 steelhead are significantly larger than those of rainbow, size of otolith nuclei does not change with 

 growth of either race, and there are no differences in size of otolith nuclei due to sex or origin (wild vs. 

 hatchery). Thus, size of otolith nuclei provides a means to differentiate effectively juvenile steelhead 

 trout and juvenile rainbow trout regardless of sex or origin. Results also indicated that steelhead 

 mature at a larger size than rainbow, egg size is directly related to body size of dam in both races, and 

 size of otolith nuclei is likely determined by egg size. 



This paper reports on an investigation of growth 

 characteristics of the sagittae, the largest of the 

 otoliths, as a means to separate juvenile steelhead 

 trout, Salmo gairdneri, from juvenile rainbow 

 trout, S. gairdneri. The technology for such 

 differentiation is presently lacking but is neces- 

 sary for independent management of the two 

 races in streams where they coexist. In the lower 

 Deschutes River, Oreg., for example, the most in- 

 tensive fishery for rainbow trout occurs during the 

 first week in May when most steelhead smolts 

 migrate; consequently, the catch may be composed 

 of 22-80% juvenile steelhead (King 1966; Wagner 

 and Haxton 1968). Precise knowledge of this catch 

 composition at various locations and times would 

 allow fisheries managers to manipulate fishing 

 pressure so that most steelhead smolts escape 

 capture during migration. 



Previously, otoliths have been used to differen- 

 tiate stocks and races of salmonids. Kim (1963) 

 found differences in the appearance and size of 

 growth rings between spawning groups of sockeye 

 salmon, Oncorhynchus nerka. The study most 

 relevant to our investigation demonstrated that 

 winter and summer races of steelhead trout can be 

 separated on the basis of differences in size of the 

 otolith nucleus (ON) (McKern et al. 1974). 



'Technical Paper No. 3619, Oregon Agricultural Experiment 

 Station, Corvallis, OR 97331. 



■'Department of Fisheries and Wildlife, Oregon State Univer- 

 sity, Corvallis, OR 97331. 



'Research Division, Oregon Wildlife Commission, 303 Exten- 

 sion Hall, Corvallis, OR 97331. 



The latter authors found that the otolith nucleus 

 is formed early in steelhead trout embryos when 

 all or a great part of nutrition comes from the yolk, 

 and that ON size appears to be directly related to 

 egg size. Also, egg size and fish length of sal- 

 monids are often directly related (McFadden et al. 

 1965; Bulkley 1967; Galkina 1970), and steelhead 

 trout are generally larger than rainbow trout at 

 maturity. Therefore, we hypothesized that 

 steelhead ON are suflficiently larger than rainbow 

 ON to permit separation of juveniles of both races. 



We investigated this hypothesis via two series 

 of observations. In the first, an indirect test of 

 validity of the hypothesis, we compared 

 measurements of ON of fish of known race and 

 then determined whether these measurements 

 changed with growth of fish or whether they were 

 related to sex or origin. In the second series, we 

 measured body size of adult fish, egg size of ripe 

 dam, and ON size of fry hatched from these eggs, 

 to determine if correlations between these varia- 

 bles logically accounted for differences in ON 

 sizes. All investigations were conducted on adults 

 and hatchery-reared fingerlings of summer 

 steelhead trout and resident rainbow trout cap- 

 tured in 1971-73 from the lower Deschutes River. 



METHODS AND MATERIALS 



Study Area 

 The study area was the lower 100 miles of the 



Manuscript accepted December 1974. 

 FISHERY BULLETIN: VOL. 73, NO. 3, 1975. 



654 



