Abstract:
Reported are results of a study of otolith nuclei as a means to separate juvenile steelhead trout (Salmo gairdneri) from juvenile rainbow trout (S. gairdneri) which co-exist in the lower Deschutes River, Oregon. An intensive recreational fishery necessitated development of a technique for separation so that the impact
of the fishery on each race could be assessed independently. Results revealed that steelhead trout mature at a larger size than rainbow trout, egg size in both races is directly related to body size of dam, and size of otolith nucleus is positively correlated with egg size in rainbow trout. Examination of adults demonstrated, in fact, that otolith nuclei of steelhead are significantly larger than those of rainbow. Size of otolith nucleus does not change with growth of either fish, nor are there differences due to sex or origin. The validity and utility of this method to separate the races were confirmed. Regressions of length and weight of otolith on length of fish demonstrated that otoliths of juvenile steelhead grow at the same
rate as those of rainbow, whereas adult steelhead on their spawning migration are longer than rainbow for a given length or weight of otolith. A list of methods used by others to facilitate the viewing of otoliths and a discussion of the formation of the otolith nucleus are included.
