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Susan M. Shirley 

 Thomas C. Shirley 



Juneau Center for Fisheries and Ocean Sciences 

 University of Alaska- Fairbanks 

 11120 Glacier Highway 

 Juneau. AK 99801 



REEXAMINATION OF THE USE OF OTOLITH 

 NUCLEAR DIMENSIONS TO IDENTIFY 



JUVENILE ANADROMOUS AND 



NONANADROMOUS RAINBOW TROUT, 



SALMO GAJRDNERI^ 



Otoliths are a potential source of taxonomic char- 

 acteristics for identifying stocks offish (Ihssen et 

 al. 1981). Differences in dimensions of the otolith 

 nucleus have provided a basis for separating win- 

 ter from summer races of steelhead, anadromous 

 rainbow trout, Salmo gairdneri. In addition, 

 otoliths provided data from which to distinguish 

 steelhead from resident nonanadromous forms as 

 well (McKern et al. 1974; Rybock et al. 1975). 

 Neilson et al. (1985) studied the development of 

 sagittal otoliths in resident rainbow trout and 

 steelhead from south-central British Columbia, 

 and identified sources of variability in the size of 

 otolith nuclei. However, they were unable to find 

 morphometric differences between the two forms 

 of trout. They concluded that the usefulness of 

 dimensions of the otolith nucleus for separating 

 steelhead from resident rainbow trout was much 

 more limited than that suggested by Rybock et al. 

 (1975) for rainbow trout in the Deschutes River, 

 Oregon. 



The difference in mean length of the otolith 

 nuclei between the rainbow trout studied by Ry- 

 bock et al. (1975) and those studied by Neilson et 

 al. (1985) suggested either population differences 

 or differences in defining the nuclear boundary. 

 These disparate results, which led to opposite con- 

 clusions, limit the usefulness of measurements of 

 otolith nuclei for the racial identification of juve- 

 nile rainbow trout until the source of these differ- 

 ences is better understood. Consequently, to de- 

 termine whether juveniles of the two forms could 

 be distinguished by differences in dimensions of 

 otolith nuclei, we measured the nuclei in sagittae 

 from steelhead and resident rainbow trout col- 

 lected from the same Deschutes River, OR, loca- 

 tions used by Rybock et al. (1975). We used the 

 definitions proposed by Rybock et al. and by Neil- 

 son et al. (1985), and compared our measure- 

 ments for the two forms with each other and with 

 published values. 



Methods 



Resident rainbow trout and steelhead were col- 



iQregon State University Agricultural Experiment Station 

 Technical Paper No. 8279. 



160 



FISHERY BULLETIN: VOL. 86, NO. 1, 1988. 



