CRITICAL SWIMMING VELOCITY AND ASSOCIATED MORPHOLOGY OF JUVENILE COASTAL CUTTHROAT TROUT (ONCORHYNCHUS-CLARKI-CLARKI), STEELHEAD TROUT (ONCORHYNCHUS-MYKISS), AND THEIR HYBRIDS

Critical swimming velocities were determined for two hatchery stocks o f cutthroat trout (Oncorhynchus clarki clarki), one hatchery stock of steelhead trout (Oncorhynchus mykiss), and reciprocal cutthroat-steelh ead hybrids from both cutthroat stocks. Steelhead had the highest crit ical swimming velocity (7.69 body lengths/s), cutthroat had the lowest (5.58 and 6.69 body lengths/s for the two stocks), and hybrids were i ntermediate (7.14 and 7.22 body lengths/s). Consistent with the signif icant differences in swimming performance, principal components analys is revealed that cutthroat had shorter and narrower caudal regions tha n steelhead and that hybrids were morphologically intermediate. The mo rphology of hatchety populations compared with that of wild juvenile c utthroat and steelhead allowed us to determine if the interspecific mo rphological differences observed in hatchery-reared fish were represen tative of the species and did not result from hatchery practices, and to assess the validity of extrapolating the swimming performance resul ts to wild fish Although wild fish were generally more fusiform than h atchery fish of both species, the relative differences between cutthro at and steelhead shapes were the same for both hatchery-reared and wil d fish. We conclude that hybrids are intermediate to the parental spec ies in both swimming performance and morphology and that the potential for a competitive advantage exists.