Some Morphological and Biochemical Changes in Steelhead Trout During the Parr–Smolt Transformation

Abstract

This study was initiated to determine changes in chemical composition, coefficient of condition, and body morphology for juvenile steelhead trout from premigratory through postmigratory periods, January through July. Winter-run steelhead trout, originating from Big Creek and the Sandy River, in Oregon, were used as experimental animals.A discriminant analysis indicated that coefficient of condition and fork length were nearly as powerful in separating smolts from parr as were 10 morphological measurements.The parr–smolt transformation in winter steelhead is size-dependent and characterized by a marked decrease in body depth and coefficient of condition. Changes in depth measurements and coefficient of condition provided the most meaningful measure of morphological change at the time of parr–smolt transformation.During the migratory period, lipid content was substantially lower than in the premigratory period for migrant-sized fish (≥ 160 mm) but remained relatively constant in nonmigrants (≤ 159 mm) throughout all periods. Seasonal changes in moisture and protein levels occurred in the larger fish, although moisture and protein content for the smaller fish remained relatively constant. The amount of ash did not change within a size group.The coefficient of condition of migrant-sized fish declined continuously from February through May but increased in June and July. The mean coefficient of condition for nonmigrants declined from December through February, but remained relatively stable from March through June, with an upward trend in July.