Growth, Survival, and Migration Patterns of Juvenile Adfluvial Bonneville Cutthroat Trout in Tributaries of Strawberry Reservoir, Utah

Abstract

Adfluvial Bonneville cutthroat trout Oncorhynchus clarki utah were introduced into Strawberry Reservoir after a rotenone treatment in 1990. Naturally reproduced fry either migrated to the reservoir immediately after emergence (migrants) or remained in the tributaries for 1–2 years (nonmigrants). In 1995, fry emerged from mid-August to early September. Migrants moved downstream almost exclusively between 2100 and 0300 hours. Migrants were always the same size as newly emerged fry, whereas the mean size of nonmigrant fry increased throughout the emergence period. Most fry followed the nonmigrant strategy. Nonmigrant fry grew from 0.12 g (26 mm) at emergence to an average 1.2 g (50 mm) by early November. Mean densities of nonmigrant fry in low-velocity microhabitats along the stream margins ranged from 5.5 to 11.0 fry/m2 during the 6 weeks after first emergence. Mean focal point velocity of nonmigrant fry in microhabitats was 3.3 cm/s (SD = 1.4, N = 42) and the mean focal point depth was 11.8 cm (SD = 6.5, N = 80). By late September, larger fry (>50 mm) shifted from the stream margins into the main channel. Growth and survival of migrants and nonmigrants were compared at low, ambient, and high fry densities in enclosure experiments to test whether migrants were inferior competitors in the stream. Growth and survival were similar for cohabiting migrants and nonmigrants within and among the three fry density treatments in the enclosure experiments. Growth was not affected by fry densities after 3 weeks; however, growth declined similarly for both migrant and nonmigrant fry in the high-density treatment compared with the low- and medium-density treatments after 6 weeks. Since no competitive differences were evident among the migrants and nonmigrants, emigration could be an innate response by some fraction of the population rather than the result of inferior competitors being forced downstream.