EFFECTS OF FLUCTUATING FLOWS ON THE POPULATION-DYNAMICS OF RAINBOW-TROUT IN THE SPOKANE RIVER OF IDAHO

Rainbow trout (Oncorhynchus mykiss) were studied in 1985 and 1986 in the Spokane River, Idaho, to evaluate possible factors limiting abundance and survival. A mark and recapture estimate made in the fall 1985 was 19,029 fish and standing crop was estimated at 5.2 g/m2 in a 7.9 km reach. Rainbow trout growth rates were above average for the region with fish averaging 140-155 mm in length by the end of the first year. Estimates of total instantaneous rate of mortality (Z) were 1.35 using catch curves and 1.13 to 1.44 based on tag returns. Exploitation corrected for nonresponse was 8.9 percent making instantaneous rates of fishing mortality (F) 0.16 and natural mortality (M) 1.19. Annual survival for recruited fish was less than 30 percent. Post spawning, over winter mortality, high zinc concentrations, high summer temperatures and/or low summer flows may have contributed to the high natural mortality. Spawning commenced in the Idaho reach about 1 April. Variable year-class strengths appeared to correlate with river flow between the spawning period (early spring) and establishment of feeding territories (1 July) during the first summer. Fry production was minimal in 1986. Sampling young-of-the-year trout in the fall and examination of the hydrograph could provide fishery managers with two years advance planning to supplement weak year-classes.