Graduate Thesis Or Dissertation
 

Factors influencing spawning migration of bull trout (Salvelinus confluentus) in the North Fork Skokomish River, Olympic National Park, Washington

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  • Distribution and life history characteristics of lacustrine-adfluvial bull trout (Salvelinus confluentus) were described in the North Fork Skokomish River Basin (including Lake Cushman, a reservoir) from 1994 to 1996. Day snorkeling was conducted in the river to determine initiation of the bull trout spawning migration, abundance of spawners, and duration of spawning. Declining photoperiod, increased river discharge, and decreased water temperature appeared to influence timing of migration and spawning. Lacustrine-adfluvial bull trout typically entered the North Fork Skokomish River in October although some fish entered as early as May. Mean lengths of spawners consistently increased from June to December 1996, and early migrating bull trout were shorter than those fish that entered after river discharge increased in October. The presence of two phases of the spawning migration may be indicative of two populations spawning in the river. Bull trout spawned between mid-September and December in the river and tributaries after water temperatures declined. All spawning occurred at temperatures less than 7.5°C. Comparisons with studies of other lacustrine-adfiuvial bull trout populations that inhabit river and reservoir complexes suggested that bull trout exhibit specific migratory strategies related to local environmental conditions. In the North Fork Skokomish River, changes in abundance of bull trout, mountain whitefish (Prosopium wilhamsom), cutthroat trout (Oncorhynchus dark), and rainbow trout (Oncorhynchus mykiss) revealed distinct temporal segregation among these species. Olympic National Park, a designated Biosphere Reserve, contains one of the largest remaining areas of relatively pristine habitat in the range of bull trout. Knowledge of responses of bull trout to changes in river discharge and temperature from relatively undisturbed systems, such as the North Fork Skokomish River, may be useful in understanding patterns observed in degraded environments.
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