This study developed a field method to examine emergence success of fry of westslope cutthroat trout Oncorhynchus clarki lewisi in relation to varying levels of fine substrate materials in a natural stream environment. We attempted to simulate natural incubation conditions in a stream by constructing cells with particle sizes and egg pockets characteristic of natural westslope cutthroat trout redds. We found a significant inverse relationship (r2 = 0.72, P < 0.005, N = 17) between fry emergence success, as measured by fry emergence traps, and the percentage of substrate materials less than 6.35 mm in diameter. Mean fry emergence success was 76, 55, 39, 34, 26, and 4%, respectively, in cells containing 0, 10, 20, 30, 40, and 50% substrate materials less than 6.35 mm. There were no significant differences in length or weight of fry emerging from the six gravel mixtures. Using the methods and results presented in this study, as well as previous laboratory results, resource managers could develop substrate quality guidelines for westslope cutthroat trout reproduction in streams affected by land management practices.
This is the seventh year of data documenting trends in fish populations and aquatic habitat characteristics in the Coal Creek drainage. Although estimated westslope cutthroat and juvenile bull trout population sizes remained similar, other fish population statistics varied considerably between sites over the period of record. In future analyses, these fluctuation patterns may correlate better with land use variables than actual fish population estimates. Westslope cutthroat trout shoved smaller fluctuations in most population statistics than juvenile bull trout in identical sections. Estimated carryover of juvenile bull trout from Age I to II has average 42 percent and Age II to III 17 vi
We reviewed the potential cumulative effects of human activities on bull trout in the Flathead Lake and River system of northwest Montana. Bull trout are the largest fish native to the Flathead drainage, attaining a length of up to one meter and a weight of 10 kg. This species migrates from Flathead Lake up to 250 km upstream to spawn in cold headwater tributaries with groundwater upwelling and clean gravels. Spawning and rearing habitats are limited and vulnerable to damage by deposition of fine sediments. Potential threats to bull trout habitat and populations include timber harvest, road building, mining, residential and agricultural development, hydropower construction and operation, harvest of fish by anglers, and non-native fish species. Management and mitigation options include application of best management practices and riparian guidelines for logging, habitat protection and improvement, improved fish passage to blocked areas, fishing regulations and integrative management of fish populations. Because of reduction in habitat, continued potential for habitat degradation, and limited options for management, the bull trout population is vulnerable and should be closely monitored to detect signs of decline. KEY TERMS: Migratory bull trout; spawning and rearing habitat; land management; sediment deposition; hydropower development; fisheries management and mitigation options.^D 0°m^S COLLECTS WV 2 r m DramXTICW Montana STA rr^" ti^N A >to6N 6
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