8191 - Laserfiche WebLink

Home Browse Search

the control reach. The numbers of whitefish increased dramatically in the altered reaches during the fall spawning season. Apparently the water velocities in the altered reaches was suitable for spawning and these areas contained clean gravel which may also have attracted the fish. While mountain whitefish may be displaced for a short time during the following and actual alteration, they appeared to quickly repopulate the altered area if suitable habitat remained or was created by the alteration. The bulldozed reaches became extensive riffle environments or glides that were utilized by the white- fish especially during spawning but these areas were not used by brown trout. Such areas lacked the overhead and instream cover that was required by the brown trout while the depth, velocity, and broken water surface required by the whitefish were still available in altered reaches. Although the whitefish apparently spawned in altered reaches, few young fish were found in these sections - - either there was low survival in these reaches or the young fish used other reaches of the river. The brown trout populations were not self- sustaining in the bulldozed sites either because of few spawning adults or little recruitment and survival of age 0 trout. Dredging apparently had less affect on the survival of age 0 brown trout but repeated dredging would adver- sely affect future recruitment based on the trout population response during this study. Populations, biomass, and production of brown trout and mountain whitefish appeared to be directly related to the proportion of the stream reach that contained pools - higher in reaches with more pools and lower in reaches that were lacking pools. Both the brown trout and the mountain whitefish occupied pools during the low streamflows occurring in late summer and early fall as well as during the winter. Fish would leave areas that lacked pools during low stream- flows and would return when streamflows became normal. The unstable stream bottom in altered reaches resulted in a continually changing habitat for fish; therefore, the variation in the density of fish or the age composition of the population merely reflected the amount of suitable habitat for a particular life stage(s). No differences were observed in the growth of either brown trout or mountain whitefish in altered reaches when compared to unaltered reaches. However, the fish moved to more suitable habitats as the streamflows became low and returned when conditions were suitable again. The annual pro- duction of brown trout flesh in the Blacksmith Fork and Logan Rivers is summarized in Figure 6. The highest values occurred in the control site and the dredged site on the Blacksmith Fork River - 72.7 and 73.8 kg/ha/yr where pools, overhead cover, and undercut banks were most abundant. The production was more than double (25.5 kg/ha/yr) in the total bulldozed site on the Logan River where pools were beginning to become reestablished in the lower part of this reach when compared with the area upstream that was stable (10.0 kg/ha/yr). A similar result occurred between the recently bulldozed and old bulldozed reaches on the Blacksmith Fork River (Figure 6). The production of mountain whitefish flesh in the Blacksmith Fork and Logan Rivers is summarized in Figure 7. Since the growth rates of the mountain whitefish were similar between sites, the differences in estimated production was the result of different mean biomasses during the production period. Therefore, the large increase in the density (and biomass) in the bulldozed reach of the Logan River during the fall spawning movement as well as the high numbers of fish at other times (Figure 4) resulted in a high annual production. The alteration of this reach provided 3