Graduate Thesis Or Dissertation
 

Ecological implications of trout introductions to lakes of the Selway Bitterroot Wilderness, Idaho

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  • The widespread introduction of trout to naturally fishless mountain lakes in the western United States has been accompanied by little research. The ecological role of trout populations occurring in 91 lakes of the central Selway Bitterroot Wilderness, Idaho, was examined with respect to 1) the sampling variability of biological and chemical lake characteristics measured, 2) possible effects of trout on biotic communities of crustacean zooplankton, macro-invertebrates, amphibians and reptiles, and 3) the relation of fish population characteristics (condition and maximum body length) to stocking rate, angling pressure, natural recruitment and lake habitat characteristics. Based on 22 lakes surveyed two or three times over a three year period, sampling variability was relatively low for individual species, biotic communities and length and weight of the largest fish in the population. Water conductivity measured at the lake surface from shore was the most reliable index of water quality, exhibiting low seasonal and duplicate sampling variability and a high correlation with alkalinity samples. Based on Detrended Correspondence Analysis (DCA), an ordination method used to organize the 91 lakes by the presence or absence of taxa, the composition of indigenous biotic communities was strongly related to the presence or absence of fish. The Multi-Response Permutation Procedure indicated that the difference in communities was statistically significant. Fewer taxa were sampled in lakes with fish and the taxa expected to be most vulnerable to predation due to their large size and frequent occurrence in open water were rarely found in lakes containing fish. DCA ordinations indicated that characteristics of the fish population (fish species, condition and maximum body length) were also related to the composition of the biotic community. This was probably due to the bottom up:top down trophic level interactions of predator and prey; fish affecting the structure of biotic communities by predation and prey affecting growth and condition of fish by their abundance and availability as determined by both physical habitat characteristics and impacts of predation by fish. Average condition and maximum body length of fish populations were related to stocking rate, natural recruitment, angling pressure and lake habitat variables by stepwise multiple regression. Average condition and maximum body length increased with decreasing level of natural recruitment for all fish population classes, except brook trout (all with high natural recruitment) and populations with no natural recruitment. Average fish condition increased with increasing angling pressure (measured by campsite impact and access distance ratings). Maximum body length of fish increased in relation to habitat variables, particularly presence of the large (2-3 mm) calanoid copepod, Diaptomus sp. Stocking rate was not related to average condition or maximum body length for any fish population class tested except one, where it may represent a spurious correlation. The findings of this study suggest the need to reassess high lake research and management policies that have promoted 1) widespread stocking of un-surveyed lakes on a regular basis and 2) considered stocking rate adjustments as an effective means of manipulating fish populations. Management direction can now be based on a recognition of the potential usefulness of one-time sampling of biological and chemical lake characteristics, the potential for significant impacts of fish on indigenous biotic communities, and the importance of natural recruitment, angling pressure and habitat variables in determining characteristics of the fish population. Recommendations include the identification and maintenance without further stocking of fishless lakes and lakes containing self sustaining trout populations (wild trout lakes).
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