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Glacier National Park

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Surviving populations of native cutthroat trout (Oncorynchus clarki lewisi) in isolated lakes of Glacier National Park are being assigned a potentially important role in a recovery program of cutthroat trout fishery in the Lake McDonald basin. One population of native trout inhabits Avalance Lake; yet its indigenous nature is questioned, because of the absence of other fish species that normally occur in association with the trout, and because the creek giving access to Avalanche Lake from Lake McDonald is considered too precipitous for naturally dispersing fish. This study aims to find out whether the trout population in Avalanche Lake was established following a deliberate but unrecorded introduction sometime during the last century, or much earlier through successful natural immigration. Evidence is to be derived from examination of fossil zooplankton assemblages contained in the lake's profundal bottom sediments. The approach is based on the idea that introduction of planktivorous fish to a fishless lake must have caused significant changes in the zooplankton community. Visual, size-selective predation by planktivorous fish typically results in preferential removal of larger prey types and/or of prey that tend to stay in the upper water column during daytime. There are two groups of zooplankters in Glacier National Park lakes whose populations are likely to be affected, and whose buried remains preserve well enough to trace these changes in the sedimentary record: the limnetic cladoceran Daphnia, and the planktonic larvae of Chaoborus, a dipterous insect. In Daphnia, changes associated with fish introduction may include a shift to smaller body-size in resident species or a replacement altogether of large species by smaller ones. In Chaoborus, species that lack the habit of diurnal vertical migration are rapidly eliminated, and may then be replaced by congeners that do migrate.