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Ecology and life history differences of bull trout (Salvelinus confluentus) living above and below barriers Ladell, Jason J.

Abstract

Bull trout populations in the Arrow Lakes exhibit two distinct life history patterns depending on whether they live above or below waterfall barriers. Migratory populations utilize the lower reaches of tributaries for spawning and rearing but resident populations carry out their entire life history upstream of waterfall barriers. Few studies have examined how these populations differ in terms of habitat preference or life history pattern. The specific objectives of this study were thus twofold. The first was to describe and quantify differences between habitat availability and use by resident versus migratory bull trout, and the second was to describe how certain life history parameters varied between the two populations. The physical environments of these two habitats showed distinct variation in temperature and physical habitat variables including stream size, water velocity, pool composition and available cover. Maximum summer water temperature and average summer water temperature were both lower above the barriers. However, bull trout utilized similar habitats irrespective of whether they were above or below barriers, and appeared to maintain positions in shallow, low velocity areas of the stream. Daytime cover associated with above-barrier fish was typically logjams or other instream woody debris, while below-barrier cover usually consisted of large boulder substrates. Densities of bull trout at all sites were similar to those reported from other populations and were temporally variable within and between sites. Movements of bull trout determined by mark-recapture rates showed that above-barrier fish exhibited a more restricted movement pattern than below-barrier fish. Evidence of life history differentiation between bull trout populations occurring above and below barriers included differences in growth rates, average lengths, morphology, and meristics. This study showed that bull trout populations occurring above and below waterfall barriers in the tributaries of the Arrow Lakes inhabit different physical environments, and that certain aspects of their life histories appear to reflect local adaptation to these distinct ecosystems.

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