Habitat selection in Brook Trout, Salvelinus fontinalis

Knight, Thomas W. (2000) Habitat selection in Brook Trout, Salvelinus fontinalis. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

I combined field and laboratory experiments to test theories of density-dependent habitat selection and habitat preference in brook trout (Salvetinus fontinalis) from Cape Race, Newfoundland, Canada. My primary objective was to test whether theory allows us to correctly infer the behaviour of individuals in a population based only on field census data. Secondarily, I tested whether patterns of habitat use are influenced by the habitat composition of a river as indicated by models of evolution in heterogeneous landscapes. -- Theories of density-dependent habitat selection predict that competitive interference for preferred sites should produce curvilinear or non-linear relationships in population density. I tested the theory by manipulating population densities of brook trout in four separate stream enclosures containing flat and riffle habitats. Regressions of density in the paired habitats (isodars) were curvilinear, suggesting that brook trout are site-dependent habitat selectors. Body-size distributions between flat and riffle habitats were consistent with the hypothesis of site pre-emption by larger, presumably dominant individuals. The isodar analysis, based only on density data, revealed the competitive behaviours that are believed to underlie brook trout distributions. -- To test whether individual behaviour is consistent with the hypothesis of site dependence, I used observations of brook trout in an experimental stream tank. Brook trout were introduced into the stream tank over a range of population densities, at two flow treatments, and their precise location was mapped at consistent intervals over 2 to 3 days. My observations support the site-dependent model of habitat selection, confirming 3 a priori predictions: 1) brook trout recognise and respond to differences in site quality; 2) individuals select the highest quality site available; 3) larger, presumably dominant, individuals occupy the sites of highest quality. Observed habitat-selecting behaviour is consistent with behaviour inferred from population census data, further supporting ideas of density-dependent habitat selection and strengthening the theoretical basis of isodar analysis. -- Finally, I tested whether habitat use differs between populations as predicted by theoretical models of evolution among heterogeneous landscapes. Habitat preferences of brook trout captured from two isolated rivers were observed in an experimental stream tank. Individuals from a river composed primarily of flat habitat used the flat portion of the stream tank significantly more than brook trout from a river composed primarily of riffle habitat. I conclude that habitat preference in salmonid fish may evolve partially in response to the landscape composition of a given watershed.

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