Spatial Distribution and Behavioural Movements of Migratory Trout Salmo trutta in a Lake District Stream

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Abstract
(1) As the sample variance to mean relationship for 18 year-classes (1967-84) of migratory trout, Salmo trutta L., followed Taylor''s Power Law, their spatial distribution was density-dependent. As expected, there were significant changes in the relative level of clumping at different stages in the life cycle. (2) The first spatial change, a decrease in relative clumping soon after the end of the alevin stage in April, was related to the behavioural movements of the fry emerging from the nest, usually at night. About 81% of fry in field experiments rarely fed, soon lost weight after their yolk was exhausted, drifted downstream chiefly at night and died. The others started to feed and either remained near their birthplace or migrated, usually upstream. The proportion migrating was density-dependent, increasing from < 1% to c. 12% as the number of fry per nest increased. Few fry emigrated out of the stream and therefore population losses in spring were chiefly due to mortality. (3) The second spatial change, an increase in relative clumping in the first winter of the life cycle, was related to the behavioural movements of first-year trout when they migrated to deeper pools in the stream. A few first-year trout emigrated out of the stream in winter, but population losses in the first summer and winter of the life cycle were chiefly due to mortality, usually within the stream, rather than migration. (4) There were no major spatial changes for second-year trout in the second spring, summer and winter of the life cycle. The third spatial change, a slight increase in the relative clumping of third-year trout, was related to their increased mobility just before they emigrated from the stream as smolts. Few second and third-year trout died in the stream and most moribund fish moved out of the stream. (5) The implications of this investigation are discussed and it is concluded that the parameters of the Power Law are not only species-specific but also life-stage specific. The present investigation has shown that such parameter changes can be used to detect major spatial changes that can be related to behavioural movements.