The Effect of Density on Mortality in Juvenile Atlantic Salmon (Salmo salar)

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Abstract
The mortality of jovenile Atlantic salmon (S. salar L.) was investigated in 16 study sites in the upper Wye [Wales, UK] catchment during the period June, 1975-Nov., 1976. Within each site the instantaneous mortality rate remained remarkably constant over the period of study, and the mortality rate at each site was directly related to the estimated density of fry on June 1st. With an estimate of the fry density on June 1st, it was possible to predict the in-site survivorship of juvenile salmon. The number of survivors decreased above the optimum initial density, as indicated by a dome-shaped recruitment curve atypical of territorial species. Assuming that smolt production is equivalent to the numbers of juvenile salmon surviving to 2 yr old, the maximum smolt production of 0.043 m-2 (95% confidence interval 0.019-0.097) was estimated to be attained from a fry density on June 1st of 0.75 m-2. Many sites on the River Wye appear to sustain fry densities which are too high for maximum smolt production. The optimum stocking density for unfed fry is estimated to be 1 m-2, considerably lower than is common practice.

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