Feeding Intensity, Growth Rates, and the Establishment of Life-History Patterns in Juvenile Atlantic Salmon Salmo salar

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Abstract
(1) Under good growing conditions, populations of juvenile Atlantic salmon develop a bimodal size-frequency distribution by late autumn of their first year; the larger fish in the upper modal group (UMG) will metamorphose into the sea-going phase the following spring, a year ahead of fish in the lower modal group (LMG). The feeding behaviour and growth rates of individually-marked fish from a sibling laboratory population were studied during the period when these two distinct life-history patterns become apparent. (2) The length-frequency distribution of the laboratory population was unimodal in August, significantly skewed by September, and clearly bimodal by December. Bimodality was a result of fish which joined the UMG putting on a brief growth spurt in late September/early October, at a time when water temperatures were falling and growth rates of LMG fish were decreasing. (3) Studies of the feeding behaviour of single fish in laboratory flume tanks revealed that the growth rate changes could be linked to changes in appetite rather than any external factors. The appetites of the two types of fish were initially similar in August, but then diverged: UMG fish showed a marked increase in appetite through to October, whereas the appetite of LMG fish decreased, with a small peak in October. (4) The results indicate that the fast and slow developing life-history patterns become fixed prior to September of the first year, and the corresponding changes in appeptite and growth rate are thereafter under internal control. The October peak in appetite does not appear to be related to natural food availability, and may instead allow fish to obtain nutritional reserves prior to the winter when feeding is constrained.