Repeated measurements of food intake made on juvenile Arctic charr, Salvelinus alpinus, held under different rearing conditions enabled examination of the effects of environmental manipulations on both intra– and inter–individual variations in food intake to be made. This permitted the assessment of the influences of differential food acquisition on individual growth rates and biomass gain. When charr were held in isolation individual fish showed relatively little day–to–day variability in food intake and inter–individual differences in intake were small (‘base–fine’ values). All fish exhibited positive rates of growth and the overall range was narrow. Nevertheless, there was a highly significant positive correlation between food intake and growth, indicating that those individuals that consumed the greatest quantities of food were also those that had the highest rates of weight gain. The rearing of charr in groups led to increases in both intra– and inter–individual variations in food intake to levels considerably above ‘base–line’. This increased variability in food intake was reflected in rates of weight gain being more variable amongst the charr reared in groups, with fish that lost weight often being recorded. Manipulation of the rearing environment had marked influences upon intra–individual variability in food intake, inter–individual differences in food acquisition and rates of weight gain. High stocking densities and exposure of the fish to moderate water currents were most effective in reducing levels of variability to approach those observed under ‘base–line’ conditions.

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There are three main methods used in the study of feeding behaviour and food consumption in captive populations of fish. These are direct observation, the recording of feeding activity using 'on-demand' feeders, and the quantitative determination of gastrointestinal content using a non-invasive 'labelled feed' technique. Observational techniques have been widely used in studies of foraging behaviour and feeding responses, but these methods are usually confined to the study of fish held in small groups. 'Labelled feed' techniques are well suited to the monitoring of feed intake of individual fish held in large groups, but cannot be used for the continuous monitoring of feeding behaviour. They are, therefore, not particularly suitable for the collection of information about short-term changes in feeding rhythms, or possible die1 changes in feeding activity. Studies of feeding activity may best be carried out using computer-operated 'on-demand' feeding systems, but the data collected using these systems give no information about individual feed consumption. Examples are presented showing how data collected using the three methods can be used to provide insights into how various abiotic and biotic factors influence fish feeding. K E Y WORDS: demand feeders, feeding behaviour, food intake, growth studies, X-radiography ~ ~ ~~~