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JOBLING ET AL. REVIEWS IN FISHERIES SCIENCE <br />CONTINUOUS LIGHT <br />12 h food intake 24 h food intake <br />Atlantic salmon f 34 g) Arctic charr (39 g) <br />~ 5 <br />+I <br />~= 2 4 <br />w <br />rn <br />t~ 3 <br />E <br />Q 1 2 <br />Z <br />O 1 <br />O <br />0 0 <br />WITHOUT WITH WITHOUT WITH <br />ACCESS ACCESS ACCESS ACCESS <br />FIGURE 3. Feed intake of Atlantic salmon and Arctic charr exposed to conditions either with or without <br />access to feed on the tank bottom. <br />tion of this type can be used to provide an indirect assessment of the social <br />environment existing within a group of fish with respect to the establishment of <br />feeding hierarchies (Carter et al., 1992; McCarthy et al., 1992). The coefficient of <br />variation (CV = [SD/mean] x 100) can be used as an expression of variability within <br />a data set, so calculations of CVs provide the background information required for <br />the assessment of different rearing conditions on intraindividual (CV,) and <br />interindividual (CVO) variations in feed intake. Because a prerequisite for the <br />achievement of rapid rates of growth and a uniform body size of the fish at harvest <br />is an even distribution of feed among fish within a group, any inequalities in feed <br />acquisition between individuals would be predicted to lead to increased size <br />disparity and depressed rates of biomass gain of the group of fish as a whole <br />(MeGarthy et al., 1992). Thus, an examination of CVrs and CVs obtained foF fish held <br />under various rearing conditions should allow assessments to be made of the <br />importance of differences in feed acquisition in governing rates of growth and the <br />overall efficiency of production. <br />These types of studies provide the kinds of information required for the <br />optimization of growth and production of fish under aquaeulture conditions. The <br />value of the X-radiographic method in such studies can be demonstrated by using <br />data collected in a study of the effects of prolonged exercise on feed-growth <br />relationships as an example. A.n experiment was conducted with groups of individu- <br />ally marked fish forced to swim at different speeds. On completion of the growth <br />trial, the average feed intake (average of four feed intake measurements) and long- <br />term growth rates (specific growth rates [SGR] for the 63-day growth period) were <br />calculated for individual fish. The relationship between feed intake EFI) and SGR is <br />232 <br />