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JOBLING ET AL. REVIEWS IN FISHERIES SCIENCE previous meals must be evacuated before additional measurements can be under- taken. This problem can be overcome by allowing sufficient time to pass between measurements, and in long-term studies designed to examine feed-growth relation- ships in the Arctic chary, the estimation of feed intake at 3-week intervals is suitable (Christiansen and Jobling, 1990; Christiansen et al., 1992). The X-radiographic method can give accurate estimates of feed intake under a wide range of experimental conditions, but underestimation can be expected both at high temperatures, when rates of feed transit are increased, and under conditions when a long period elapses between the start of the measurement period and the photographing of the fish. In both cases, there would be the risk that some particles would be evacuated before exposure of the X-ray plates. Thus, it is of vital importance that workers wishing to use the method carry out adequate preliminary studies in order to test the possible limitations of the method with given species held under given conditions. IV. DAILY FEED INTAKE The X-radiographic method allows information to be collected about the feed intake of previously unstarved, relatively undisturbed fish and has, therefore, a number of advantages over the methods traditionally used in studies of feeding behavior and growth. Direct observation and video-recording have been widely used in laboratory studies, but the monitoring of feed intake using such techniques is limited to groups containing small numbers of fish. Thus, these techniques have been rarely used to study feeding behavior and feed intake in large groups of fish held under conditions resembling those found in commercial production facilities. Where observational techniques have been used, they have been restricted to either general examinations of behavioral responses to feed presentation, or to studies on focal individuals (Thorpe and Huntingford, 1992). Information about feed intake by groups of fish in a laboratory setting can be obtained by feeding the fish by hand and ensuring that there is little or no feed wastage. The information gathered refers, however, only to the total amount of feed consumed by the group of fish, and provides no information about differences in feed acquisition among individuals. In large-scale growth trials, the provision of feed by hand may not give an adequate reflection of feed intake because there will inevitably be a certain amount of feed waste. By combining knowledge about the amounts of feed provided with measurements of feed intake obtained using the X-radiographic method, it is possible to gain insights into the amounts of feed wastage that result from different feeding practices (Thorpe et al., 1990). When fish are fed by hand at predetermined time intervals, the feeding regime used may impose restrictions upon the fish. For example, it is known that the timing of feed provision can influence the rhythmicity of a range of physiological and behavioral processes and, thus, have consequences for growth and production performance (Boujard and Leatherland, 1992). It would seem that the only ways in which such influences could be avoided would be either to allow the fish to feed using on-demand feeding systems, or to provide feed in excess from automatic 226