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SUMMARY: Demand‐feeding behavior of juvenile yellowtails was examined under two different light conditions: in an experimental room and in the outdoors. In the indoor experiment, five groups of 20 juvenile yellowtails (mean bodyweight, 16.0 g) were allocated individually to five 200 L cylindrical tanks set up in the experimental room. A rod‐type tactile switch was used for activating the feeding device which delivered 5–10 pellets per actuation. Each tank was illuminated by an overhead fluorescent lamp with a LD 12:12 photoperiod regime. In the outdoor experiment, 69 juvenile yellowtails (mean bodyweight, 79.8 g) were allocated to a 7000 L cylindrical tank. A switch with a string that ends in a rubber, pellet‐like knob was used. The feeding device delivered 90–120 pellets by activation of the switch. In both experiments, yellowtail learned demand‐feeding within 1–3 days. The number of feeder actuations gradually increased during the experimental period in both experiments, presumably relating both to the rate of learning and to growth of the fish. The yellowtails in the indoor system showed a clear diurnal feeding pattern synchronizing to the given photoperiod, whereas those in the outdoor system showed a clear nocturnal feeding pattern also synchronizing to the outdoor natural photoperiod. Thus, the results showed that yellowtail has an ability for demand‐feeding both in light and dark phases. These exact opposite results may have been caused by the difference in intensity of the luminance between the inside of the experimental room and the outdoors.

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