Nocturnal/diurnal demand-feeding pattern of yellowtail Seriola quinqueradiata under different keeping conditions

Demand-feeding behavior of juvenile yellowtails was examined under two diff erent 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 u p in the experimental room. A rod-type tactile switch was used for activati ng the feeding device which delivered 5-10 pellets per actuation. Each tank was illuminated by an overhead fluorescent lamp with a LD 12:12 photoperio d regime. In the outdoor experiment, 69 juvenile yellowtails (mean bodyweig ht, 79.8 g) were allocated to a 7000 L cylindrical tank. A switch with a st ring that ends in a rubber, pellet-like knob was used. The feeding device d elivered 90-120 pellets by activation of the switch. In both experiments, y ellowtail learned demand-feeding within 1-3 days. The number of feeder actu ations gradually increased during the experimental period in both experimen ts, presumably relating both to the rate of learning and to growth of the f ish. The yellowtails in the indoor system showed a clear diurnal feeding pa ttern synchronizing to the given photoperiod, whereas those in the outdoor system showed a clear nocturnal feeding pattern also synchronizing to the o utdoor natural photoperiod. Thus, the results showed that yellowtail has an ability for demand-feeding both in light and dark phases. These exact oppo site results may have been caused by the difference in intensity of the lum inance between the inside of the experimental room and the outdoors.