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The existence of a food-entrainable circadian oscillator (FEO) was examined in goldfish Carassius auratus. Single goldfish were exposed to light-dark (LD)

Self-feeding of grouped ayu Plecoglossus altivelis altivelis was examined under an artificial light-dark (LD) cycle and natural day length using a trigger with a photosensitive sensor. In Experiment 1, approximately 15 juvenile fish (mean body weight, 0.6 g) were kept in 60-L glass tanks (four tanks) under LD 12:12 (lighting period 06:00-18:00 hours) and self-feeding and locomotor activities were simultaneously recorded. Self-feeding was detected within one day and feeding activities were strongly synchronized to the LD cycle, with almost strictly diurnal feeding (98.2%). Although locomotor activities in the upper part of the tank were also greatest during the light phase (76.8%), the activity profiles were different from those of self-feeding, suggesting the validity of the photosensitive sensor. In Experiment 2, 25 fish (mean body weight, 15 g) were kept in outdoor 1000-L tanks (four tanks) under natural day length and water temperatures in May, and self-feeding was recorded from mid-August. Stable self-feeding was observed from early September in all the tanks. Nearly all food demands were diurnal, and usually crepuscular. Somatic growth was seen in all tanks. These results demonstrate that the self-feeding system using a photosensitive sensor is applicable for grouped juvenile ayu as small as 0.6 g and adult fish reared under natural conditions.

Background and feeding activity in the bar¢n £ounder T Sunuma et al.

Individual variability of self-feeding activity in group-reared barfin flounder Verasper moseri was examined under natural photoperiod and water temperature. A self-feeder with a string trigger and a switch sensor, and a charge-coupled device (CCD) camera were placed in each of six tanks. Activation of the self-feeder resulted in delivery of three pellets (approximately 0.2 g in total), and each activation was recorded by an event logger. Selffeeding behavior of tagged fish (three or five fish per tank) was recorded continuously on video for 24 h once a week for 4-12 weeks. In all tanks, the fish learned self-feeding within 2 days, were mostly nocturnal feeders, and immediately consumed all the pellets provided by the feeder. All the fish gained body weight during the experimental period. In three tanks, significant differences were observed in the number of feeder activations per 24 h among individuals, i.e., one fish mainly triggered the switch. In contrast, in the other three tanks, no significant differences were observed in the number of feeder activations per 24 h among individuals. These results indicate that barfin flounder attain appropriate growth by using the self-feeding systems independently of whether or not individual differences in the number of self-feeding activities were observed.

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