Mitsushi Yamashita

Self-feeding behavior of yellowtail, Seriola quinqueradiata, in net cages: diel and seasonal patterns and influences of environmental factors

Abstract The self-feeding pattern of yellowtail, Seriola quinqueradiata , maintained in floating net cages was examined throughout a year, and the influence of seasonal changes of the photoperiod and water temperature on the feeding pattern was investigated. Two groups of 50 yellowtail (initial mean body weight about 50 and 80 g, respectively) were kept in two experimental net cages (2×3×4 m deep), with a self-feeding device installed on each. It was possible to maintain yellowtail in net cages using a self-feeder throughout almost a year. The number of daily food demands was greatly affected by the seasonal changes in water temperature. Yellowtail showed high self-feeding activity, above 18 °C; depressing water temperatures did not influence the activity when the water temperature was over 18 °C. However, the activity decreased when the water temperature fell below 18 °C. The feeding pattern in a 24-h period was greatly affected by the temporal changes in light intensity. Annual observations revealed that yellowtail showed generally crepuscular plus nocturnal feeding behavior and had two peaks of feeding activity a day. These peaks appeared at dawn and dusk; moreover, a clear association between the most active time of self-feeding and the period of the greatest change in light intensity was observed. The results suggest that a change in light intensity might stimulate the appetite of yellowtail or that there is a light level at which yellowtail prefer to eat.

Development of new self-feeding system for mass rearing of ayu Plecoglossus altivelis altivelis under artificial and natural light-dark cycles

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.