Yoshitaka Sakakura

Ontogeny of aggressive behaviour in schools of yellowtail, Seriola quinqueradiata

We studied ontogenetic changes in social interactions, especially in aggressive behaviour of the migratory marine yellowtail, Seriola quinqueradiata (Carangidae), and compared these to morphological and physiological changes. No agonistic interactions were observed during the larva period until 10 mm in total length (TL), at approximately 20 days after hatching. Typical shivering behaviour with ‘J-posture’ was observed during metamorphosis, when fin rays and calcification of vertebra were completed and there was an increase of tissue thyroid hormone. The onset of aggressive behaviour was just after metamorphosis to the juvenile period, and coincided with a significant increase in tissue cortisol levels. The onset of schooling behaviour was at 12 mm TL, slightly after the onset of aggressive behaviour. From observations of individual aggressive behaviour within juvenile schools, we found three categories of social rank: dominants (10–20%), intermediates (10–20%), and subordinates (60–80%). There was an inverse relationship between social rank and cortisol concentration. Otoliths of dominant fish in 8 experimental groups were labeled and the fish were returned to their groups. Six labeled dominants appeared after 1 day and three after 1 week rearing, respectively, indicating that social rank was maintained for at least 1 week (binomial distribution, p < 0.05). Dominants were larger than subordinates after 1 day rearing, whereas dominants were smaller after 1 week rearing. From long-term rearing experiments using individual otolith marking, larvae that showed the ‘J-posture’ more frequently tended to become dominants after metamorphosis, indicating a positive correlation between the ‘J-posture’ and aggressive behaviour. Synthesizing all results from behavioural experiments, we generated a behavioural model for the triggering mechanism of aggressive behaviour and size selection of school members.

Competition for space and food in rainbow trout, Oncorhynchus mykiss, as related to ascorbic acid status

Abstract Whether nutrition affects behavior, and thus survival potential of hatchery trout, is largely unknown. Vitamin C has been shown to affect behavior in marine fish, prompting us to investigate a possible role of this vitamin in behavior in rainbow trout. Two groups of rainbow trout were maintained on either 0 or 450 mg/kg dietary vitamin C for 4 months during which the average weight increased from 13.1±0.1 to 107±4.9 g. At the end of this period, liver vitamin C levels were significantly higher in fish fed the vitamin supplemented diet (93.9±17.2 μg/g) vs. the vitamin C-free diet (1.9±1.1 μg/g). No effects on growth or mortality were observed, nor were external signs of scurvy visible. In six behavioral trials, three to four fish of each diet (average weight 89.2±4.3 g) were combined in a single 280 l aquarium. Fish were video-taped to determine their vertical positioning in the tank both after a 3-h acclimation period and immediately after a subsequent feeding. Fish positions were divided into five zones: resting on bottom (1), lower 3rd (2), mid 3rd (3), upper 3rd (4) or breaking the surface (5). Fish fed the vitamin C supplemented diet showed the highest preference for zones 1 and 2, whereas, fish fed the unsupplemented diet were found mostly in zones 1–3. Significantly less fish fed the vitamin C supplemented diet were found in zone 3 than unsupplemented fish. Fish holding the position closest to the surface before feeding in each trial did not respond to the addition of feed. These findings led us to conclude that: (a) the positions in the tank closest to the surface were occupied by submissive individuals that lacked a normal feeding response, and (b) vitamin C supplementation enhanced the competitiveness of trout, expressed as a better ability to maintain preferential tank positions near the bottom.

Distribution of Leptocephali and Other Fish Larvae around the Tanegashima Island, with Special Reference to the Coastal Migration of Anguilla japonica

種子島沿岸での、1991年11月(広島大学豊潮丸)および12月(東京大学淡青丸)の採集により、計242尾のウナギ目レプトケファルス幼生と計5520尾の他目の仔稚魚が採集された。ニホンウナギの幼生は採集されなかった。採集されたレプトケファレス幼生の54.5%をギンアナゴ Gnathophis nystromi nystremi が占めた。ギンアナゴ幼生は種子島の西岸および南岸の水深100-10mの測点で、水深や昼夜を間わず採集された。ギンアナゴ幼生、および他目の仔稚魚の中で優先したネズミギス Gonorynchus abbreviatus・ソコイワシ Bathyltagus ochotensis・ハダカイワシ科 Myctophidae spp. は、いずれも通常は沖合い水域に出現する魚種である。当該海域の水温・塩分濃度は表層から底層まで極めて均一であり、しかも同じ時期の黒潮の水温・塩分濃度とよく一致した。以上より、種子島の西岸と南岸には黒潮水が直接流れ込むことが明らかとなった。沖合い性の仔稚魚は黒潮によって運ばれ、種子島沿岸に達するものと推察される。同時に行なわれた、種子島西海岸長浜なぎさにおける採集では多数のニホンウナギのシラスが採集された。