The Sakasa-ami is a kind of Otoshi-ami (Set net), the entrance of net being open offshore, and the leader net (Kaki-ami) placed in the offshore of the main net. To estimate the fish behavior when they encountered the leader net, the catch by Sakasa-ami was compared with that of the ordinary type of Otoshi-ami, one is Naka-ami and the other is Oki-ami which is located offshore of the Sakasa-ami. A common inshore leader net was placed between Naka-ami and Sakasa-ami. The catch data of Autumn season of 1977 and 1979 were analysed, and the results obtained are summarized as follows:
1) When Tsubaiso Serbia quinqueradiata, young of yellow tail (less than 20cm in body length) encountered the leader net, they showed a tendency to proceed inshore or offshore, while Fukuragi larger type of same species (20-40cm), had a tendency to proceed offshore.
2) Small size of horse mackerel Trachurus japonicus (body length 10cm) showed a tendency to proceed inshore and offshore in the same ratio, but the larger type of same species (15-30 cm) had a tendency to proceed offshore.
3) Barracuda Sphyraena Pinguis seemed to proceed inshore when they encountered the leader net.

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In order to examine gross-anatomical changes of the brain components of yellow-tail with their body-growth, an observation was made on the specimens of four stages, larval, young, immature and adult. Total number of the specimens used are 102, the total length of them ranging from 10mm to 1, 100mm.
The main results obtained are summarized as follows.
1) Most remarkable changes of the following two stages.
(a) Notable growth of each of the corpus cerebella, valvula cerebelli and eminentia granularis was observed in the fish of a transitional stage of their behaviour shifted from a rather planktonic life at the sea surface to a life travelling under floating seaweeds; total length of the fish at this stage was measured to be 30-50mm.
(b) Conspicuous growth of the corpus cerebelli, valvula cerebelli, eminentia granularis and crista cerebelli was found in the fish of the other transitional their behaviour shifted from a life travelling under floating seaweeds to free-swimming life; total length of these fish was 100-150mm.
2) These results suggest that in yellow-tail there exists an intimate correlation between the behavioural modification and the morphological changes of some brain components, especially of the corpus cerebelli and valvula cerebelli.

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