The role of circadian rhythm on the fish school of larval stage

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The process being formed a fish school was determined to correlate with the circadian rhythm of fish larvae and physical flow condition of the sea. The horizontal distribution density of the anchovy larvae was indicated statistically as the concentrated pattern in the surface layer. The pattern was changed by the vertical shear flow condition which was shown as the different flow direction in each depth. Vertical shear flow was measured two different layers, surface and a 10 m depth using 8 drogues. The drogues in the surface layer drifted toward northwest while in the 10 m layer they drifted toward south. This fact suggests that if the fish larvae migrate vertically in accord with the circadian rhythm they do not be dispersed by the flow since in night they rise to the surface and drift toward northern direction while in day time they can return toward southern direction in deeper layer. A new apparatus was developed to measure automatically the behavior of fish larvae throughout the day.