Sampling Leptocephali with Reference to the Diel Vertical Migration and the Gears

In order to determine the most efficient sampling method of Anguilla japonica leptocephali, the diel change of vertical distribution of leptocephali and sampling efficiency of gears were examined in a fixed water mass marked by drifting buoy with drag in the Western Pacific in September 1986. Forty-min horizontal tows by 3 m IKMT were conducted by night and day at 7 towing depth layers between 10 and 400 m deep. Number of leptocephali taken by night was 2.6 times larger (60 fish) than those sampled during daytime (23 fish). Most of leptocephali (85 %) of night sample were taken at shallow layers, 10, 20, 50 and 80 m deep, while no leptocephali occurred at layers shallower than 80 m by day. All of 10 leptocephali of A. japonica were also taken at shallow layers between 10 and 50 m deep by night. Upper limit of daytime sample and the layers of maximum catch by night roughly coincided with a sharp thermocline observed at 70–80 m deep. A. japonica leptocephali seem to be obtained most efficiently by a horizontal night tow at layer just up to a thermocline. Comparison of sampling efficiency between IKMT (8.7 m² mouth opening, 0.5 mm mesh) and Hexagon Net (10.4 m2 mouth, 1–8 mm mesh) showed that Hexagon Net caused a bias in sampling leptocephali smaller than 25 mm TL, while IKMT collected fewer leptocephali exceeding 40 mm TL than Hexagon Net.