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The vertical distribution of juvenile skipjack tuna Katsuwonus pelamis was investigated in the tropical western Pacific (0-25°N and 130-160°E) based on midwater trawl sampling from October to December in 1992 to 1996. Most juveniles were sampled between depths of 40 and 120 m, that is, at depths ranging immediately above and below the thermocline, and at temperatures between 20 and 30°C. Relatively lower temperatures were observed in the pelagic zone of the research area from 1992 to 1994 (period of a shallow thermocline), in contrast to relatively higher temperatures from 1995 to 1996 (with a deep thermocline). The vertical distribution of skipjack juveniles became shallower from 1992 to 1993, whereas it became deeper in 1995 and 1996. These findings suggest that the vertical distribution of skipjack tuna during the juvenile period changed annually relative to the vertical temperature profile. Moreover, fluctuations in vertical temperature are believed to affect the expansion or contraction of the vertical habitat of skipjack juveniles in the pelagic zone. The mean standard length of juveniles collected in 1994 at a depth of 80-100 m in the North Equatorial Counter Current area was significantly larger than that of juveniles collected at 40-60 m. These findings suggest that the vertical distribution of juvenile skipjack tuna becomes deeper in line with their growth. Discipline: Fisheries Additional key words: annual change, habitat, North Equatorial Current, North Equatorial Counter Current, thermocline Present address: 5 Fisheries Management and Oceanography Division, Seikai National Fisheries Research Institute, Japan Fisheries Research and Education Agency (Nagasaki, Nagasaki 851-2213, Japan) *Corresponding author: e-mail katsuwo@affrc.go.jp Received 22 February 2016; accepted 29 June 2016. Introduction The pelagic zone of the tropical Pacific Ocean is known as the main spawning ground of skipjack tuna Katsuwonus pelamis (Matsumoto et al. 1984). The region also ranks as the world’s largest fishing ground of skipjack stock (FAO 2014). Distribution patterns and recruitment levels of skipjack tuna in the region fluctuate with large environmental changes, such as El Niño and La Niña (Bertignac et al. 1998, Lehodey 2001, Lehodey et al. 1997, Lehodey et al. 1998, Lehodey et al. 2003). To explore this phenomenon, research must be conducted on the early life ecology of this species. Previous studies have reported that tuna larvae no longer than 10 mm (and less than 10 days old after hatching) are mainly distributed in the epipelagic zone of the depth above 60 m, where the sea surface temperature is higher than 24°C (Strasburg 1960, Ueyanagi 1969). However, very little is known about the vertical and

Vertical Distribution of Juvenile Skipjack Tuna <i>Katsuwonus pelamis</i> in The Tropical Western Pacific Ocean