Vertical distribution of juvenile skipjack tuna Katsuwonus pelamis in the tropical western Pacific ocean

Abstract

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.