Distribution of walleye pollock (Theragra chalcogramma) larvae and juveniles off the northern Hokkaido coast from the Sea of Japan to the Sea of Okhotsk

Abstract

We examined the early life history of walleye pollock, Theragra chalcogramma, in the Sea of Japan off the coast of western Hokkaido and the Sea of Okhotsk. The distribution patterns of larvae and juveniles were determined by net samplings usingan FMT (Framed Midwater Trawl) net carried out from the spawning grounds to the nursery grounds every April from 2005 to 2008. Larvae and juveniles were not caught in the spawning grounds; however, 175 (1.2 inds/1000m³ in 2005), 3331 (30.3 in 2006), 18 (0.2 in 2007) and 146 (1.1 in 2008) larvae andjuveniles were caught in the areas from the north of Ishikari Bay to the Soya region in each year. This distribution pattern suggested that the larvae and juveniles were transported from the spawning grounds to the nursery grounds with the Tsushima Warm Current that flows northward. The water temperature at a depth of 50m in the areas where larvae were mainly distributed ranged from 4 to 7℃. The highest catch of larvae and juveniles was observed in the cold year (2006) with a small mean body length of 14mm, while less larvae and juveniles were caught in warmer years with a larger mean body length of 21mm. The high water temperature in the spawning grounds was suboptimal for egg development and may have resulted in the smallest catch in 2007. These size differences in April among years were thought to be caused by differences in the hatching period.