Effects of water temperature and prey density on recent growth of chub mackerel Scomber japonicus larvae and juveniles along the Pacific coast of Boso-Kashimanada

The effects of temperature and prey density on the recent growth rate (RGR) of chub mackerel, Scomber japonicus, were examined. Scomber spp. larvae and juveniles were collected along the Boso-Kashimanada Pacific coast in Japan between March and July in 2015 and 2016. Larvae and juveniles identified as S. japonicus using DNA were subjected to stomach-content and otolith-increment analyses. The effects of temperature and prey density on RGR were analyzed using a generalized linear mixed model (GLMM), which showed that temperature positively affected the RGR of larvae in the body length (BL) classes<5 mm, 5-7 mm, 7-9 mm and 9-11 mm. Prey density also affected growth in the<5 mm, 5-7 mm, 7-9 mm and>11 mm classes and effects of prey density on RGR for larger (>11 mm) larvae and juveniles were stronger than those for smaller (<9 mm) larvae. S. japonicus larvae transform into juveniles at around 12 mm BL, and the increased feed demands of juveniles likely lead to prey-density-dependent growth. The otolith increment widths of larvae and juveniles were significantly narrower than those during the same stages in 1-year-old S. japonicus collected from purse seine fisheries operated in the Joban or Kashimanada areas, suggesting growth-dependent mortality.