Distribution, growth and mortality of larval jack mackerel Trachurus japonicus in the southern East China Sea in relation to oceanographic conditions

Abstract

Distribution, growth and mortality of jack mackerel Trachurus japonicus larvae <10 mm standard length (SL) were investigated in relation to habitat conditions in the southern East China Sea (ECS) in 2005, 2007 and 2009. In all the years, the center of larval distribution shifted northeastward with growth, indicating northeastward entrainment by the Kuroshio branch current. In 2005, the mean habitat temperature of the larvae was ∼2°C lower than the other two years. Mean density of the major prey item Paracalanus spp. in the larval habitat varied interannually, with the highest and lowest values in 2007 and 2005, respectively. The mean absolute growth rate and weight-specific instantaneous growth coefficient (Gw) were highest in 2007 (0.30 mm d−1 and 0.217 d−1, respectively) and lowest in 2005 (0.19 mm d−1 and 0.120 d−1, respectively), corresponding with the habitat temperature and food availability. There was no significant interannual difference in the daily instantaneous mortality coefficient (Z) which ranged from 0.107 d−1 in 2009 to 0.136 d−1 in 2007. The ratio Gw/Z, an index of stage-specific larval survival, was highest (1.60) in 2007 and lowest (1.02) in 2005. The lowest Gw/Z in 2005 corresponded with the lowest recruitment of the juveniles (mainly 50–75 mm SL) in the ECS in 2005. However, Gw/Z in 2007 and 2009 did not clearly correspond with recruitment levels, suggesting that a high Gw/Z is a necessary, but may not be a sufficient, criterion of a strong year-class.