Spatial Distribution and Growth Variability of Juveniles of Two Myctophid Species (Myctophum asperum and Symbolophorus evermanni) in Relation to Environmental Factors in the Kuroshio Current System in Winter

Abstract

The distribution and growth rate of juveniles of two myctophid species (Myctophum asperum and Symbolophorus evermanni) were examined in relation to environmental factors, based on samples collected in the Kuroshio Current system during winter from 2020 to 2022. The growth rate of juveniles was compared among three oceanic regions: the inshore side of the Kuroshio axis, the Kuroshio axis, and the offshore side of the Kuroshio axis in relation to sea surface temperature (SST), sea surface salinity (SSS), chlorophyll-a concentration (CHL), and biomass of zooplankton (ZPB). M. asperum juveniles were distributed mainly in the inshore region and the Kuroshio axis of higher CHL and ZPB, whereas S. evermanni juveniles were distributed mainly in the Kuroshio axis and in the offshore regions of higher SST and SSS. No significant difference in growth rate was found among the three regions for M. asperum juveniles, whereas the growth rate of S. evermanni juveniles in the Kuroshio axis was higher than in the offshore regions. No environmental factors were selected for growth rate of M. asperum in the GLMM, whereas SST was selected for S. evermanni. M. asperum juveniles can be considered to have a higher tolerance for variability in environmental factors, whereas water temperature could be the driver for the distribution and growth rate of S. evermanni. Lastly, we discuss possible interspecific interactions among noncommercial fish (myctophids) and commercial fish (Pacific saury Cololabis saira).