Spatio-temporal distribution of Benthosema pterotum larvae in the shelf region of the southern East China Sea in summer

Abstract

Benthosema pterotum is one of the most abundant myctophids on the continental shelf of the East China Sea (ECS) in summer. It is a key species in food web in the ECS because they feed on some copepods and also are one of the important foods for some economical species. In this study, biological samples and hydrological eniveronment data were simultaneously collected on nine cruises by ORI net onboard R/V Ocean Researcher 1 and 5 from 2006 to 2017 in summer on the continental shelf of the southern ECS. In addition, some biological samples were collected by mid-water trawl in I-Lan Bay in March to June 2016 to 2017. This study was tried to investigate the spatial distribution of B. pterotum larvae associated with the environmental factors on the continental shelf of the southern ECS. Stable isotope analysis and stomach contents were also used to analyze the prey composition and estimate the trophic position of B. pterotum. Generalized additive models (GAM) and habitat suitability index (HSI) were further used to evaluate potential habitat of B. pterotum. The annual hydrography in the southern ECS varied differently and possibly influenced by the exchange and/or interaction of the Kuroshio Current (KC), Taiwan Strait Current (TWC), Yellow Sea and East China Sea Mixed Water (YESMW), Changjiang Diluted Water (CDW) from 2006 to 2017. A total of 6,690 larvae of B. pterotum were collected during the 9 cruises. The densities were varied from 0.88 to 2488.19 ind./1000m3 by year. The total density in the developmental stage of flexion larvae was abundant, but less in juvenile. According to the HSI results, the preflexion and flexion larvae were mainly observed in the north of Taiwan and the center shelf region of the ECS, the postflexion larvae and juvenile were distributed in the south of Changjiang River mouth and coastal waters. Salinity, temperature, biomass of zooplankton, and chlorophyll a concentration were first four significant factors to affect the distribution of B. pterotum larval on the continental shelf of the ECS by the GAM results. The distribution of B. pterotum will move to the waters with low temperature and salinity, as it was grow up gradually. The stomach contents analysis suggest Calanoa was the major prey in the southern ESC and I-Lan Bay, even it their trophic position using the stable isotope analysis were 3.2-3.8 and 3.7-3.9 and showed significantly different between the southern ESC and I-Lan Bay. Finally, the trophic position of B. pterotum was changed with growth. It suggested that B. pterotum might be changed their habitat for feeding during the summer period.