Ontogenetic habitat differences in Benthosema pterotum during summer in the shelf region of the southern East China Sea

Abstract

Benthosema pterotum is one of the most abundant myctophids and an important species link in the food web between producers and consumers in the continental shelf of the southern East China Sea (ECS). This study used R/V Ocean Researcher 1 and 5 to investigate the spatial distribution of B. pterotum larvae and the associated environmental factors affecting the continental shelf of the southern ECS between 2006 and 2017. A generalized additive model (GAM) was used to evaluate potential habitats of B. pterotum. In total, 6690 B. pterotum larvae were collected during nine cruises. The density varied from 0.88 to 2488.19 individuals per 1000 m3 (ind. 1000 m−3) during the study period. The median density was highest in June 2009 (65.68 ind. 1000 m−3) and lowest in July 2010 (5.78 ind. 1000 m−3); it was highest at 18.42 ind. 1000 m−3 in flexion larvae but lowest at 7.01 ind. 1000 m−3 juveniles. The density of B. pterotum in juveniles did not significantly differ according to different sampling depths and between daytime and nighttime. These results suggest that diel vertical migration commences in the juvenile stage. The GAM results indicated that influential variables, namely salinity, temperature, zooplankton biomass, and chlorophyll a concentration affected B. pterotum larval distribution on the continental shelf of the southern ECS. Benthosema pterotum in the preflexion and flexion stages inhabit optimum temperatures ranging from 28.2 to 28.8 °C and salinity value ranges of 32–33.6 in the waters of the south and central shelf region of the southern ECS (25–29° N, 121–125° E). They then moved to relatively low temperature and low salinity areas in the waters of the north and peripheral shelf region. In addition, some juveniles moved to the south of the Changjiang River estuary, which has high primary productivity. In short, B. pterotum larvae's patterns of growth and habitat distribution might relate to a survival strategy that seeks to optimize environmental factors and food availability during summer.