Population connectivity in a highly migratory fish, Japanese Spanish mackerel (Scomberomorus niphonius), along the Chinese coast, implications from otolith chemistry

Abstract

Connectivity is a vital component of metapopulation and fishery ecology, influencing fundamental processes such as population dynamics, evolution, and community responses to climate change. The Japanese Spanish mackerel, Scomberomorus niphonius, is an economically important species in the temperate and subtropical waters of the Northwestern Pacific Ocean, especially in the China seas. As a highly migratory species, the population connectivity of S. niphonius is not only limited to dispersal in the pelagic larval stage but throughout the whole life history, and then determines the natal origin and population structure. However, knowledge about population connectivity is scarce and, at present, is no formal management strategy to ensure the long-term sustainability of the S. niphonius fishery in China. In this study, otolith chemistry was applied to investigate the connectivity among four spawning grounds along a latitudinal gradient spanning much of the known species range in the Yellow Sea and the East China Sea [Qingdao (QD), Lvsi (LS), Xiangshan (XS) and Fuzhou (FZ)]. Otoliths from age-1 spawning or spent individuals were analyzed from the core to the margin by using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). In each sampled life stage, otolith elements are similar among fish from different spawning grounds, but otolith chemical compositions vary greatly between life stages. Decreasing Ba:Ca ratio indicates ontogenetic migrations from relatively shallow coastal water during larval growth to relatively deep offshore waters with increasing age. The random forest classification and non-metric multi-dimensional scaling show a strong overlap in otolith elemental composition. The hereby results indicate a large-scale connectivity in the life history of S. niphonius: the immature fish mix to a large extent when they feed and overwinter in the extensive offshore waters; adults spawned in the same spawning ground have multiple natal origins and fish caught off QD may even be of FZ origin. Clustering of near core chemistry pointed to three sources, with southern grounds dominated by a single cluster while northern grounds dominated by two or three clusters. From these results, it was suggested that the local mackerel assemblages on which fisheries rely in the Yellow Sea are collectively influenced by the recruitment of all the spawning assemblages along China coastal waters instead of self-sustaining.