Spatiotemporal variability in the occurrence of juvenile Japanese jack mackerel Trachurus japonicus along coastal areas of the Kuroshio Current

Abstract

To understand the population structure of the Japanese jack mackerel Trachurus japonicus in coastal areas adjacent to the Kuroshio Current (referred to as the “CAK”), we analyzed size composition and commercial landing data of juvenile fish in these areas for the period 2005–2015. Trachurus japonicus does not undergo population‐scale spawning migration, and thus, the connectivity between the spawning and juvenile/adult habitat areas is important. Therefore, our primary aim was to assess the origin of juveniles landed in a number of subareas, including those spawned in local spawning grounds in January–May in the western part of the CAK (w‐CAK), those spawned in May–July in the eastern part (e‐CAK), and those spawned in February–March in the remote spawning ground in the southern East China Sea (s‐ECS). Fishing periods starting in spring (spring onset) were commonly observed in the CAK, which involved relatively small size classes (50–100 mm fork length [FL]). Back estimates based on the growth rate of T. japonicus suggested that the contributions from the s‐ECS probably dominated most of the spring onsets in April–June because the smallest size class (50–70 mm FL) occurred almost exclusively in April–May. In autumn, onset signals were associated with the landing of juveniles from the local spawning ground in an eastern subarea of the e‐CAK. Despite the asymmetric transport and migration flows between the habitat areas of T. japonicus, its population levels may be sustained because the local and remote spawning grounds are used in different seasons.

Development of species‐specific multiplex real‐time PCR assays for tracing the small pelagic fishes of North Pacific with environmental DNA

“…For example, a major spawning ground of Japanese jack mackerel was found offshore of northeast Taiwan in the East China Sea during winter to spring (Sassa et al, 2006), and it was thought that juvenile mackerel migrated from the spawning ground to the coastal areas of the Sea of Japan and the Pacific. However, there are also coastal spawning subpopulations in the Sea of Japan and Pacific; thus, the ratio between the locally spawned subpopulation and migrant subpopulation is unclear (Ishikawa et al, 2021). As our group is currently sampling the eDNA samples of the migration routes of small pelagic fishes including jack mackerel, the developed qPCR assays will allow us to better resolve the migratory behavior of the subpopulations.…”

Effect of interannual variations of Kuroshio–Tsushima Warm Current system on the transportation of juvenile Japanese jack mackerel (Trachurus japonicus) to the Pacific coast of Japan

“…The annual landing in the East China Sea and the Sea of Japan is approximately five times larger than that in the Pacific coastal region (Yasuda et al, 2021; Yoda et al, 2021). T. japonicus has many spawning grounds in the southern East China Sea and the Japanese coastal regions (Ishikawa et al, 2021; Sassa et al, 2006; Takahashi et al, 2022). The major spawning regions are located off northeastern Taiwan in the southern East China Sea (in and around the rectangle region indicated by character N in Figure 1a,b) and of western Kyushu, especially around Goto Islands (Figure 1b) (Sassa et al, 2009).…”

“…Since the individual number of age‐0 T. japonicus also decreased (Figure 2b), T. japonicus recruitment has been considered to decline after 2000 (Yasuda et al, 2021). Within the region along the Pacific coast where drifted larvae arrive from the major spawning regions of the southern East China Sea, T. japonicus was interpreted to have sources from several spawning grounds (Ishikawa et al, 2021; Xie & Watanabe, 2007). This fact means that interannual variability in the abundance of age‐0 T. japonicus in the Pacific coastal waters of Japan results from the interaction between recruitment variability in the local spawning population and the population obtained from larval dispersal processes via Kuroshio from the southern East China Sea (e.g., Ishikawa et al, 2021).…”

“…Within the region along the Pacific coast where drifted larvae arrive from the major spawning regions of the southern East China Sea, T. japonicus was interpreted to have sources from several spawning grounds (Ishikawa et al, 2021; Xie & Watanabe, 2007). This fact means that interannual variability in the abundance of age‐0 T. japonicus in the Pacific coastal waters of Japan results from the interaction between recruitment variability in the local spawning population and the population obtained from larval dispersal processes via Kuroshio from the southern East China Sea (e.g., Ishikawa et al, 2021). Therefore, the interannual change in the larval transportation of T. japonicus from the southern East China Sea could cause its recruitment to decrease.…”

“…Larvae of T. japonicus in the waters of western Kyushu usually spawn in April and are believed to recruit into the Sea of Japan (Figure 1a) (Sassa et al, 2016;Takahashi et al, 2012Takahashi et al, , 2022. In comparison, those spawned from February to March in the southern East China Sea recruit into the East China Sea and the Pacific coastal waters of western Japan (Ishikawa et al, 2021;Kasai et al, 2008;Sassa et al, 2016).…”