[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-P07] Open ocean environmental DNA revealed environment dependent distribution pattern of small pelagic fish in the Kuroshio

*Zeshu Yu^1,2, Shin-ichi Ito¹, Marty Kwok-Shing Wong^1,3, Jun Inoue¹, Sk. Istiaque Ahemd¹, Tomihiko Higuchi¹, Susumu Hyodo¹, Sachihiko Itoh¹, Kosei Komatsu¹, Hiroaki Saito¹ (1. Atmosphere and Ocean Research Institute, The University of Tokyo, 2.The University of Tokyo, 3.Toho University)

Keywords:environmental DNA, fish distribution, Kuroshio, small pelagic fish

Environmental DNA (eDNA) is increasingly used to non-invasively monitor aquatic animals in freshwater and coastal areas. Since full investigation of marine fish distributions is difficult due to huge volume of the ocean in addition to stormy weather, researchers also began to use eDNA methods for marine fish. This research applied eDNA methods to detect small pelagic fish which widely distribute in the open ocean and play critical roles in fisheries and ecosystems. The Kuroshio Current system was chosen as the testbed because it is known to influence spawning and recruitment of the small pelagic fish. We chose four small pelagic fish species (Sardinops melanostictus, Engraulis japonicus, Scomber japonicus, and Scomber australasicus) as the target fish, because their large resource fluctuation and that they are known to be influenced by the strong frontal structure.
To obtain the occurrence and density of target fish, OceanDNA method was performed on seawater samples collected from three cruises (KS-18-5, KS-21-8, KS-21-11) by Shinsei-Maru and KH-20-9 cruise by Hakuho-Maru in the Kuroshio Current system. Water samples were collected from 0 to 300 m in KS-18-5 (May, 2018); from 0 to 200 m in KS-21-8 (May, 2021) and KS-21-11 (June, 2021); from 5 to150 m in KH-20-9 (September-October). 0 m water samples were collected by a bucket and all other water samples were collected through Niskin bottles combined with Conductivity Temperature Depth (CTD) instrument. Water samples were filtered by Sterivex DNA filter (0.22 or 0.45μm) and the filters were stored at -30 °C, then DNA was extracted from filter after the cruise. The environment information was recorded by CTD instrument.
We examined the relationships between occurrence or density (eDNA quantity) of target fish and temperature, temperature gradient, salinity, salinity gradient, chlorophyll-a concentration, oxygen concentration by generalized additive models (GAMs). For Scomber japonicus and Scomber australasicus, we further examined the effects of prey fish using GAMs. Temperature is found to be an important factor, especially for Sardinops melanostictus and Engraulis japonicus. On the other hand, Scomber japonicus and Scomber australasicus were more common where more prey fish (especially Engraulis japonicus) existed. We also examined the influence of the Kuroshio axis on target fish distribution using the samples collected in KH-20-9 and KS-21-11 that crossed the Kuroshio. As for the result, four target fish can be observed in areas on the near shore side of the Kuroshio or around Kuroshio axis while hardly ever on the offshore side. In summary, we found the distribution of four target fish here seems to be largely determined by environmental factors like temperature and Kuroshio current, and showed the possible important influence of prey fish on the small pelagic fish distribution for the first time.