Optimizing the sampling effort for detecting fish environmental DNA in the open ocean

Environmental DNA (eDNA) metabarcoding is a promising method for fish biodiversity monitoring in various environments. Especially in the open ocean, where the conventional method like net sampling is hardly conducted, adopting the eDNA approach is expected to provide new insight into hidden fish biodiversity and contribute to evaluating the effect of ongoing global climate change on the ocean ecosystem. However, considering the scarce distribution of fish eDNA in the open ocean, eDNA analysis is prone to underestimate fish species richness due to false-negative errors unless the sampling protocol is appropriately designed. Therefore, this study aims to evaluate how the filtration volume and filter pore size affect the detection of fish species using eDNA metabarcoding. To achieve this aim, we performed replicated eDNA sampling in sites from the subtropic and subarctic Northwestern Pacific Ocean and the Arctic Chukchi Sea during the R/V Mirai (Japan Agency for Marine-Earth Science and Technology) Arctic expedition held from August to October 2020. Subsequently, we evaluated the effect of the sampling protocol on fish eDNA metabarcoding. This study will facilitate establishing the appropriate sampling strategy for describing the biodiversity and ecology of fish in the open ocean using eDNA.