Assessment of fish diversity in the coastal waters off Nodaedo Island, Tongyeong, Korea, using an underwater visual census and environmental DNA metabarcoding

Environmental DNA (eDNA) analysis is a promising method for monitoring marine biodiversity. However, it is necessary to verify whether eDNA analysis provides an accurate measure of biodiversity in species-rich ecosystems such as coastal waters as part of a long-term monitoring campaign. Therefore, both an underwater visual census (UVC) and eDNA metabarcoding were conducted monthly from June 2018 to May 2019 to investigate fish diversity off the coast of Nodaedo Island, Tongyeong, South Korea. A total of 16,036 fish were observed in the UVC and classified into 69 species. Perciformes was the most dominant group with 31 species. eDNA metabarcoding identified 68 operational taxonomic units and 738,217 reads, encompassing 18 orders, 42 families, and 68 species. The Gobiidae family was the most dominant taxon with five species. The UVC was able to detect species that were otherwise not identified using eDNA due to the low number of individuals or analytical limitations, while eDNA metabarcoding was suitable for detecting smaller species, pelagic species, and those that inhabit microhabitats. Our findings demonstrated that these two methods can be used to complement each other and gain insights into the structure of fish communities in various coastal environments. Though eDNA metabarcoding represents a fast and efficient method for surveying fish species diversity in coastal waters, it should be carefully compared with other traditional survey methods to ensure that it is suitable as an effective complementary measure.