Detection and quantification of two commercial flatfishes (Solea solea and Pleuronectes platessa) in the North Sea using environmental DNA

Sustainable fisheries management requires regular scientific monitoring of fish stocks. When information on certain fish stocks is limited, environmental DNA (eDNA) holds promise to complement traditional monitoring surveys. However, a better understanding of how eDNA concentrations relate to fish abundance and biomass is needed. Here, eDNA quantification of two commercially important flatfish species in the North-East Atlantic, common sole (Solea solea) and European plaice (Pleuronectes platessa), was assessed. First, species-specific, probe-based assays for plaice and sole targeting the mitochondrial cytochrome b and cytochrome c oxidase subunit I gene, respectively, were developed (for sole) and validated (for both species). Subsequently, two mesocosm experiments revealed a significant and positive relationship between both abundance and biomass and eDNA concentrations for both species at three eDNA emission time periods (5 min, 1 h, and 24 h). Larger plaice shed significantly more eDNA (copies L-1) than smaller conspecifics. Finally, eDNA was obtained from seawater collected during research surveys in the Belgian part of the North Sea in spring 2020 (i.e., local scale) and the southwestern North Sea in autumn 2020 and 2021 (i.e., regional scale). eDNA concentrations were compared to the observed abundance (individuals per km(2)) and fish density in terms of biomass (kg per km(2)) as observed in the trawl at the same station. Local eDNA concentrations of both sole and plaice were positively correlated with observed abundance and fish density. The correlation between regional eDNA concentrations and fish density was positive and significant for sole in 2020 and 2021 and for plaice in 2020, but not in 2021. The correlation between regional eDNA concentrations and observed abundance was positive and significant for sole and plaice in 2020, but not in 2021. These results illustrate the potential of eDNA to estimate abundance and biomass parameters for stock assessments of flatfishes in the North Sea.