Species-specific detection and quantification of environmental DNA from marine fishes in the Baltic Sea

Biomass assesment of fish stocks is a difficult task that often involves costly fisheries trawl surveys. Trawl surveys rarely return replicate samples, as trawling attempts are expensive and difficult to reproduce. Furthermore, traditional benthic trawling is often detrimental to habitats and the organisms associated with the sea bottom. The rapidly developing field of environmental DNA (eDNA) analysis offers a new approach to non-invasive monitoring of fish. In the present study, we develop and test species-specific primers and probes for qPCR detection of eDNA from Atlantic herring (Clupea harengus), Atlantic cod (Gadus morhua), European flounder (Platichthys flesus), European plaice (Pleuronectes platessa), and Atlantic mackerel (Scomber scombrus) in the Baltic Sea. A recently published qPCR system was applied for European eel (Anguilla anguilla). Filtered water samples were collected during a stratified benthic trawl survey, enabling parallel comparisons of eDNA concentrations with biomass caught by trawling. No significant correlation was found between eDNA concentrations and the biomass of fish caught by the trawl, although an association was observed between the measured concentrations of eDNA and the known distributions and main abundances of cod, herring, plaice and flounder. This indicates that while eDNA concentrations may not be directly comparable to results from existing methods - likely because aquatic eDNA concentrations are not controlled by the same combinations of factors as e.g. trawl biomass catch - eDNA analysis could prove a useful supplement for monitoring fish stocks in the future.