A Net with No Holes: A novel approach to marine biodiversity assessment by combining eDNA and hydro-acoustics

Standardised monitoring of fisheries stocks is vital for their effective management. The use of hydro-acoustic sounders to identify schools of fish has been an established survey method for decades. While the 'identity' of fish species detected is indirectly inferred from the frequency response, backscatter strength, the schooling behaviour, and the location of fish in the water column, validation is typically performed by means of trawling activities. However, these are time-consuming and therefore expensive, and they require specialist equipment and can therefore not be conducted from all acoustic platforms. In addition, the resulting catch composition is often affected by species and age specific catchability which is difficult to quantify and may not representative for the species composition of the acoustically observed targets. Developing an environmental DNA (eDNA) approach could revolutionises standardised surveys with its power to verify fish species and population composition. This study investigates the potential for using eDNA as a qualitative and quantitative tool for the assessment of marine pelagic communities. To investigate these possibilities, we collected eDNA seawater samples during the PELTIC 2018 survey on the Cefas Endeavour around the South coast of the UK, from surface water during trawls, and from a CTD Rosette at pre-determined sampling stations. The PELTIC survey is used to map and quantify economically as well as ecologically important pelagic fish species including; sardine (Sardina pilchardus), sprat (Sprattus sprattus), European anchovy (Engraulis encrasicolus), Atlantic herring (Clupea harengus) and Atlantic mackerel (Scomber scombrus). DNA metabarcoding of 12S mitochondrial amplicons is being used to target these, and other data-limited fish species, and the spatial faunal composition of species will be compared to the hydro-acoustic and trawl data. The patterns obtained will be examined to achieve a better understanding of pelagic communities around the South-West of Britain and to devise improved approaches for the monitoring of marine biodiversity.