Evaluation of filter and extraction kits for eDNA-based quantification of Northern pike density

Abstract: Multi-mesh gillnets are the most common gear in Swedish fish monitoring programs. The method works for many fish species but has limitations. The popular game fish Northern pike (Esox lucius) represents one of the species where the gillnet strategy has difficulties to reliably estimate density and to accurately provide data for reliable status assessments. Environmental DNA (eDNA) could be a useful complementary tool to standard surveys if it can provide reliable estimates of pike density. The primary aim of this study was to investigate if eDNA could be used to quantify different densities of pike. This was tested in an experimental setup, using two outdoor tanks. One tank contained one pike and the second tank had eight pikes. Furthermore, another aim of the study was to investigate which filter combination and extraction kits were the best to use for the quantification of pike density. The underlying premise of quantitative eDNA is that there is a correlation between the amount of eDNA detected in the water and the actual biomass or abundance of the species. The results revealed that it was possible to separate different pike densities, regardless of filter combination and extraction kits. The experiment also revealed that adding a GF/A filter to a CN filter did not yield more compare to what a single CN filter retrieved. This facilitates future studies due to faster filtration when using one filter and not two. Moreover, the experiment showed that Chelex as an extraction method yielded the most eDNA compared to the other two kits: Blood and Tissue and PowerWater. Nevertheless, these kits can be good choices for projects that do not have time for method development. PowerWater was the only extraction kit that did not need to be diluted and could perform qualitative amplifications in the qPCR, while Blood and Tissue was slightly faster and cheaper than PowerWater. Taken together, the benefits of Chelex makes it a strong candidate method for future monitoring of pike densities. Although there are several questions to address before eDNA can practically be used for monitoring pike.