Does Aqueous Edna Concentration Indicate Fish Density? Intrinsic and Methodological Considerations

Interest in aqueous macrobial eDNA is high because it can indicate the occurrence and abundance of aquatic species. The hypothesized positive correlation between eDNA concentration and organism density has been demonstrated by two studies to date: one in controlled experiments and one in both controlled and natural experiments. To further test this hypothesis we used species-specific qPCR to measure fish eDNA concentration across gradients of fish density in experimental ponds and natural water bodies. To investigate the properties and processes of eDNA we also measured the size distribution of eDNA-containing particles and quantified eDNA in sediments. Our results illustrate how the property of particle size and the processes of settling and re-suspension can affect the path from organism density to aqueous eDNA concentration. Methodological issues including particle capture yield and PCR inhibition are also addressed. These findings allowed us to test our hypothesis more effectively and further described the sometimes complex fate of eDNA-containing material. We show that aqueous eDNA concentration can indicate fish density in semi-natural and natural habitats. We also suggest a generally optimal strategy for capturing, extracting, and quantifying aqueous macrobial eDNA.