Interspecific allometric scaling in eDNA production among northwestern Atlantic bony fishes reflects physiological allometric scaling

Relating environmental DNA (eDNA) signal strength to organism abundance requires a fundamental understanding of eDNA production. A number of studies have demonstrated that eDNA production may scale allometrically-that is, larger organisms tend to exhibit lower mass-specific eDNA production rates, likely due to allometric scaling in key processes related to eDNA production (e.g., surface area, excretion/egestion). While most previous studies have examined intraspecific allometry, physiological rates and organism surface area also scale allometrically across species. We therefore hypothesize that eDNA production will similarly exhibit interspecific allometric scaling. To evaluate this hypothesis, we reanalyzed previously published eDNA data from Stoeckle et al. (ICES Journal of Marine Science, 78(1), 293-304, 2021) which compared metabarcoding read count to organism count and biomass data obtained from trawl surveys off the New Jersey coast. Using a Bayesian model, we empirically estimated the value of the allometric scaling coefficient ("b") for Northwestern Atlantic bony fishes to be 0.77 (credible interval = 0.64-0.92), although our model failed to converge for Chondrichthyan species. We found that integrating allometry significantly improved correlations between organism abundance and metabarcoding read count relative to traditional metrics of abundance (density and biomass) for bony fishes. Although substantial unexplained variation remains in the relationship between read count and organism abundance, our study provides evidence that eDNA production may scale allometrically across species in some contexts. Future studies investigating the relationship between eDNA signal strength and metrics of fish abundance could potentially be improved by accounting for allometry; to this end, we developed an online tool that can facilitate the integration of allometry in eDNA/abundance relationships.