Correlating eDNA indices of relative biomass and abundance with those derived from electrofishing for two ecologically disparate Australian freshwater species

In response to increasing global interest in the utility of environmental DNA (eDNA) for non-invasively monitoring fish populations, the relationships between eDNA concentrations and electrofishing-derived relative biomasses and abundances were assessed for two ecologically distinct Australian native freshwater species: Murray cod (Maccullochella peelii) and bony herring (Nematalosa erebi). Species-specific eDNA assays targeting mitochondrial 12S rRNA were applied using quantitative PCR (qPCR) to filtered water samples collected concurrently with boat electrofishing at five sites in each of three rivers in New South Wales during autumn, 2021 and 2022. While eDNA concentrations were low, we identified significant positive correlations with relative biomass and abundance for both species, albeit strongest for bony herring, which unlike the mostly solitary Murray cod, occur in dense schools. Regardless of species, daily river discharge was negatively correlated with eDNA concentrations, possibly owing to dilution effects. The results support the feasibility of using eDNA to monitor fish populations in dynamic riverine systems, although future work is required to refine sampling protocols and better investigate the effects of various biotic and abiotic factors that might influence relationships.