Poor Utility of Environmental DNA for Estimating the Biomass of a Threatened Freshwater Teleost; but Clear Direction for Future Candidate Assessments

Environmental DNA (eDNA) can potentially facilitate estimating the abundance of teleosts and might therefore be used for stock assessments. Nevertheless, relationships are species-specific—reflecting several abiotic and biotic factors that require understanding prior to ongoing research investment. Here, we investigated potential relationships between eDNA concentrations and the weights of an endemic and threatened Australian teleost, Murray cod, Maccullochella peelii to determine if eDNA could be used to estimate biomass. Three controlled aquaria experiments using juvenile fish were conducted to investigate the time to eDNA stabilisation in tanks following stocking (aquaria experiment 1), and then if water temperature (aquaria experiment 2) and/or fish weight (aquaria experiment 3) explained variability among eDNA concentrations in tanks. In aquaria experiment 1, eDNA concentrations stabilised nine days after stocking, but there were no significant effects of water temperature or fish weight in aquaria experiments 2 and 3. The latter result was verified during a mesocosm experiment using brood stock held in semi-natural outdoor ponds. During a fourth aquaria experiment (experiment 4), ten similar-sized juveniles were held under identical conditions in individual replicate tanks, with significant variability in eDNA concentrations that was not explained by individual fish weights. When all aquaria data were collectively assessed, there was a significant (but not useful) quadratic relationship between eDNA concentrations and fish weight. We conclude eDNA is not suitable for estimating the weight/biomass of Murray cod. Assessing the suitability of future candidate species might benefit from similar initial aquaria experiments under controlled conditions to test the independence of individual fish weight on variability among eDNA concentrations in tanks.