The effective management of fisheries resources implicitly requires accurate abundance estimates. But, even after more than a century of effort, this prerequisite need remains one of the greatest challenges facing fisheries scientists. Traditional methods of estimating abundance include parametric and empirical approaches that often rely on extensive time-series of size-at-age data (e.g. from otoliths). Acquiring these data can be expensive and therefore restricted to species that are most economically important or of conservation concern. Due to the cost of independent sampling, indicators of abundance usually involve fishery-dependent data, which have numerous biases that can affect accurately quantifying population trends.
The clear need for accurate and rapid stock assessments is increasingly apparent in the current climate of competing commercial, recreational and conservation interests. Specifically, marine protected areas have been implemented across all NSW coastal bioregions and are a source of political and public contention as a result of perceived inequalities in access to (and allocation of) fisheries resources, particularly among species shared by commercial and recreational fishers. Also, recent fish kills in NSW due to hypoxic water have sparked debate over the cause of poor water quality and longer-term effects on freshwater habitats. Rapidly quantifying existing and/or changing stock abundances of key species will be essential to inform recovery actions, and advise on the sustainable exploitation of commercial and recreational species. Sampling eDNA could meet these needs by directly facilitating cost-effective biomass and/or relative abundance estimates for population assessments in NSW where fishery-dependent data are scarce or unavailable.
Final report
The literature review identified 63 relevant papers investigating relationships between eDNA and biomass and/or abundance, with biases towards salmonids and cyprinids, and mostly from the United States and Japan. These studies predominantly utilised species-specific detection methods and reported positive correlations for various fish species in the field (60% of publications), controlled environments (25%) or a combination of both (14%). However, the review also highlighted uncertainties regarding absolute biomass or abundance estimates due to environmental factors affecting DNA dynamics in freshwater and marine ecosystems—emphasising the need for controlled pilot studies to address these limitations. Consequently, the design of phase-2a aquaria experiments assessed the importance of key factors such as water temperature on eDNA concentrations.
During the phase-2a aquaria work, species-specific assays were developed for Murray Cod, Mulloway, and Golden Perch. The results indicated there was a significant quadratic relationship between biomass and eDNA concentration for Murray Cod, whereby eDNA increased in tanks containing up to 180 g of Murray Cod, but then decreased as Murray Cod weights exceeded 200 g. This outcome was attributed to excessive variability in DNA shedding rates among individuals and possibly exacerbated by a species-specific response to confinement. In contrast, promising findings were observed for Golden Perch and Mulloway, with significant positive relationships between eDNA concentrations and biomass, despite some variability in shedding rates among the latter species.
Phase-2b field trials further validated the potential of eDNA as an indicator of fish biomass, and in some cases, abundance. Notably, experiments with Golden Perch revealed a useful significant positive quadratic relationship between eDNA concentration and biomass, while the Mulloway trials showed a significant positive relationship between eDNA concentration and biomass and an almost significant positive relationship with abundance, but (for both) only during autumn. Bony Bream also had useful positive linear relationships between their eDNA concentration and biomass and abundance and a significant negative relationship between eDNA and flow. Similarly, and despite few attributable impacts detected in aquaria, in their natural environment Murray Cod biomass (but not abundance) was characterised by a significant (but weaker) positive linear relationship with eDNA concentration, as well as a significant but weak negative relationship with flow.
