The extinction of the New Zealand grayling (Prototroctes oxyrhynchus) provides a cautionary precedent for its only congener, the Australian grayling (P. maraena), whose area of occupancy has reportedly declined by 62% since 1993. For highly dispersive amphidromous species, abundance can decline continuously while occupancy remains high, because larval dispersal continues to generate occasional recruits across river basins even as source populations erode. This decoupling means distribution-based metrics may underestimate extinction risk, and effective management requires a network-scale framework rather than basin-by-basin intervention. Building on earlier theoretical work, here we apply such a framework to Australian grayling across south-eastern Australia.
We developed a spatially explicit, time-discrete metapopulation model spanning New South Wales and Victoria, in which subbasin carrying capacity is allocated from species distribution model predictions and a coastal Gaussian dispersal kernel governs larval exchange between catchments. Barriers (dams and weirs) are incorporated as permeability penalties scaled by effective barrier density per subbasin. Using this model we conducted three management experiments: 1) ranking the demographic importance of individual subbasins by simulating their removal from the network, 2) evaluating progressive barrier removal to identify where fish passage restoration delivers the greatest population benefit, and 3) testing targeted fish stocking (an option recently made technically feasible through hatchery development) to identify where supplementation most improves network-wide persistence.
Outcomes were evaluated across multiple metrics spanning abundance, occupancy, and extinction timing. Several catchments consistently emerged as high-priority, suggesting robust targets for conservation investment. The magnitude of expected benefit was sensitive to dispersal kernel width and barrier density, reinforcing that landscape connectivity is as important as local habitat quality for long-term persistence. Future refinements incorporating oceanographic currents, and improved barrier severity data would strengthen this as a decision-support framework for prioritising conservation investment in amphidromous fish.