A management support framework for subtidal rocky-reef communities on the east coast of Tasmania

Abstract

We developed a local and regional scale simulation model of the important dynamical features of seaweed-based rocky reef systems in eastern Tasmania. Climate-driven range extension of the spiny sea urchin ( Centrostephanus rodgersii ) from Australia’s mainland into eastern Tasmanian waters presents a major threat to the productivity of subtidal rocky-reef communities because overgrazing of native seaweed beds by this sea urchin can create and maintain extensive areas of ‘barrens’ habitat. This causes major loss of production, biodiversity and physical structure. In particular, the two most valuable fisheries in Tasmania, blacklip abalone ( Haliotis rubra ) and southern rock lobster ( Jasus edwardsii ), are not commercially viable on C. rodgersii barrens. Additionally, the fishing depletion of large rock lobsters, the main natural predator of C. rodgersii , facilitates the establishment of urchin barrens. Based upon empirical observations, field experiments and information about larval dispersal derived from an ocean circulation model, we developed a model that captures the regional dynamics of temperate reef communities on the East coast of Tasmania. Our model is hierarchically structured as a connected network of local models, which can each produce ecosystem shifts from productive kelp bed to sea urchin barrens at the scale of individual reefs. Connectivity between local reefs essentially captures large-scale dispersal of C. rodgersii larvae. Through simulations, we test the effectiveness of alternative management options (reduction in lobster fishing, temporary no-take zones as a series of ‘rolling closures’ and harvesting of the urchin) to minimise the risk of serious ecological impacts of the invasive urchin, i.e. to maintain native reef communities in their original state or restore the productive seaweed habitat from the barren state. Through model forecasts, we assess the trade-off between alternative management strategies in terms of costs and benefits for the fisheries and risks of further sea urchin barrens formation.