Rebuilding Ecosystem Resilience: assessment of management options to minimise formation of ‘barrens’ habitat by the long-spined sea urchin (Centrostephanus rodgersii) in Tasmania
University of Tasmania (UTAS)
There is clearly potential for C. rodgersii barrens to cover ~50% of nearshore reefs on the east coast of Tasmania, as is already the case in NSW and the Kent Group in Bass Strait. This would reduce both the Tasmanian abalone and rock lobster fisheries by ~15%, with a loss of value totalling ~$25M (before processing). The need for a management response is self evident. Large rock lobsters (=135 mm CL) are the key predators of C. rodgersii in Tasmania, and experiments have shown clearly they can prevent sea urchin populations from building to the point where overgrazing occurs. There is urgent need to assess the viability of controlling C. rodgersii populations through changing current management of the rock lobster fishery, and through targeted removal by divers as a tactical response on small scales. However, before management instruments are invoked in an attempt to minimise the risk of further development of barrens habitat or rehabilitate existing barrens, it is imperative to carefully evaluate the effectiveness of potential management strategies. The proposed research will provide the necessary information and knowledge base to enable robust management decisions. The proposed work has strong support from managers and the fishing industry in Tasmania, is acknowledged as a high priority by the relevant RAGs, and addresses several high priorities on both the State and TAFI strategic research plans.
1. To assess the effectiveness of translocating large rock lobsters (=135 mm CL) en masse as means of preventing formation of incipient barrens and rehabilitating incipient and extensive barrens.
2. To assess the effectiveness of a range of management options (e.g. imposing upper size limits and spatial management) in building the biomass of large (=135 mm CL) rock lobsters to levels sufficient to limit C. rodgersii populations.
3. To assess the effectiveness of killing or removing sea urchins by abalone divers during the conduct of their fishing activity, as a means of preventing formation of incipient barrens and rehabilitating incipient and extensive barrens.
By overgrazing seaweeds and sessile invertebrates, essentially back to bare rock, the advent of the long‐spined sea urchin Centrostephanus rodgersii in eastern Tasmanian waters poses a significant threat to the integrity, productivity and biodiversity of shallow (<40 m) rocky reef systems and the valuable fisheries (principally abalone and rock lobster) that they support. The present research examined means of managing this threat at small, medium and large spatial scales.
Divers have the opportunity to limit C. rodgersii densities at local scales by culling or harvesting to prevent formation or expansion of urchins ‘barrens’ habitat at incipient stages when barrens occur as small patches in seaweed beds. To ensure sufficient time for seaweed recovery in cleared patches, local control in this way requires that sea urchins show a high fidelity to their particular incipient barrens patch so that once a patch is cleared of sea urchins there is little likelihood of it being quickly recolonised by other individuals from nearby patches. We found that on all types of barrens habitat C. rodgersii is highly nocturnal in behaviour, and has a strong tendency to return to its home crevice at the end of each night. Individuals in incipient barrens patches show strong fidelity to their patch over periods of several months, with little tendency to cross the boundary between barrens and seaweed cover, such that mean net movement in small patches is less than 1 m in 3 months. Accordingly, there is little tendency to migrate among patches, which is explained in part by laboratory experiments indicating that C. rodgersii lacks a directional chemosensory response to either macroalgae or conspecifics. Thus, urchin behaviour suggests that localised culling is likely to be effective in rehabilitating existing incipient barrens patches and reducing risk of further patches forming.
However, this outcome is unlikely to be achieved by the activity of professional divers culling urchins while fishing for abalone. Our trials indicate that abalone divers are motivated primarily by catching abalone. Thus, while they can be effective at culling urchins from the individual incipient barrens patches they encounter so that seaweeds recover in these particular patches, the number of patches they are able to visit while fishing through an area is small so that the overall effect of their culling activity within the area that they fish is not detectable except at the scale of individual patches visited. Given typical revisitation times to fish in a given area, divers culling urchins while fishing abalone are unlikely to provide meaningful local control of urchin populations. In this context, systematic and targeted harvesting of urchins as an independent industry, or killing urchins with quicklime or by deploying divers whose sole task is to cull urchins, is likely to be much more effective (but at added cost).
Abalone divers culling C. rodgersii while fishing can be successful in helping to regenerate seaweed cover on particular targeted barrens patches, but this is unlikely to have any significant effect in controlling urchins at the level of dive sites or reefs. Abalone divers should be encouraged to cull C. rodgersii while fishing.
Keywords: Sea urchin, Centrostephanus rodgersii, rock lobster, Jasus edwardsii, abalone, Haliotis rubra, sea urchin barrens habitat, ecosystem based management, modelling, stock rebuilding, maximum economic yield.