Sustainable overfishing of longspined sea urchins to protect key abalone habitat

The climate-driven range-extension and population explosion of the Longspined Sea Urchin, Centrostephanus rodgersii, is degrading key abalone habitat across south-eastern Australia. Off the island state of Tasmania, an increase from zero to 20 million urchins within four decades has resulted in 15% of the eastern coast becoming unproductive urchin barren, with predictions barrens could increase to more than 50%. Sustainable overfishing of the urchin has become a key management objective to prevent reef destruction and substantial negative impact on abalone populations. Harvest subsidies have facilitated the urchin fishery to rapidly increase to 500 t pa representing a harvest fraction of 6.5% within the fishable range. Urchin abundances are in decline in three of nine regions where harvest fractions are up to 10%. A size structured stock assessment model has shown densities would be almost doubled in key regions if no commercial urchin fishing had occurred. For each region, we explore projections of harvest intensity and filter this by potential abalone productivity to provide a framework for possible management goals, indicating regions where an ecological density target could be obtained through fishing, or regions where intervention would likely be unnecessary depending on the goal. Carefully management of the urchin fishery is required into the future, as a collapse from overfishing would result in processor shutdown and termination of the key control mechanism. Harvest strategies need to facilitate sustainable overfishing; fishing urchin populations beyond MSY but not to levels where fisheries collapse.