Remediating kelp habitat destruction to restore ecosystem services by curbing urchin proliferation in southeastern...

Shallow, rocky reef habitats are key components of temperate marine ecosystems, providing shelter, feeding and breeding habitat for a wide variety of organisms. In these ecosystems, urchins occur alongside other benthic species, such as abalone, competing for food and space. As active grazers, urchins are habitat engineers, whereas gastropods like abalone are habitat responders. Increasing abundance of the long-spined urchin (Centrostephanus rodgersii), in eastern Victoria, on Australia's Great Southern Reef, under the influence of the warm East Australian Current, has led to overgrazing of macroalgae, deforesting kelp canopies to form urchin barrens with greatly reduced biodiversity. This in turn has weakened the resilience of abalone populations and limited productivity from commercial stocks. A collaborative government-industry reef restoration strategy was implemented in response through an urchin removal trial at Gunshot Reef in eastern Victoria. Divers removed urchins from vegetated areas, clearing them up to 100 m from the kelp/barren junction in all directions, resulting in greatly reduced urchin density. Video-transect surveys before and after urchin removal assessed the effectiveness of reducing urchin density and promoting subsequent habitat recovery. Spatiotemporal patterns in divers' effort were evaluated by geospatial analysis of vessel-diver GPS tracking. Findings indicated positive signs of site-scale habitat recovery progression over ∼2.5 years post-treatment, with increased macro, turfing, and coralline algae. The cost of urchin removal, in 2019-2020, was AUD$9385 Ha^-1 suggesting further work to evaluate the cost-benefit trade-offs among alternative remediation approaches including maintenance harvesting and broader potential for targeted urchin control, although regional applicability requires further evaluation.