Long-term outcomes of passive and active restoration approaches following a vessel grounding in Hawaii, USA

In February 2010, the cargo vessel M/V Vogetrader ran aground on a forereef in Oahu, Hawaii. Baseline surveys documented considerable damage to coral communities. Several restoration actions were implemented in 2013, including active restoration (rubble removal, coral outplanting) and passive restoration (natural recovery), with the goal of returning corals to their pre-disturbance state. In 2022, repeated surveys were conducted across three injury zones that varied in the severity of impact and the restoration actions employed to provide a rare assessment of restoration outcomes a decade post-grounding. We found coral recovery to be contingent on the severity of impact and the quality of the impacted habitat, not the amount of active restoration. Despite rubble removal efforts, present-day rubble cover was significantly higher at the impact sites compared to the reference sites and appeared to constrain recovery in the injury zone where grounding impacts destabilized the reef framework. Outplant efforts did not increase coral density or mean size relative to natural recovery sites, though this may be the result of an ineffective outplant design rather than failed outplanting as a whole. The sites closest to returning to a pre-disturbance state were the passive restoration sites. This, however, likely reflects the low severity of grounding impacts and the marginal (e.g., small and sparse) population of corals at these sites. These findings suggest that the extent of active restoration actions should be carefully and intentionally scaled to the severity and spatial extent of impact (with greater impacted areas receiving greater amounts of restoration), and that with sufficient time, marginal reef habitats with a low impact severity can likely recover from passive restoration alone.