Successful eradication of a non-indigenous marine bivalve from a subtidal soft-sediment environment

P>1. Biofouling, the accumulation of biological growth on submerged surfaces such vessel hulls and artificial structures, is an important transport pathway that can facilitate the establishment of marine non-indigenous species in new locations. Despite efforts to develop effective tools to eradicate newly established populations before they become widespread and beyond control, eradication successes are scarce in the marine environment. This paper describes a dredge-based eradication of the brown mussel Perna perna from a deep (c. 44 m) soft-sediment habitat in central New Zealand, following the discovery of this species amongst biofouling organisms physically removed (i.e. defouled) from a drilling rig. 2. We evaluated the efficacy of dredging in removing P. perna and other target species, and determined whether a density-based eradication success criterion had been achieved. The catchability coefficient (q) of the defouled material was estimated using catch data, and the dredge efficiency (E) was determined. Initial and remaining mussel densities were then calculated using estimates of E. The reliability of these estimates was tested by simulations. 3. A total of 227 dredge tows covering c. 94% of a 12 center dot 6 ha target area were undertaken, and an estimated 35 tonnes of material defouled from the rig was dredged from the seabed and disposed of in a landfill. 4. Estimates of q and E were 0 center dot 0054 and 0 center dot 30 (respectively) and mussel densities at the completion of the eradication programme were estimated to be c. 0 center dot 5 m-2, well below the success criterion of 10 m-2. From our simulations, it was estimated that 71 dredge tows would be required to remove 50% of the initial population, whilst 232 dredge tows would be needed to achieve a 90% reduction in population size. 5. Synthesis and applications. The eradication of non-indigenous bivalves from a relatively deep (> 40 m) soft-sediment environment is unprecedented and highlights that, with appropriate tools and other resources, eradication is feasible even in challenging circumstances. Where complete elimination of a pest is not feasible, alternative density-based success criteria can be developed that, if achieved, can effectively mitigate risks. This study highlights the need for further development of both vector treatment options and pest eradication tools, and improved policy surrounding in-water defouling in the coastal environment.