Factors affecting the recovery of soft-sediment mussel reefs in the Firth of Thames, New Zealand

Bivalve reefs are vital ecosystem engineers but have declined or disappeared in many regions. In the Firth of Thames (FOT), north-east New Zealand, overfishing, sedimentation or both led to the virtual extinction of extensive reefs of green-lipped mussels (Perna canaliculus). The mussel reefs have not recovered since commercial fishing ceased in 1968, possibly because the muddy sediments that replaced the reefs are an unsuitable habitat for adult mussels. To test this hypothesis, we transplanted mussels into cages on the seafloor for 500 days at three sites along a turbidity gradient (average visibility 0.8-4.7 m) within the mussel reefs' former range for 500 days. Results showed that 68% of individuals survived the experiment and grew an average of 19 mm in length. Survivorship and growth did not differ between sites. However, at the completion of the experiment, mussels from the least turbid site were in better condition (condition index 15) than those from the most turbid site (condition index 10). Our results suggest that the current lack of recovery of mussel reefs in the FOT is attributable to low recruitment and survivorship of juvenile mussels. Restoration of mussel reefs and the ecosystem services that they provide may therefore be possible.