Population dynamics of restored green-lipped mussel (Perna canaliculus) beds in the Hauraki Gulf, New Zealand

Abstract

The widespread degradation of biogenic habitats created by bivalve beds has spurred numerous restoration initiatives worldwide. The success of those restoration initiatives depends on the persistence of restored populations which in turn relies on robust and frequent assessments of population dynamics to identify potential limitations to the persistence of those populations. The research presented in thesis aimed to examine the potential for restoration of the nearly extirpated green-lipped mussel in the Hauraki Gulf, New Zealand and to develop a foundation for best practice methods in restoration of this, and other mussel species. With the deployment of seven experimental mussel beds and multiple field and laboratory experiments, this body of research addresses the questions of; 1) whether transplanted mussels are persistent in restored mussel beds, 2) to what extent sea star predation contributes to mortality of adult and juvenile mussels, 3) whether the addition of attachment substrate enhances the persistence of adult and juvenile mussels, and 4) whether recruitment at the restored mussel bed site is limited by the amount of available settlers. Experimental mussel beds exhibited high mortality, with only 26.2% survival of originally estimated abundance after 25 months. This decline was attributed to unsustainable levels of mortality of adult mussels combined with a near absence of recruiting mussels. Predation by sea stars was estimated to have removed 30.1% of the mussels over the 25 month study, contributing to 40% of the overall mortality observed for experimental mussel beds. However, the large sea stars inhabiting the beds did not preferentially select for juvenile mussels. Providing attachment substrate was found to not enhance the persistence of transplanted adult mussels. However, juvenile mussels preferentially attached to adult mussels and had higher survival in the presence of a sea star predator compared to either mussel shell or unmodified substrate. The settlement of mussels on artificial collectors within the restoration site was greater for collectors within mussel beds than on the soft-sediment but was lower than previously observed on artificial collectors placed in the vicinity of natural populations of green-lipped mussels elsewhere. The overall findings of this thesis suggest that sea star predation and lack of recruitment will limit the success of future restoration efforts. Therefore, further investigation and development of techniques for overcoming these limitations will be necessary for enabling effective restoration of sustainable beds of green-lipped mussels to the Hauraki Gulf in the future.