The retention of juvenile Perna canaliculus in aquaculture

Abstract

The green-lipped mussel Perna canaliculus is the most important aquaculture species in New Zealand. The poor retention of seed mussels during the early stages of aquaculture is a major problem facing the industry and a constraint to the security of production and sustainable growth of the industry. Few studies have quantified the retention of seed mussels, consequently the timing, magnitude and causes of mussel losses are a considerable research gap that hinders management of this valuable resource. The research presented in this thesis used largely fieldbased approaches to address the knowledge gap surrounding losses of seed mussels during early aquaculture. Spatial and temporal patterns of mussel losses and biofouling development were assessed in two separate experiments. The retention of Kaitaia spat, the most important source of mussel juveniles in New Zealand, and the effects of the substratum to which the juveniles are attached at seeding were investigated throughout early aquaculture production. The efficacy of placing adult mussels alongside seeded juveniles to prevent subsequent losses of the juveniles was assessed with a separate experiment, which included an assessment of biofouling development in relation to retention of juvenile mussels. Finally, three laboratory based experiments tested whether losses observed in earlier experimental work could be occurring immediately upon seeding after relay to the mussel farm. Across the field experiments, losses of juvenile mussels were greatest between deployment and the first sampling point (42.9 – 72.9 %, depending on experiment), and in all experiments, losses continued throughout early aquaculture with 64.4 – 84.9 % being lost at the end of early production. Test factors, except duration of deployment, and correlation analyses suggested only small, transient effects of the factors of interest that were tested experimentally in this study. By contrast, secondary settlement behaviour of the juveniles, and to a lesser extent their mortality, was shown to have a major role in mussel losses from aquaculture. Indeed, most mussels detaching from their substratum in the laboratory experiments were alive and re-settled when given the opportunity. Size-analyses indicate that secondary settlement has an inverse relationship with size and the propensity for this behaviour reduces after 1.75 mm in shell length.The findings of this thesis give insight into the timing, magnitude and causes of losses in the aquaculture of P. canaliculus to allow targeted research to further understand the biology of this species and mitigate against its loss from aquaculture to achieve the goal of efficient and sustainable development of the industry.