Magnitude and timing of seed losses in mussel (Perna canaliculus) aquaculture

Losses of juvenile mussels seeded into aquaculture systems are a major constraint on the stability and growth of the New Zealand green-lipped mussel (Perna canaliculus) industry. Yet, the timing and magnitude of losses following seeding and during grow-out are unknown. In this study, losses of wild-sourced juvenile mussels were assessed 19, 40, and 89 days following deployment onto a mussel farm. In New Zealand, these wild juvenile mussels are attached to a variety of substrates, mostly macroalgae, hydroids or terrestrially derived material, when they are seeded into aquaculture systems. This study examined juvenile losses over time from coarse, fine and mixed macroalgae and unmanipulated substrate. Secondary settlement of mussels from their position at seeding to adjacent un-seeded sections of rope was followed to estimate migration. The number of dead mussels was quantified to assess the relative contributions of mortality and migration to mussel losses. In addition, small-scale variations in water quality that might be associated with the breakdown of the macroalgal substrates, such as dissolved oxygen and nutrient concentrations, were determined. Losses of mussels were initially very high (52.8–72.9% of the starting density at 19 days) and continued throughout the aquaculture nursery stage (40 days = 69.6–76.7%, 89 days = 74.4–84.9%). Overall retention was greatest for mussels deployed on coarse macroalgae (24.6%), but there were no consistent differences among substrates across deployment durations. At the end of the experiment, at least 20% of the estimated starting number of mussels had moved from their initial position at seeding. There were also high numbers (8.1% of the initial seeding density) of small dead mussels found soon after deployment. However, around 40% of the initial seeding density of juvenile mussels remained unaccounted for at the end of the experiment. These results highlight the immense scale of post-seeding losses in mussel aquaculture and the complexity of underlying processes driving these losses.

Keywords: Secondary settlement; Recruitment; Kaitaia spat; Mortality; Green-lipped mussel; Retention; Seeding efficiency