The importance of physical characteristics of settlement substrate to the retention and fine-scale movements of Perna canaliculus spat in suspended longline aquaculture

Abstract

The early stages of mussel aquaculture can be extremely inefficient, with the majority of seed mussels often lost shortly after seeding out onto coastal farm structures. These high losses of seed mussels (conversely known as poor spat retention) are extremely costly to the mussel aquaculture industry, while constraining production and the growth of the industry. The causes of poor spat retention are unclear, though it appears to be predominantly caused by secondary settlement behaviour, which is a process which enables juveniles to actively settle and resettle among a variety of settlement substrata. One factor that appears to play an important role in the settlement of a range of commercially important mussel species is the physical complexity of settlement substrata. Smaller seed mussels settle preferentially onto finely branching filamentous substrata, whereas larger seed mussels prefer coarser substrates. In this study the seed of the green-lipped mussel, Perna canaliculus, were seeded onto grow ropes with a range of physical characteristics to determine whether modifying the structure of seeding materials could improve seed mussel retention. Modifying the physical characteristics of seeding substrata had no impact on mussel retention; with high losses observed from grow ropes in each treatment. Seed mussels were highly mobile following seeding out and rapidly began to migrate among seeding substrata. The majority of seed losses occurred while these small-scale migrations were taking place, suggesting these movements maybe a major contributor to the loss of seed mussels from farms. These results suggest new approaches for seeding out mussels that do not require small-scale migrations of mussels between substrata should be developed and tested.