UNDERSTANDING THE ISSUE OF POOR SPAT RETENTION ON NEW ZEALAND'S GREENSHELL™ MUSSEL Perna canaliculus FARMS

The early stages of Greenshell™ mussel (Perna canaliculus ) aquaculture in New Zealand are highly inefficient, with the majority of seed mussels ('spat') lost from farming substrata shortly after seeding out.  These high spat losses (conversely known as poor spat retention) are extremely costly to the Greenshell™ industry, where they constrain production and limit  industry  growth.  The causes of poor spat retention are unclear, though it appears to be  predominantly caused by secondary settlement behaviour , which is a prominent feature in juveniles of this species. However, the factors that trigger secondary settlement behaviour are poorly understood, and therefore, cannot be managed . To date,  few studies have addressed the issue of poor spat retention directly, and as a result, no solutions to the problem currently exist.  Through a series of field experiments, this study sought to identify some dominant causes of poor spat retention on Greenshell™ mussel farms under typical farming conditions.

Four  experiments  were conducted;  each  measuring spat retention across multiple farm sites  under typical farming conditions. In each experiment  a series of experimental droppers were seeded out with spat from a single cohort, and their performance was measured over time.

In each experiment, spat retention was poor (frequently < 30%), and the majority of spat losses took place early on in the production cycle, shortly after seeding out (Fig. 1). Spat losses were also consistent among farm sites, indicating that local environmental conditions likely played little role in influencing spat retention.  The timing of spat losses in relation seeding substrata breakdown suggest that changes in the arrangements for seeding out could be used to reduce spat losses.