The loss of spat following seeding onto coastal Greenshell™ mussel (Perna canaliculus) farms

Abstract

The early stages of mussel aquaculture can be extremely inefficient, with large numbers of seed mussels, also known as spat, lost from production shortly after seeding out. The exact causes of these losses are unclear, although factors such as local environmental conditions at farm sites, fish predation, mortality and the secondary settlement behaviour of spat are likely to play a role. Spat losses are an acute problem for Greenshell™ mussel (Perna canaliculus) aquaculture in New Zealand, where up to 100% of spat can be lost within a few months of seeding onto coastal farms. However, the timing and pattern of spat losses on Greenshell™ farms has not been well documented. This study aimed to quantify spat losses to determine if they occur consistently at individual locations at different times of the year, or if they vary among spat from different sources (i.e., Kaitaia and line-caught spat). Wild spat harvested from two sources were seeded onto commercial Greenshell™ mussel farms in four experiments, which measured their subsequent performance over 3 months, while also measuring a range of environmental conditions for comparison. Spat losses were considerable (frequently in excess of 70%) and occurred early in the production cycle (within the first month following seeding out). Local environmental conditions appeared to play a minimal role in influencing secondary settlement behaviour, as spat retention varied little among individual farm sites, and between experiments conducted at different times of the year. However, the movement of spat among the substrata used for seeding spat suggests that improvements in spat retention may come from altering the arrangements for deploying spat to mussel farms, such as seeding out with larger, single seed spat.