The impact of seeding density on spat losses on New Zealand's Greenshell™ mussel (Perna canaliculus) farms

Abstract

High losses of seed mussels (spat) during the early stages of mussel aquaculture is a major problem that results in the inefficient use of spat, which severely disrupts production and ultimately constrains industry growth. These losses are thought to be caused by a variety of factors including predation, mortality, the natural migratory behaviour of mussel spat and inappropriate culture conditions, such as excessive seeding density. Mussel farmers frequently seed out with high densities of spat in anticipation of the subsequent losses of spat. In this study, we measured the impact of three different seeding densities on nursery dropper rope (low – 11,782 spat m−1, medium – 17,640 spat m−1, and high – 59,242 spat m−1) on subsequent spat losses on two Greenshell™ mussel (Perna canaliculus) farms in New Zealand. Seeding out at the high and medium initial seeding densities resulted in greater spat losses (93.5% and 90.0%, respectively) over 59 days compared with the low initial density (49.5%). Furthermore, despite the significant differences in initial seeding densities among the three treatments, similar numbers of spat (i.e., ~ 3000 spat m−1) remained on dropper ropes in each treatment at the two farm sites after 59 days. Growth of spat over 59 days did not appear to be affected by initial seeding density that might otherwise be expected if there was competition for resources (e.g., space and food) associated with the experimental densities. These results suggest that the optimal seeding density of spat on Greenshell™ farms may be much lower than current seeding densities, and that farmers may be able to significantly improve spat retention and the efficiency of the use of spat by simply reducing their seeding densities.