Effect of water velocity on growth and retention of cultured Greenshell™ mussel spat, Perna canaliculus (Gmelin, 1791)

The Greenshell TM mussel, Perna canaliculus , is a commercially important species forming New Zealand’s largest aquaculture export product. Losses of P. canaliculus spat from culture ropes between larval settlement and the time mussels reach initial reseeding size ( c. 10 mm) are common. To test whether water velocity affects growth and retention of post-settlement P. canaliculus spat, and whether there is a threshold effect of increasing velocities on spat migration, a laboratory-based experiment was conducted. Spat were grown for 8 weeks in experimental tanks on culture ropes at four velocities typical of velocities within mussel farms (40, 10, 4, and 1 cm s −1 ). Spat migration was observed at all the velocities tested, but the number of spat migrating decreased as water velocity increased. Spat retention was highest at the 40 cm s −1 velocity. Mean spat size increased significantly with increasing water velocity. At the highest velocity tested (40 cm s −1 ), migration increased as the spat grew suggesting that migration was density-driven. The results of this experiment indicate the potential for mussel farmers to reduce seeded spat loss and to increase the growth rate of spat by modification of the water flow within their farms, or placement of seeded ropes in locations of certain existing water velocities, and indicate that velocities in the range 15–40 cm s −1 promote higher spat growth and spat retention for P. canaliculus .