Predation on hatchery reared scallop spat (Pecten maximus L.) by the ballan wrasse (Labrus bergylta) in controlled experiments and in sea ranching

Sea ranching of scallops (Pecten maximus) in Norway has been associated with great losses caused by predation by the edible crab (Cancer pagurus,). This predation is now hindered by the development of a fence that prevents intrusions of crabs, which has resulted in high survival of scallops in sea ranching. Because crab predation is radically reduced, the opportunity of seeding scallops directly from hatchery to bottom culture emerged. A straightaway release of scallops from hatchery will greatly reduce the labor effort and cost associated with operation of intermediate culture. In the summer of 2002 Helland Skjell AS carried out preliminary experiments with early transfer of spat from intermediate culture to bottom culture. The seeding of scallops from intermediate culture attracted fish and by seeding 30-mm spat it was observed that the ballan wrasse (Labrus bergylta) nab on the scallops. Tank and field experiments were conducted to test if ballan wrasses predate on hatchery reared scallop spat. Ballan wrasses were offered spat from 15 to 35 mm in shell height. Predation was recorded in 20 of 40 tanks and predation was greater than 20% in seven tanks. In these seven tanks the mean predation frequencies significantly decreased with increasing shell height. Predation was not recorded for spat larger than 30 mm in shell height. There was also indication of size dependent predation from the field experiment, but other fishes than ballan wrasse may be the predator.