Predation of hatchery-reared scallop spat (Pecten maximus L.) by the Ballan wrasse (Labrus bergylta)—consequences for sea ranching

Fish predation on scallops has received relatively little attention compared to the primary predators sea stars and crabs. Available knowledge of fish predation is mainly based on observations from scallop beds and fish stomach analysis. These are the first controlled experiments conducted to test if fish (Ballan wrasse, Labrus bergylta) prey upon on hatchery-reared scallop spat. Under laboratory conditions Ballan wrasse from 22 to 40.5 cm in length were offered spat from 15 to 34 mm in shell height at a density of 50–103 individuals m− 2. Predation was recorded in 15 out of 35 tanks. The mean predation frequency for all tanks was 0.10. The mean predation frequency for the 15 predation tanks was 0.17 and the mean size class predation frequency was 0.53 (15–19 mm), 0.16 (20–24 mm), 0.03 (25–29 mm) and 0 (30–34 mm) (n = 15). The mean predation frequency was significantly different between spat of 15–19 and 20–24 mm in shell height. No significant difference in predation frequency was found between larger spat. There was also indication of size-dependent predation from a field experiment, although this experiment was not conclusive. Results from this study indicate that farmers may seed spat larger than 30 mm in shell height for sea ranching with a minor risk of predation from Ballan wrasse.