Large-scale raceway nursery for improved scallop (Pecten maximus) spat production

Variable survival of small hatchery-produced Pecten maximus after transfer to sea has been considered a constraint to scallop spat production in Norway. A land-based raceway nursery was introduced as an intermediate step to bridge hatchery post-larval growth and growout in the sea to increase yield and stabilise spat production. The nursery was of a flow-through type with 10 parallel raceways (10 m × 0.6 m × 1.0 m). Seawater (8-19 °C) was filtered through 100 μm mesh and pumped into the nursery at a rate of 6 m3 min−1 (2 cm s−1). Efficiency of the nursery was evaluated based on different growth trials comparing initial spat size, settlement technology and nursery type. Scallops settled either on downwelling screens or collector bags in the hatchery. Spat settled on screens were placed on trays before transfer to nursery, while collector bags were placed directly into raceways. The nursery facilitated growth of scallop spat from 1.5 mm until saleable seed >15 mm shell-height. Growth rates were 1.4-2.3% day−1 for spat of initial size 3, 5 and 7 mm, and 4.2-5.2% day−1 for the fastest growing spat of mean initial size <2 mm. The filter removed fouling organisms and predators successfully, and resulted in stable survival on land (85%), while survival varied from 36 to 84% in the sea. Yield of spat >5 mm per collector bag varied from 46 to 2780, and was not affected by settlement conditions (colour of bag, water circulation and light conditions). Yield of competent larvae from trays averaged 18.9% on land and 14.2% in the sea and was superior to yield from collector bags (2.8%). The downwelling screen-tray method resulted in higher output per unit raceway than did the settlement and growth on collector bags and was thus regarded the most economical method. The land-based nursery had about 20% higher operational costs than the sea-based, but higher survival counterbalanced the extra costs. Using intermediate land-based nursery was proven advantageous to achieve a more cost-effective and stable production of 15-20 mm P. maximus spat available to scallop growers in Norway.