Effects of environmental conditions on culturing scallop spat (Pecten maximus)

The great scallop, Pecten maximus, is a valuable bivalve species attractive for human consumption. Scallop aquaculture is increasing, but availability of spat is a bottleneck to the development of a viable culture industry in many countries. The collection of P. maximus spat from the wild has been insufficient, and intensive hatchery production is considered an optional strategy for a predictable spat supply to growers in Norway. Large variations in growth and survival during the post-larval stages are experienced, and temperature, salinity and food availability have been identified as critical environmental factors during the nursery growth stage. A nursery bridges the gap between intensive culture in the hatchery and growout in the sea, and covers the rearing from metamorphosis (ca. 0.25 mm) until the commercial spat size of 15-20 mm. Transfer between production stages and grow-out sites is required in scallop culture. The present study filled an information gap related to effects of environmental factors on small scallops (ca. 2 mm) in the transition phase between hatchery and nursery. Different transfer strategies were investigated to test the capability of scallop spat to tolerate transfer and adapt to new environmental conditions. Experimental studies with hatchery reared P. maximus showed that viability through the various spat stages could be improved by employing adequate culture and transfer strategies. Transfer of spat of different settlement age to nursery was approved to further increase the spat yield. Site selection and timing of transfer are of major importance, and the observed suboptimal and lethal environments should be avoided. Higher rearing temperature (18 vs. 15 °C) resulted in increased growth and survival, while salinity of 20 and 25 affected growth, survival and byssal attachment negatively. Exposure to low temperatures was a main factor affecting the performance of small spat. Acclimation of 1-2 mm spat to a lower temperature than in the hatchery improved post-transfer survival to seawater of <7 °C. So did transfer of larger (4 mm) spat. Alternatively, spat could be transferred to a land-based nursery using solar heated and fertilised “poll” water as food source to give an earlier start of the production season in the spring. The growth was similar to in sea-based nursery. Dry transportation of small spat was shown feasible up to 12-24 hours. The change to "poll" food conditions resulted in a significant decrease of lipids, but except for sterol content, the nutritional status at transfer was not associated with growth and survival in the land-based nursery. Increased water recirculation rates in the hatchery rearing tanks hardly affected the spat performance and the chemical composition, thus supporting more efficient utilisation of algae.