G. Christophersen, O. Strand
Abstract:
The scallop Pecten maximus is commonly found in sub-littoral coastal marine waters. In Norway spat supply of the scallop, P. maximus, rely on hatchery production. At 2 mm shell height the postlarvae are transferred from the hatchery to a nursery for growing to 15 mm. A landlocked heliothermic marine basin may be used as water supply and food-production system for a land-based nursery. In this environment, however, lower salinity and ligher temperature compared to sea conditions are likely to occur. The effects of reduced salinity on survival, growth and behaviour of scallop spat of 1-2 mm size were studied at 15 and 18 C for a period of four weeks. The scallops were exposed to the experimental salinities of 20, 25 and 30 ppt. The result showed an overall higher mortality of the scallops at 15 C than at 18 C. The mortality rate for scallops held at 20 ppt was higher than for scallops at 25 and 30 ppt. The shell of spat held at a salinity of 20 ppt became very fragile and damaged easily, resulting in negative estimated growth rates. Growth rates were higher at 30 ppt than at 25 ppt, and higher at temperature of 18 C than 15 C. No byssal attachments were observed at 20 ppt. At 25 ppt the attachment rate were around 20% throughout the experimental period at both temperatures. In 30 ppt and 15 C water the attachment rate decreased from 100 to 60% during three weeks, but were never less than 80% in 18 C. The results show that scallop spat cultivated in water with freshwater impact may suffer high mortality after a few weeks exposure to salinity lower than 25 ppt. The major part of the scallops survived 25 ppt for more than three weeks, which is sufficient to re-establish marine salinity after a brackish water situation. However, growth rates of the scallop spat are retarded at salinity of 25 ppt compared to salinity of 30 ppt.
(Centre for Studies of Environment and Resources, University of Bergen, Bergen High-Technology Centre, 5020 Bergen, Norway)
***************