Effect of reduced salinity on the great scallop ( Pecten maximus) spat at two rearing temperatures

Abstract

The effects of reduced salinity on scallop, Pecten maximus, spat of 1.7 mm shell height and 31.5 μg ash free dry weight were studied at 15 and 18 °C. Mortality, growth, byssus attachment, activity behaviour and clearance rate were studied at salinities of 30, 25 and 20 for a period of 25 days. The mortality of scallops held in salinities of 30 and 25 at 18 °C was 8%, which was significantly lower than for the spat kept at the other salinity–temperature combinations. Mean final mortalities at 15 °C were 59% for spat held at salinity of 20 and 25% for spat held at 25 and 30. At 18 °C and salinity of 20 final mortality was 26%. At salinity of 20, the increase in mortality rate commenced 1–2 weeks earlier at 15 °C than at 18 °C. Higher growth rates at temperature of 18 °C than at 15 °C were observed at salinities of 25 and 30. The shell of spat held in salinity of 20 became very thin and damaged easily, resulting in negative shell growth, while ash free dry weight growth was positive. No byssal attachments were observed in salinity of 20, and about 20% attachment occurred in 25. In salinity of 30 at 15 °C, the attachment rate decreased from 100% to 55%, while at 18 °C the attachment remained 83–97%. The general activity level was affected by low salinity as the foot movements decreased, the mantel retracted from the shell margin and gaping increased. Cultivation of scallop spat at sites influenced by brackish water (e.g., Norwegian polls) may result in high mortality when exposed to salinity lower than 25 and retarded growth may occur below 30. Locations exposed to salinities below 25 for an extended period should be avoided for nursery growth of P. maximus spat.