commercial fishery is at depths >50 m (>165 ft). In colder waters of Maine 

 scallops are sometimes found close to the low water mark. 



Pelagic larvae live in the water column of the marine and high salinity areas 

 (>20 ppt) of the estuarine system. In laboratory experiments temperatures 

 above 66°F (19°C) over an extended period of time were fatal to larvae. 



The juvenile and adult scallops live in subtidal marine waters and in areas of 

 comparatively high salinity (approximately 20 to 25 ppt) in estuarine systems. 

 Estuarine populations are generally found in deep channels where temperatures 

 and salinity are least variable (Welch 1950) . They live on unconsolidated 

 sediments, usually sand or gravel, and to a lesser degree on rocky bottoms. 



Factors of Abundance 



Temperature is the most critical natural factor limiting the distribution and 

 abundance of the sea scallop. High summer water temperatures of 68 to 74 °F 

 (20 to 23.5 °C) limit the distribution of adult scallops to deeper waters in 

 the southern region of the species' range (Long Island and further south; 

 Bourne 1964). The maximum temperature for larvae is about 19° C (66 °F; 

 Culliney 1974). The water temperature must reach a minimum level of 46 °F (8 

 C; Posgay and Norman 1958), 49°F (9.5°C; Dickie 1955) or 51°F (14°C; Culliney 

 1974) for spawning to occur. In the northern part of their range, only the 

 shallower waters of New England and the Maritime Provinces of Canada are warm 

 enough to meet this minimum temperature requirement. 



Scallops have a limited ability to withstand reduced salinities; hence, they 

 are not found in areas of low salinity (<20 ppt) in estuaries. 



Sporadic large-scale mortality has been observed in beds of sea scallops in 

 Maine (as large fluctuations in landings corroborate; see figure 12-3) and 

 elsewhere, but the cause has not been determined. Medcof and Bourne (1962) 

 suggest that sudden, extreme change in temperature may contribute 

 significantly to natural mortality. They estimate the rate of natural 

 mortality in scallops over 3 years old is 10% of the population, based on 

 numbers of living and newly-dead individuals in dredge catches. Merrill and 

 Posgay (1964) derived the same rate for offshore populations on Georges Bank. 



Scallops can live at least 8 years (Baird 1967). Predators include Atlantic 

 cod, American plaice, Atlantic wolffish, the northern starfish ( Asterias 

 vulgaris ) , and the common sun-star ( Crossaster papposus ) . The extent to which 

 predators affect populations of scallops is unknown. 



Human Impacts 



Fishing indirectly may lead to high mortality in scallop populations through 

 disrupting the bottom sediment by dragging, and through exposing and damaging 

 discarded small scallops. Medcof and Bourne (1962) estimate that fishing 

 mortality may reach an annual rate of 10% in the inshore populations in Nova 

 Scotia . 



Other factors potentially affecting scallop distribution and abundance are oil 

 spills, dredging, spoil disposal, and discharge of heated effluents or 

 contaminants . 



12-12 



