Causes of Mass Mortalities of Scallops (Placopecten magellanicus) in the Southwestern Gulf of St. Lawrence

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Abstract
In 9 of the years since 1928 there have been midsummer mass mortalities of scallops in the southwestern Gulf of St. Lawrence. There is evidence that sudden, great increases in water temperature sometimes exceeded the upper incipient lethal temperatures of scallops and that these were directly responsible for the more devastating, sudden and widespread mortalities (up to 80%). Unusual and unexplained concentrations of predators, mainly starfish, are considered to have been responsible for several of the slower and less spectacular mass mortalities (up to 25%). Aquarium tests showed that, in spite of well developed escape reactions, scallops are nevertheless often killed by starfish. Smaller, sudden temperature increases (too small to be lethal) and sudden decreases are common in the southwestern Gulf and both can cause temperature debility in scallops. Presumably debility hampers escape from enemies, and magnifies effects of predation. It probably helped to produce the minor mass mortalities but so far its importance has not been assessed.

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