Fluctuations in Abundance of the Giant Scallop, Placopecten magellanicus (Gmelin), in the Digby Area of the Bay of Fundy

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Abstract
Fluctuations in total landings and in catch per boat have characterized the Digby scallop fishery since it began in 1920. Although changes in fishing methods have been partly responsible for early changes in catch, their influence in recent years has been small and changes in abundance have been primarily responsible for fluctuations in the fishery since it reached its full development in the mid-1930''s. Changes in abundance are assessed from analyses of catch records, special "census-fishing" techniques, submarine photography and marking experiments. Estimates from the different methods correspond. Scallops are recruited into the catchable population as 6-year-old year-classes. Abundance is high when these recruited year-classes are strong, but is low when they are weak. Abundance in any year is correlated with water temperature 6 years previously. Both abundance and the strength of individual year-classes are correlated with water temperatures which prevailed at the time the scallops were present as pelagic larvae. Hence changes in the abundance of the catchable scallop stocks result from the combined action of temperature and circulation on the pelagic larvae. Low temperatures retard larval development, and are indicative of great exchange of the water in the Bay of Fundy with outside water masses. This apparently leads to heavy losses of the larvae from the Bay, poor sets on the parent beds, weak year-classes and low abundance of the catchable stocks of six-years later. High temperatures speed larval development and are indicative of a closed Fundy circulation which holds the larvae in the vicinity of the parent beds. This leads to good sets, strong year-classes and high abundance of the catchable stocks 6 years larer.

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