Seasonal relationships between size, bottom temperature, and distribution of cod are described for the western Gulf of St. Lawrence and Nova Scotia Banks.In summer, in the western Gulf of St. Lawrence, cod are distributed from 35 to 145 m at bottom temperatures from −0° to 6 °C. They are most abundant at about 100 m where the temperature is around 1 °C. In winter they are concentrated in 130–180 m along the western slope of the Laurentian Channel at bottom temperatures from 1° to 3 °C.On the Nova Scotia Banks cod are less abundant than in the Gulf of St. Lawrence. They are found mainly around Banquereau, Middle Ground, and the northern edges of Sable Island Bank. Further to the west cod are replaced by haddock as the dominant species. Nova Scotia Banks cod are found in shallower and warmer waters than Gulf cod, both in summer and winter. In summer they are present from 65 to 110 m at bottom temperatures varying from about 1° to 8 °C. In winter they are taken primarily at 90–135 m at bottom temperatures from 2° to 4 °C.Area and depth distributions of commercial catches reflect the seasonal pattern of cod migrations and distributions demonstrated in surveys and tagging studies.
Sampling aboard northern New Brunswick draggers from 1956 to 1961 shows a substantial reduction in the percentage by numbers and by weight of the catch of cod discarded at sea. The total numbers of cod discarded in the southwestern Gulf of St. Lawrence, which amounted to about 7 million in 1956, have been reduced to an estimated 1 million in 1961. Two factors are mainly responsible for this reduction: large-mesh nets which, following an ICNAF recommendation, became effective in 1957, and reduction in sizes retained for landing. The mesh effect is believed to be 1.3 times greater than the cull effect in reducing cod discards in 1961. Discards of American plaice have increased during the same period, due to larger recruitment of young year classes. Discards of species other than cod and plaice are negligible because they are only a small part of the catch. Survival experiments show that most cod and plaice are dead when returned to the sea. The use of large-mesh codends may result in long-term increase in the total cod landings in the southwestern Gulf of St. Lawrence.
During the winters of 1959 to 1962, 2459 commercial-sized cod were tagged off the Canadian coast south of the Laurentian Channel. Most returns came from the same general area. Very few cod crossed the Laurentian Channel to Newfoundland or Gulf of St. Lawrence North Shore grounds, or the Scotian Gulf to grounds off western Nova Scotia. Several cod populations were observed, each moving south in autumn and north in spring months. Fish tagged off central Nova Scotia did not migrate as far as the Gulf of St. Lawrence, but those tagged off Cape Breton migrated into the southwestern Gulf for summer months. Fish marked in the middle of the area were intermediate in their migration pattern.Temperature and feeding appear to be the important factors associated with cod migrations. Cod remain in temperatures of 1° to 4 °C during winter months by moving to deeper water. In spring, cod disperse and feed in shoaler water. The cod of the Western-Emerald Banks area move east over the Scotian Shelf. The Gulf cod move up through the cold-water layer and north to inshore feeding near the Magdalen Islands and the Gaspé peninsula.
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