Use of parasites as tags in delineating stocks of Atlantic cod (Gadus morhua) from the southern Gulf of St. Lawrence and the Cape Breton Shelf

Parasites were inventoried on and in 470 Atlantic cod (Gadus morhua) collected from the southern Gulf of St. Lawrence, NAFO (North Atlantic Fisheries Organization) division 4T, and Cape Breton Shelf (NAFO subdivision 4Vn) in September of 2004 and 2005, and August 2006, respectively. Forward stepwise discriminant function analysis (DFA) of the 4T samples indicated that the nematodes, Hysterothylacium aduncum, and Pseudoterranova decipiens and the acanthocephalans Echinorhynchus gadi and Corynosoma strumosum were significant in the classification of cod to eastern or western 4T. Cross-validation yielded a classification efficiency of 74% overall, thereby supporting the findings of earlier mark-recapture studies which have indicated that 4T cod are comprised of discrete eastern and western spawning groups. Further analyses indicated that western 4T cod was distinct not only from eastern 4T cod, but also differed from cod from the Smokey Channel and Cape Breton Shelf slopewaters (4Vn), with rates of misclassification ranging from 6% to 9%. Hence, “parasite tags” may prove useful for monitoring the exploitation of cod from western 4T during their annual migration through the Cabot Strait and while over-wintering along the edge of the Cape Breton Shelf where mixing with eastern 4T and 4Vn cod may occur. Similar results could be achieved with migrant cod from eastern 4T through analyses of parasite markers together with host meristic or morphometric parameters.