Relationship between cod fishery activities and the population of herring gulls on the North Shore of the Gulf of St Lawrence, Québec, Canada

The history of herring gull (Larus argentatus) populations nesting along the North Shore of the Gulf of St Lawrence and the commercial fishing activities there are closely related. Coastal cod (Gadus morhua) supply the main fishery in this part of the Gulf and traditionally have resulted in significant amounts of fish offal being discarded at sea and in fishing ports. Using commercial catch data of cod landings, and information from herring gull surveys conducted in the migratory bird sanctuaries on the North Shore from 1925 to 1993, the relationship between the cod fishery and herring gull populations was investigated. Between 1925 and 1975 the fisheries harvested a mean of 5 234 t of cod annually, with high and low catches of 11 000 and 1 700 t, respectively. During the same period, the herring gull population in the sanctuaries increased from 650 to 8 000 pairs. After 1975, and until 1993, the annual mean harvest was 5771 t peaking in 1983 at 11 500 t. There was then a steady decline until a complete fishery collapse in 1993. During the same period, the herring gull population increased from 8 000 pairs in 1975 to 14 000 pairs in 1988, but then dropped dramatically in most sanctuaries and was estimated at only 3000 pairs in 1993. This general decline of gulls appears to be related to the decrease in commercial fishing activities in the region, but not uniformly so.