Predation by Harp Seals in Atlantic Canada: Preliminary Consumption Estimates for Arctic Cod, Capelin and Atlantic Cod

Abstract

Current estimates of harp seal (Phoca groenlandica) abundance indicate that the total population size in eastern Canada in 1994 was approximately 4.8 million (95% C.I. 4.1– 5.0 million) animals. To estimate the consumption of important fish prey by harp seals off the coast of Newfoundland and in the Gulf of St. Lawrence, a model incorporating agespecific estimates of energy requirements, population size, seasonal distribution and diets was developed. Total annual prey consumption increased from 3.6 million to 6.9 million tons between 1981 and 1994. The proportions of prey obtained in the Arctic and eastern Newfoundland/southern Labrador areas were 46% and 40%, respectively, while 14% was consumed in the Gulf of St. Lawrence. Arctic cod (Boreogadus saida) and capelin (Mallotus villosus) were the major prey off eastern Newfoundland while capelin was the most important in the Gulf. Based on an average diet, harp seals consumed an estimated total of 2.8 million tons, including 1.2 million tons (95% C.I. 735 000–1.7 million) of Arctic cod, 620 000 tons (95% C.I. 288 000–1.0 million) of capelin and 88 000 tons (95% C.I. 46 000– 140 000) of Atlantic cod (Gadus morhua) off the eastern coast of Newfoundland in 1994. In the Gulf, harp seals consumed an estimated 445 000 tons (95% C.I. 208 000–727 000) of capelin, 20 000 tons (95% C.I. 0–48 000) of Arctic cod, and 54 000 tons (95% C.I. 14 000–102 000) of Atlantic cod out of a total of 961 000 tons of prey. Incorporating seasonal, geographic and annual variations in the diet provided additional information on trends in consumption. Basic assumptions of the model were varied to assess its sensitivity. Changes in the energetic costs of activity and growth, abundance, residency period, or the proportion of energy obtained from offshore areas can affect estimates of total consumption significantly.