Using mark-recapture methods to estimate pup production of harp seals (Phoca groenlandica) in the Greenland Sea

Abstract

During the period 1968-1991 16,917 harp seal pups have been tagged in the Greenland Sea. From 1976 until 1991, there have been 468 recoveries. 73 of these, mostly drowned in fishing gear or part of Inuit catches at Greenland, are from areas outside the Greenland Sea, broadly covering the Norwegian coast, Iceland, East and West Greenland and Newfoundland. Recaptures made one year or more after tagging during the commercial catch operations in the Greenland Sea have been used to calcuiate pup production for several years during the period 1977-1991 by the Chapman mark-recapture estimator. These estimates, ranging from 40,000 to 115,000 but with no power to reveal any trend, show features that makes one suspecting that they might be seriously biased. The bias might be caused by violations of the randomness assumption and non-uniform distribution of tagged animals. A mechanism of non-permanent emigration is suggested to explain the source of the bias, but no data are available to correct it. A remedy that might be of limited vaiue in management, would be to truncate the recapture data by using data collected from the fifth year onwards only, assuming that temporary emigrated seals have retumed to the population upon reaching sexual maturity. Also inherent in the estimates is a bias of some + 5% caused by tag loss.