TAG LOSS IN GREY SEALS (HALICHOERUS-GRYPUS) AND POTENTIAL EFFECTS ON POPULATION ESTIMATES

Cumulative tag loss among 4064 grey seal (Halichoerus grypus) pups obs erved on Sable Island was less than 1% during the first 5 months of li fe, 13% by the end of the first year, and continued to increase with a ge. Cumulative tag loss among grey seals aged 6 and older was over 40. 0%. A double-tagging study indicated that pre-punching of flippers, th e colour of tags applied, and application to the left or right flipper significantly affected tag loss. The addition of a flag to the tag ha d no significant effect. A comparison of scientific observations of ta g loss with commercial bounty return data indicated that casual observ ers probably missed tags on double-tagged animals. Overlooked tags in single-tagging experiments result in population overestimates. In doub le-tagging experiments, overlooked tags result in correction factor ov erestimates and population underestimates. Age-specific tag loss corre ction factors should be used if population estimates include more than one age group.