Modeling tag loss in New Zealand fur seal pups

Mark-recapture studies of pinnipeds commonly use double-tagging to reduce bias of parameter estimates and to allow estimation of tag retention rates. However, most tag retention estimates assume independence of tag loss. Here we were able to identify when individual New Zealand fur seal (Arctocephalus forsteri) pups had lost both tags; therefore, we tested the assumption of no association between the tag-loss rates of left and right tags. We also tested for differences in tag retention among three different types of plastic tag (Allflex® cattle, mini and button tags), between two attachment types (i.e., fixed or swivel), and whether retention varied among years and colonies sampled. We found strong evidence of within-individual tag loss association for most tags in most years, but little evidence that this varied among colonies. We found that ignoring within-individual association of tag loss led to a bias in estimated tag retention of 7.4-10.1%. Smaller rocks and greater crevice and ledge densities in colonies were associated with lower probabilities of tag retention. We suggest researchers should attempt to use permanent marks in combination with tags to assess unbiased estimates of tag retention.