Evaluating the performance of PIT tags and ear tags in a capture‐recapture experiment

Abstract

Capture‐recapture techniques are the most common method for gathering data on abundance and survival of mammals. A crucial assumption of all capture‐recapture models is that animals do not lose their identification tags. In this study, ferrets (Mustela furo) were trapped in 16, six‐day sessions over a 22‐month period and tagged with both passive integrated transponders (PITs) and ear tags. Comparative population and ferret survival estimates, using records provided by both ear tags and PIT tags, were evaluated using the Jolly‐Seber model to quantify any bias associated with their use. While all ferrets implanted with PIT tags were positively identified, 6% of 98 ferrets lost their ear tags, 3.5% of ear tags on 262 recapture occasions were misread, and mean ear tag retention was 86 days compared with 108 days for PIT tags. None of these differences were significant. Although the PIT tagging system provided the most accurate population estimates, PIT tags are 14 times more expensive than ear tags and, thus, the benefits of increased accuracy must be weighed against the greater economic costs. For short‐term abundance and survival studies, where ear tag loss is relatively small, ear tags remain the most effective method for marking ferrets.