Shedding rates and retention performance of conventional dart tags in large pelagic sharks: Insights from a double-tagging experiment on blue shark (Prionace glauca)

Abstract

Tagging studies are a fundamental tool for understanding fish population dynamics. Choosing the right tag type, however, is of major importance, as properties such as shedding rate can affect estimates of mortality, abundance, and movement rates. Here, we provide tag-specific recapture rates for blue sharks (Prionace glauca) and assess the retention performance of plastic and stainless-steel dart tags based on shedding rates from a double-tagging experiment. In total, 4648 sharks were tagged, of which 67 were recaptured. Single-barb (SB) and double-barb (DB) plastic tags yielded similar recapture rates, which were almost eight times lower compared to stainless-steel dart tags (M). Shedding rates from recaptured double-tagged sharks were 54%, 8% and 0% for SB, DB and M tags, respectively, with the shedding probability of SB tags positively correlated with time. Double-tagging results provided critical insight into the potential loss of information through tag shedding: the overall recapture rate from this study would drop from 1.4% to 1.0%, and above half of all long-term (≥ 1 year) and large-scale (≥ 1000 km) recaptures would have been lost through shedding of SB tags. This study showcases the utility of double-tagging experiments to assess the performance of different tag types. We conclude that M tags outperform SB tags in retention performance and ease of application, and recommend that future conventional tagging studies focused on large pelagic sharks implement the use of this tag type, as they minimize tag loss through shedding and maximize the probability of obtaining long-term and large-scale recaptures.