Development of new methods for attaching pop-up satellite archival tags to blue sharks (Prionace glauca)

Abstract

Pop-up satellite archival tags (PSATs) have been widely used to study the behavior, habitat selection, and survival rates of sharks. Longitudinal data on shark behavior is needed to understand seasonal migration, spatiotemporal shifts of distribution with the ontogenetic/reproductive stage and the long-term impact of anthropogenic activities. Therefore, long-lasting techniques to secure PSATs to the body of the subject with little harm to its survival are essential. We developed four methods for externally attaching PSATs on the deck to blue sharks (Prionace glauca) in the northwestern Pacific and evaluated the performance of several different methods in terms of tag retention and post-release mortality. Analysis of time-to-event data suggested differences in tag retention rates (here, ‘survival rates’) among the attachment methods. Our Method C, whereby an umbrella-type dart was inserted into the dorsal musculature, penetrating the vertical septum, and the tag body was positioned along anteroposterior body axis and secured to the first dorsal fin base using a cable tie, provided the best performance among the four methods in terms of greater restricted mean survival time (RMST), higher rate of days at liberty, moderate post-release mortality, and less time required for attachment. A meta-analysis based on data published to date indicated that the ‘one-point attachment’ method has been the main approach with blue sharks, and that the tag survival rate differs largely depending on the type of dart head and tether configuration even in the same approach. The RMST from Method C was slightly greater than that with one-point attachment method with an ‘umbrella dart–monofilament or fluorocarbon tether’ combination, which was the most frequently used across previous studies, but the one-point attachment method with a ‘Wilton dart (large or small) or titanium dart–stainless steel tether’ combination outperformed methods compared. Combining available information on the biology of this species with the results of the current study, we propose the following procedure as the most effective to decrease drag and prevent swinging of the tag: pretreat the tag with an anti-biofouling agent; position the tag along the anteroposterior axis of the shark's body; allow the dart to penetrate the vertical septum of the subject; and, additional fixation of the tag to the dorsal fin. Although further data are needed to evaluate some aspects (e. g., dart-head type and tagging location), application of appropriate steps depending on the situation will increase the probability of obtaining long-term data for this highly migratory species.