Movements and habitat use of juvenile silky sharks in the Pacific Ocean inform conservation strategies

Understanding the habitat use and behavior of commercially exploited species throughout ontogeny is useful for devising effective management and conservation strategies. Differences in habitat use can often be exploited to separate target and non-target species, and determinations of home range size can inform the proper scale of conservation actions. In tropical tuna purse seine fisheries in the Pacific Ocean, juvenile silky sharks, Carcharhinus falciformis, comprise a large proportion of the total elasmobranch bycatch. There is now growing recognition of declines in silky shark populations and the need for international collaboration in conservation efforts. Yet, very little is known about the movement behavior or habitat use for this species. In this study, movement behavior of juvenile silky sharks was investigated using pop-up satellite archival tags placed on sharks that were captured during chartered research cruises on a commercial tuna purse seine fishing vessel using drifting Fish Aggregating Devices (FADs) in the western and central Pacific Ocean and on sharks captured using pelagic longlines in the eastern tropical Pacific. Analysis of horizontal and vertical movement behavior revealed silky sharks spend nearly 100% of their time in the shallow, warm waters of the mixed layer. Juvenile silky shark depth and thermal preferences overlapped with the preferred habitat of the primary target tuna species, indicating vulnerability to capture in purse seine and shallow-set longline fisheries throughout the tropical and subtropical regions of the Pacific Ocean where temperatures range between 24 and 29 °C. Reconstruction of horizontal movements showed dispersal between adjacent national jurisdictions and international waters, highlighting the need for international collaborations in the implementation of conservation measures.