Date Published: June 6, 2018
Publisher: Public Library of Science
Author(s): Catarina C. Santos, Rui Coelho, George Tserpes.
The smooth hammerhead shark, Sphyrna zygaena, is a cosmopolitan semipelagic shark captured as bycatch in pelagic oceanic fisheries, especially pelagic longlines targeting swordfish and/or tunas. From 2012 to 2016, eight smooth hammerheads were tagged with Pop-up Satellite Archival Tags in the inter-tropical region of the Northeast Atlantic Ocean, with successful transmissions received from seven tags (total of 319 tracking days). Results confirmed the smooth hammerhead is a highly mobile species, as the longest migration ever documented for this species (> 6600 km) was recorded. An absence of a diel vertical movement behavior was noted, with the sharks spending most of their time at surface waters (0–50 m) above 23°C. The operating depth of the pelagic longline gear was measured with Minilog Temperature and Depth Recorders, and the overlap with the species vertical distribution was calculated. The overlap is taking place mainly during the night and is higher for juveniles (~40% of overlap time). The novel information presented can now be used to contribute to the provision of sustainable management tools and serve as input for Ecological Risk Assessments for smooth hammerheads caught in Atlantic pelagic longline fisheries.
With the rapid expansion of fishing fleets and the increasing exploitation of the open ocean, many marine predators have experienced a decline over the past decades [1, 2]. Among the impacted species, large elasmobranchs (including sharks) have been of particular concern . Pelagic sharks are caught by a variety of fishing gear and are common as bycatch of pelagic longline fleets targeting mainly swordfish (Xiphias gladius) and tunas (Thunnus spp.) [4–8]. Since predators play a major role in marine communities’ structure and function, widespread decline of sharks across the world’s oceans are expected to strongly influence the equilibrium of marine ecosystems [6, 9]. Therefore, understanding habitat use and ecology of sharks is crucial to evaluate the impacts of fishing on them and throughout the food web. Additionally, for many pelagic shark species, important information on their life history and ecology is still missing, as well as survey or reliable catch data, which hinders higher level scientific-based management advice.
Understanding habitat preferences and vulnerability of smooth hammerhead sharks to fisheries is crucial to ensure successful species conservation strategies and effective management measures. The present work represents the most continuous recording of the movements and habitat use of smooth hammerhead sharks in the Atlantic Ocean. In this study, we were able to tag and track both juvenile and adult sharks, and the differences between maturity stages were analyzed and reported for the first time.