Vertical space use and thermal range of the great hammerhead (Sphyrna mokarran), (Ruppell, 1837) in the western North Atlantic

The great hammerhead (Sphyrna mokarran) is a highly mobile, large-bodied shark primarily found in coastal-pelagic and semi-oceanic waters across a circumtropical range. It is a target or by-catch species in multiple fisheries, and as a result, rapid population declines have occurred in many regions. These declines have contributed to the species being assessed as globally critically endangered on the IUCN Red List. Although conservation and management measures have yielded promising results in some regions, such as the United States, high levels of at-vessel and post-release mortality remain a major concern to the species population recovery. This examined the vertical space use and thermal range of pop-off archival satellite-tagged S. mokarran in the western North Atlantic Ocean, expanding the understanding of the ecological niche of this species and providing insight into by-catch mitigation strategies for fisheries managers. The results showed that S. mokarran predominantly used shallow depths (75% of records <30 m) and had a narrow temperature range (89% of records between 23 and 28 degrees C). Individual differences in depth use were apparent, and a strong diel cycle was observed, with sharks occupying significantly deeper depths during the daytime. Furthermore, two individuals were confirmed pregnant with one migrating from the Bahamas to South Carolina, U.S.A., providing further evidence of regional connectivity and parturition off the U.S. East Coast. The findings suggest that S. mokarran may be vulnerable to incidental capture in the western North Atlantic commercial longline fisheries due to substantial vertical overlap between the species and the gear. The results can be incorporated into conservation and management efforts to develop and/or refine mitigation measures focused on reducing the by-catch and associated mortality of this species, which can ultimately aide S. mokarran population recovery in areas with poor conservation status.

Automated summary

The great hammerhead shark is primarily found in coastal-pelagic and semi-oceanic waters across a circumtropical range and is critically endangered globally. Research shows that they predominantly use shallow depths and exhibit a strong diel cycle with significant vertical variation. These findings can aid in developing conservation and management efforts to reduce by-catch and mortality of this species.