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

Abstract

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°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.

Depredation rates and spatial overlap between Great Hammerheads and Tarpon in a recreational fishing hot spot

“…2017; Guttridge et al. 2017, 2022; Calich et al. 2018), but seasonal residency and philopatry have also been documented in Bimini, The Bahamas, during the winter months (Guttridge et al.…”

“…org/ sedar -77). They are capable of extensive annual migrations (Graham et al 2016;Chin et al 2017;Guttridge et al 2017Guttridge et al , 2022Calich et al 2018), but seasonal residency and philopatry have also been documented in Bimini, The Bahamas, during the winter months (Guttridge et al 2017). The Great Hammerheads in this study exhibited similar levels of philopatry, with multiple individuals detected in Bahia Honda for two to three spawning seasons over the 3-year study period.…”

Genomic population structure of great hammerhead sharks (Sphyrna mokarran) across the Indo-Pacific

“…Further, S. mokarran reportedly use shallow coastal environments to target prey (Chapman and Gruber 2002;Doan and Kajiura 2020) and extend across both pelagic and oceanic shelf waters (Queiroz et al 2016;Raoult et al 2019). Guttridge et al (2022) also studied the thermal and vertical range of the species, noting they primarily use shallow depths (75% at <30 m) and occupy warm waters (89% between 23 and 28°C). This range reflects the possibility for significant connectivity due to the <100 m of water connecting the Gulf of Carpentaria to the Timor Sea, Arafura Sea, Torres Strait and Gulf of Papua (National Oceanic and Atmospheric Administration 2022) as well as the continuous shallow continental shelf along Qld's Great Barrier Reef (Dudgeon et al 2009).…”

Bringing down the hammer on bycatch

“…In the exciting study by Guttridge et al (2022), pop‐off archival satellite tags were used on 18 hammerheads, with most of the tags transmitting records every 5 minutes for a programmed 11‐270 days. The results identified that the hammerheads predominantly used shallow depths, with 75% of the records registering shallower than 30 metres.…”