Vertical movements of a pelagic thresher shark (Alopias pelagicus): insights into the species’ physiological limitations and trophic ecology in the Red Sea

Abstract

The pelagic thresher shark Alopias pelagicus is an understudied elasmobranch harvested in commercial fisheries of the tropical Indo-Pacific. The species is endangered, overexploited throughout much of its range, and has a decreasing population trend. Relatively little is known about its movement ecology, precluding an informed recovery strategy. Here, we report the first results from an individual pelagic thresher shark outfitted with a pop-up satellite archival transmitting (PSAT) tag to assess its movement with respect to the species’ physiology and trophic ecology. A 19 d deployment in the Red Sea revealed that the shark conducted normal diel vertical migration, spending the majority of the day at 200-300 m in the mesopelagic zone and the majority of the night at 50-150 m in the epipelagic zone, with the extent of these movements seemingly not constrained by temperature. In contrast, the depth distribution of the shark relative to the vertical distribution of oxygen suggested that it was avoiding hypoxic conditions below 300 m even though that is where the daytime peak of acoustic backscattering occurs in the Red Sea. Telemetry data also indicated crepuscular and daytime overlap of the shark’s vertical habitat use with distinct scattering layers of small mesopelagic fishes and nighttime overlap with nearly all mesopelagic organisms in the Red Sea as these similarly undergo nightly ascents into epipelagic waters. We identify potential depths and diel periods in which pelagic thresher sharks may be most susceptible to fishery interactions, but more expansive research efforts are needed to inform effective management.

Environmental cycles and individual variation in the vertical movements of a benthic elasmobranch

“…DVM is widely documented among elasmobranchs, especially sharks (Carey et al 1990;Andrews et al 2009;Arostegui et al 2020). While flapper skate occupied a range of depths over the diel cycle, this study adds to the evidence base for DVM in flapper skate (Wearmouth and Sims 2009;Pinto and Spezia 2016) and benthic predators more widely (Sims et al 2006;Gorman et al 2012;Harrison et al 2013;Peklova et al 2014;Cott et al 2015;Humphries et al 2017).…”

“…In line with recently documented seasonal trends in average depth use (Thorburn et al 2021), there is a substantial seasonal shift in diel vertical movement patterns, with normal DVM and elevated nocturnal activity weakening in summer when skate tend to be less active and spend more time in deeper water. This points towards solar light as an important correlate of vertical movements and may have implications for their vulnerability as bycatch in other parts of their range (Bendall et al 2017), as noted for Arctic skate (Amblyraja hyperborea) (Peklova et al 2014) and other elasmobranchs (Gilman et al 2019;Siskey et al 2019;Arostegui et al 2020). There is also evidence for a role for individual variation in movement, alongside ontogenetic segregation in depth use documented previously (Thorburn et al 2021).…”

“…In other elasmobranchs, including batoids, DVM has been attributed to thermoregulation and bioenergetic efficiency (Matern et al 2000;Sims et al 2006;Papastamatiou et al 2015) and foraging (Andrews et al 2009;Arostegui et al 2020). For flapper skate, given elevated DVM in winter when vertical temperature gradients are minimal, a role for foraging seems most likely.…”

“…Over short timescales, many elasmobranchs exhibit tidal-driven movement, moving into shallower waters with incoming tides to minimise energy expenditure or predation risk or to forage (Ackerman et al 2000;Wetherbee et al 2007;Starr 2009, 2010). Over diel scales, solar light levels are often linked to vertical movement via DVM, with crepuscular or nocturnal movement into shallower or deeper water (normal and reverse DVM, respectively), and changes in vertical activity (Carey et al 1990;Andrews et al 2009;Doherty et al 2019;Arostegui et al 2020). In many cases, these movements are associated with prey availability, but links to thermoregulation and bioenergetic efficiency are also documented (Matern et al 2000;Sims et al 2006;Papastamatiou et al 2015).…”

Extensive oceanic mesopelagic habitat use of a migratory continental shark species

“…While many oceanic predators exploit mesopelagic prey resources through deep diving excursions into corresponding depth layers, others have been shown to engage in regular diel vertical migrations, tracking their prey. Such DVM has been shown for a variety of large predatory ecto-and endothermic shark species [13][14][15][16][17][18] , and distinct diel patterns also occur in large, filter-feeding sharks like basking sharks (Cetorhinus maximus) and the megamouth shark (Megachasma pelagios)-noting that only the latter seems to fully exploit the total extension of deep scattering layers [19][20][21] .…”