Vertical and horizontal movements of a silvertip shark (Carcharhinus albimarginatus) in the Fijian archipelago

Satellite telemetry is a widely used tool to define horizontal and vertical movements
of elasmobranch species across a variety of environments 1], 2]. Studies have described critical habitats, redefined management strategies, and gathered
important life-history data about these at-risk predators 3], 4]. However, very little information exists about pelagic sharks in the South Pacific
despite the region’s exportation of substantial quantities of shark fins to the Hong
Kong market. Fiji exported 76,634 kg of dried and frozen fins in 2011, though these
estimates include fish not necessarily caught in Fijian waters 5]. Fijian communities and villages are generally not targeting sharks on nearshore
reefs as no domestic market exists 6], 7]. Considering the gear types (longlines and gillnets) used by larger vessels to harvest
sharks in large quantities, it is logical that fins harvested in Fijian waters are
primarily from pelagic species.

The Republic of Fiji is home to several shark species that spend at least part of
their life history associated with reef habitats. Whitetip reef (Triaenodon obesus), grey reef (Carcharhinus amblyrhyncos), blacktip reef (Carcharhinus melanopterus), bull (Carcharhinus leucas), and silvertip (Carcharhinus albimarginatus) sharks are commonly observed species on Fiji’s reefs 8], 9]. The former four species are also known to make nearshore slope or pelagic excursions
10], 11]. There are no current studies defining movements of pelagic sharks in the Fijian
archipelago. Gathering fundamental ecological information, such as vertical and horizontal
movement data obtained through satellite tagging, is the critical first step towards
appropriate management of species in this area.

The silvertip shark is a slow-growing requiem shark, with a maximum recorded total
length (TL) of 300 cm. Males and females reach maturity between 160–180 and 160–200 cm
TL, respectively 11], 12]. Although widely distributed throughout the tropical Indian and Pacific Oceans, populations
appear fragmented with minimal dispersion 10]. The silvertip shark inhabits coastal and offshore waters, and individuals are most
commonly observed near coral reefs and nearshore reef slopes from the intertidal zone
to depths of 600–800 m 10], 11], 14]. Juveniles inhabit shallow nearshore waters, such as lagoons. In contrast, adults
occupy a larger range of habitats 10], 14]. Few studies have examined the horizontal movements of silvertip sharks. At Osprey
Reef, Australia, silvertips demonstrated both year-round residency to this isolated
seamount and unidirectional migrations, briefly being detected on neighboring reefs
(~14 km away) 14].

Silvertip sharks are an active species with a diet consisting of benthic and pelagic
teleosts, eagle rays, other sharks, and cephalopods 15]. A calculated trophic position of 4.2 traditionally classifies silvertip sharks as
an apex predator 15].

The conservation status of silvertip sharks is undecided throughout most of its range,
but based on available data, the species is currently Red-listed and classified as
‘Near Threatened’ by the International Union for Conservation of Nature (IUCN) 14]. Globally, silvertips are one of nine prominent shark species landed by high seas
longlines and net fleets, either intentionally or as by-catch, and are identified
in the Hong Kong fin trade 14], 16]. Reports of severe localized declines in Australia and Chagos, high catch rates in
commercial and artisanal fisheries in South Africa, Indonesia, Thailand, Myanmar,
Cambodia and the Philippines, coupled with its habitat-faithful nature suggest isolated
populations are vulnerable to overexploitation 14], 17], 18]. A 1996 shark survey conducted in the Chagos Archipelago noted severe declines in
sightings of silvertip sharks when compared to 1970 findings 18]. Given the species’ susceptibility to line capture, this decline was attributed to
fishing pressure 18]. However, an increase in silvertip shark abundance was recorded during a 2006 survey,
demonstrating the potential time-scales at which isolated populations can be reduced
by over-fishing and show signs of recovery 19]. Though no current population data exist for silvertip sharks within the Fijian archipelago,
the broader western Pacific tuna longline fishery reports high levels of silvertip
by-catch 20]. Additional data may provide a basis for the reclassification of this species by
the IUCN.

The cultural and governance structure of the Fijian islands offers unique protection
to nearshore reefs and their associated species (including several shark species),
which are locally managed by coastal villages that reside in the immediate area 7]. Community-based management or locally managed marine areas (LMMAs) have created
de facto protection for certain reef-associated shark species, whereas offshore waters
are, conversely, heavily fished by foreign longlining fleets 20]. It is unclear how much protection the silvertip shark obtains from Fijian LMMAs,
as silvertips are suspected to inhabit shelf or pelagic waters during some life stages
and/or routine movements 9], 10], 14]. A long-term assessment of shark relative abundance at an established Fijian provisioning
site, observed seasonality to silvertip presence, being most abundant between September
and December 8].

The lack of basic life-history data, the prevalence of silvertip sharks in the international
fin trade 16], and recent discussions by the IUCN to relist the species as “vulnerable” 14] give merit to an ecology-focused satellite tagging study. Understanding water column
use and the potential possibility to correlate such findings with fisheries-dependent
data will be a vital step in protecting this species. The aim of this study was to
describe the silvertip’s short-term depth and temperature use to obtain critical habitat
data of this poorly understood species.