|Scientific Name:||Carcharhinus albimarginatus (Rüppell, 1837)|
Carcharhinus platyrhynchus (Gilbert, 1892)
Carcharias albimarginatus Rüppell, 1837
Eulamia albimarginata (Rüppell, 1837)
Eulamia platyrhynchus Gilbert, 1892
|Taxonomic Source(s):||Weigmann, S. 2016. Annotated checklist of the living sharks, batoids and chimaeras (Chondrichthyes) of the world, with a focus on biogeographical diversity. Journal of Fish Biology 88(3): 837-1037.|
|Red List Category & Criteria:||Vulnerable A2bd ver 3.1|
|Assessor(s):||Espinoza, M., González-Medina, E., Dulvy, N.K. & Pillans, R.D.|
|Reviewer(s):||Lawson, J. & Simpfendorfer, C.|
|Facilitator/Compiler(s):||Kyne, P.M. & Walls, R.H.L.|
The Silvertip Shark (Carcharhinus albimarginatus) has a wide but patchy distribution associated with coral reefs and islands throughout the tropical Indian and Pacific Oceans. It is a large, slow-growing species with an estimated generation length of 22 years. The Silvertip Shark is subjected to high bycatch levels in high seas fisheries and in artisanal longline, gillnet, and trawl fisheries throughout its range, with its meat and fins marketable. Surveys of fish markets in Indonesia and Papua New Guinea suggest that the Silvertip Shark has undergone large declines in those waters. There is evidence that Indonesian fisheries have considerably reduced the numbers of the Silvertip Shark within the Memorandum of Understanding [MOU] 1974 Box in the Timor Sea. Catches of sharks in this area declined throughout the early 1990s to the level that Indonesian shark fishing vessels have become relatively uncommon. In the Indian Ocean there was an estimated decline in the abundance of several reef shark species (including the Silvertip Shark) of over 90% in the period between 1970 and 2006 and around the US Pacific Islands common reef sharks have declined to 3-10% of baseline levels. This species' site-specificity, patchy population, and life history characteristics indicate that remote populations that are not currently managed are highly susceptible to depletion from direct shark fisheries and illegal practices. This information, combined with actual and potential levels of exploitation throughout its range results in a global assessment of Vulnerable, based on suspected overall population decline of greater than 30% over three generations (66 years) inferred from survey data. This assessment should be revisited when reliable catch data become available. In Australia, catch and survey data from the Great Barrier Reef suggest that this species is fairly common. The Silky Shark has limited interaction with fisheries operating in tropical Queensland and the Northern Territory an therefore, the species is listed as Least Concern in the regional Australian assessment.
|Previously published Red List assessments:|
The Silvertip Shark has a tropical Indo-Pacific distribution that is wide ranging but fragmented (Ebert et al. 2013). In the Western Indian Ocean, the Silvertip Shark occurs in the Red Sea, off South Africa, Mozambique, Kenya, Madagascar, Aldabra Islands, Mauritius, Seychelles, and Chagos Archipelago (Ebert et al. 2013). In the Western Central Pacific, it occurs off Indonesia, Taiwan, Guam, New Caledonia, Philippines, Palau, Marshall, Solomon and Phoenix Islands, Tahiti, Tuvalu, and Papua New Guinea (Ebert et al. 2013). It also occurs in northern Australian waters from Carnarvon (Western Australia) to Bundaberg (Queensland), with the exception of the Gulf of Carpentaria and Arafura Sea (Jones et al. 1991, MIRC 2007, Last and Stevens 2009). In the Eastern Central Pacific, the Silvertip Shark is known from southern Baja California, south to Guatemala and Colombia, including the Revillagigedo and Clipperton Islands, Cocos Island and Galapagos Islands (Ebert et al. 2013). The species possibly also occurs in the Western Central Atlantic (Mexico, Gulf of Mexico, and Caribbean Sea; Ebert et al. 2013), although its presence in that region is unconfirmed (Grace 2001).
Native:Australia (Northern Territory, Queensland, Western Australia); British Indian Ocean Territory; Cocos (Keeling) Islands; Colombia; Costa Rica (Cocos I.); Ecuador (Galápagos); Egypt; Eritrea; French Polynesia (Society Is.); Guam; Guatemala; Indonesia (Kalimantan, Sulawesi); Kenya; Kiribati (Phoenix Is.); Madagascar; Marshall Islands; Mauritius; Mexico (Baja California Sur, Revillagigedo Is.); New Caledonia; Palau; Papua New Guinea; Philippines; Saudi Arabia; Seychelles (Aldabra, Seychelles (main island group)); Solomon Islands (Santa Cruz Is., South Solomons); South Africa (KwaZulu-Natal); Sudan; Taiwan, Province of China; Tuvalu; Yemen (North Yemen, South Yemen)
|FAO Marine Fishing Areas:|
Indian Ocean – eastern; Indian Ocean – western; Pacific – northwest; Pacific – southeast; Pacific – eastern central; Pacific – western central
|Range Map:||Click here to open the map viewer and explore range.|
The Silvertip Shark population appears to be fragmented with apparently low potential for interchange between localised stocks. Surveys of fish markets in Indonesia and Papua Guinea suggest that the Silvertip Shark has undergone large population declines in those waters (W. White, CSIRO, 02/2015, pers. comm). For example, a five-year survey of Indonesian fish landing sites only found 95 Silvertip Shark individuals out of a total of more than 21,000 sharks recorded (White 2007). Based on diving survey data, Graham et al. (2010) showed that the number of reef sharks observed per dive in Chagos Archipelago (Indian Ocean) declined from a mean of 4.2 in 1970 to 0.4 in 2006, which represents a decline of over 90% across 36 years. Species-specific data only included the period from 1996 to 2006, in which the mean number of the Silvertip Shark observed per dive actually increased from 0.03 sharks per dive to 0.12 (Graham et al. 2010). However, underwater surveys conducted between 2004 and 2010 across a much broader area of the Central Western Pacific Ocean, that is, around 46 United States Pacific Islands, estimated that common reef sharks have declined to 3-10% of baseline levels around inhabited islands (Nadon et al. 2012). Baseline was considered to be the absence of humans (Nadon et al. 2012) and theoretically could be considered to be at least three generation lengths of Silvertip Sharks (66 years). Given this large decline in the abundance of reef shark species around inhabited islands and the high exploitation levels from fisheries operating in the Indo-Pacific region, it is suspected that the Silvertip Shark have experienced population reductions of more than 30% across the Indo-Pacific Ocean over three generations.
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||The Silvertip Shark occurs on the continental shelf, offshore islands, coral reefs, and offshore banks, from surface waters to depths of 600-800 m (Compagno et al. 2005). It is also found inside lagoons, near drop-offs, and offshore (Compagno et al. 2005). The species appears to be relatively site-specific for long periods with limited dispersion, particularly at remote and isolated coral reef habitats around tropical islands or atolls (Barnett et al. 2012, Espinoza et al. 2015). On the continental shelf, larger individuals of the species tend to be more mobile and use larger areas than other common reef shark species (Barnett et al. 2012, Espinoza et al. 2015). The Silvertip Shark also exhibits strong diel behaviour, moving closer to coral reefs and using shallower habitats at night.|
Reproduction is viviparous, with a yolk sac placenta (Compagno et al. 2005, White et al. 2006, White 2007). Females give birth to 1-11 pups per litter (average six) biennially, after a 12 month gestation period (Compagno et al. 2005, White et al. 2006, Last and Stevens 2009). Size at birth is reported at 63-68 cm total length (TL) (Compagno et al. 2005) and 73-81 cm TL (White et al. 2006). Young are found in shallow water closer to shore, whereas adults are more wide-ranging (Compagno et al. 2005). Compagno et al. (2005) report that males mature at 160-180 cm TL and females at 160-199 cm TL. White et al. (2006) report that males mature at 190-200 cm TL and females at ~195 cm TL. Growth appears to be fairly slow at about 9 cm per year for juveniles (Last and Stevens 2009). It reaches a maximum size of 300 cm TL (Compagno et al. 2005). No estimate of generation length is available for the species, although it can be inferred from its congener, the Pigeye Shark (C. amboinensis), which has a similar maximum size and an estimated generation length of 21.5 years (Tillett et al. 2011).
|Generation Length (years):||21.5|
|Use and Trade:||The Silvertip Shark is utilized for its fins, meat, cartilage, liver, teeth, jaws, and skin (SEAFDEC 2006). The meat, teeth, and jaws are generally sold locally, whereas fins, skin, and cartilage are generally exported.|
The Silvertip Shark is a bycatch in high seas fisheries and in artisanal longline, gillnet, and trawl fisheries throughout its range. The number of pelagic sharks landed by fishing fleets in all oceans has increased in economic importance (Mejuto et al. 2006). However, catch statistics are generally not available (Holts 1988, Smith et al. 1998) and where they are, they are under-reported. The Silvertip Shark is one of the nine main species landed by high seas longline and net fleets. The majority of these fleets target tunas in all of the world's oceans and as a result have a large bycatch of pelagic sharks (Fowler et al. 2005); for example, the species is a known bycatch of western Pacific tuna fleets (Ward et al. 2004). The Silvertip Shark was not considered in Clarke et al.'s (2006a) analysis of the global shark fin trade, although fins of the species have been identified in the trade (Clarke et al. 2006b).
Coral reef-associated species such as the Silvertip Shark are important to fisheries in Eritrea, Sudan, Egypt, Saudi Arabia, Yemen, the Maldives, and Chagos Archipelago (500 km south of the Maldives in the Indian Ocean), where reefs dominate coastal habitats (Anderson and Simpfendorfer 2005). In this region elasmobranchs are most commonly taken as bycatch in non-target fisheries or catchall artisanal fisheries (Anderson and Simpfendorfer 2005). Finning and discarding of carcasses has been reported, especially in offshore and high seas fisheries targeting tuna (Anderson and Simpfendorfer 2005). The Silvertip Shark from the Chagos Archipelago is taken by the Mauritian Inshore Fishery, which targets grouper (subfamily Epinephelinae) and the Deepwater Snapper (Etelis carbunculus) and has operated in these waters since the early 1970s (Mees 1996). There is also an open-water fishery targeting tuna that commonly catches reef-associated sharks incidentally (Graham et al. 2010). Despite conservation measures in place in the Chagos Archipelago (see the Conservation section), illegal fishing is suspected to be a common activity in remote locations.
The Silvertip Shark is landed in local markets in Indonesia, Papua New Guinea, Myanmar, and the Philippines (Kumoru 2003, SEAFDEC 2006, White 2007). In the Philippines, it is in the top 10 most landed species by number (0.73%) and weight (2.6%) with individuals ranging in size from 210-240 cm TL and averaging 23 kg in weight (SEAFDEC 2006). In Papua New Guinea, the Silvertip Shark was taken in the longline shark fishery that operated close to seamounts and non-emergent reefs (Kumoru 2003). Based on observer data, the Silky Shark (C. falciformis) and the Oceanic Whitetip Shark (C. longimanus) dominated the shark catch (58%), but the Silvertip Shark (6.2%) was also taken in significant numbers (Kumoru 2003). Most Silvertip Shark individuals caught in this fishery were retained (88%) and the fins and meat marketable (Kumoru 2003). The fishery ceased in June 2014 due to the Western and Central Pacifc Fisheries Commission (WCPFC) requirement to not land or retain Silky Shark.
There is evidence from northern Australia that finning can cause localised depletion of shark populations. Remote populations of Silky Shark are likely highly sensitive to target fisheries for meat or fins, particularly given their limited dispersal and localised movement patterns (Barnett et al. 2012). Acoustic and baited camera survey techniques were used to census shark abundance at two northern Australian reefs: Mermaid Reef in Rowley Shoals (a Commonwealth Marine Protected Area closed to all fishing) and Scott Reef (within the Memorandum of Understanding [MOU] 1974 Box in the Timor Sea, where access by Indonesian fishers using traditional artisanal fishing techniques is permitted). Shark abundance was an order of magnitude higher on Mermaid Reef (Meekan and Cappo 2004). The Silvertip Shark, the main target of shark finning fleets in this area, was common on Mermaid Reef and absent at Scott Reef (Meekan and Cappo 2004). Fishing pressure is the most plausible explanation for differences in the composition and abundance of shark assemblages between Mermaid and Scott Reefs. Sharks preferentially targeted by fishermen, such as hammerheads (Sphyrna spp.) and the Silvertip Shark were absent from counts at Scott Reef. Furthermore, catches of reef sharks in the local area (MOU74 Box) declined throughout the early 1990s to the point that Indonesian shark fishing vessels have become relatively uncommon in this area (Wallner and McLoughlin 1996, Fox and Sen 2002, Ruppert et al. 2013). Several initiatives are underway to identify which species are being taken and in what quantities.
In the Great Barrier Reef (GBR), Queensland, Australia, the Silvertip Shark is not targeted by fisheries, but very low numbers are taken incidentally by commercial and recreational line fisheries that target Coral Trout (Plectropomus leopardus; Heupel et al. 2009). This may be a result of its strong diel and foraging behaviours around coral reefs at night when little fishing activity occurs (Espinoza et al. 2015). Conversely, poor reporting of species-specific shark landings may occur (Heupel et al. 2009), a common pattern observed in many fisheries where untargeted elasmobranch bycatch typically goes unidentified and/or unreported (Walsh et al. 2002). In addition, although most of the shark catch associated with the Coral Trout line fishery is not retained, sharks that interact with line fisheries may break off before landing or are released bearing hooks and traces, and it is unclear how this may affect the health and survival of Silvertip Sharks (Gallagher et al. 2014). However, catch-per-unit effort data from the period between 1989 and 2006 showed no evidence of increase or decline of sharks associated with the line fishery along the GBR (Heupel et al. 2009). The Silvertip Shark is not known to interact with the inshore gillnet fishery that operates in Queensland waters (Harry et al. 2011).
A ten-year survey (2000–2010) using baited remote underwater video stations (BRUVS) along the entire GBR showed that the Silvertip Shark was the second most commonly sighted species (12.2%) (Espinoza et al. 2014). This survey showed that the species is restricted to offshore coral reef habitats in the GBR. An examination of the GBR Marine Park Zoning showed higher abundances of the Silvertip Shark in reefs that were closed to fishing. Moreover, higher abundances of the species were observed in protected areas that had more and healthier coral cover, suggesting that the loss and degradation of coral reefs may negatively affect their populations (Espinoza et al. 2014). BRUVS data from 2004-2010 suggest that the abundance of the Silvertip Shark has not changed since the new re-zoning of the GBR Marine Park in 2004, despite an increase in the coverage of protected areas (Espinoza et al. 2014).
Currently there are no species-specific management measures in place. In the Chagos Archipelago, since 2006 all fishing vessels were required to report shark bycatch, finning practices were banned, and the use of wire trace was prohibited. There was also a British Indian Ocean Patrol Vessel patrolling this Archipelago to detect illegal shark fisheries, mainly from Sri Lanka (Anderson et al. 1998). However, illegal fisheries still operate in these areas as these measures are largely unenforced. Catch levels should be quantified and monitored throughout the range of the species. Silvertip Shark populations in isolated coral reef habitats subject to fishing pressure may benefit from creation of Marine Protected Areas, because their limited dispersal from those habitats likely makes them more susceptible to depletion (Compagno et al. 2005, Barnett et al. 2012). Enforcement of regulations related to fisheries catch and landings, such as finning regulations and size limits, would be the most beneficial conservation measure for the species.
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|Citation:||Espinoza, M., González-Medina, E., Dulvy, N.K. & Pillans, R.D. 2016. Carcharhinus albimarginatus. The IUCN Red List of Threatened Species 2016: e.T161526A68611084.Downloaded on 20 September 2018.|
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