|Scientific Name:||Carcharhinus falciformis (Müller & Henle, 1839)|
Aprionodon sitankaiensis Herre, 1934
Carcharhinus atrodorsus Deng, Xiong & Zhan, 1981
Carcharhinus floridanus Bigelow, Schroeder & Springer, 1943
Carcharhinus menisorrah (Müller & Henle, 1839)
Carcharias falciformis Bibron, 1839
Carcharias menisorrah Müller & Henle, 1839
Carcharius falcipinnis Lowe, 1839
Eulamia malpeloensis Fowler, 1944
Eulamia menisorrah (Müller & Henle, 1839)
Gymnorhinus pharaonis Hemprich & Ehrenberg, 1899
Prionodon tiburo (Poey, 1860)
Squalus tiburo Poey, 1860
|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:||Near Threatened ver 3.1|
|Assessor(s):||Rigby, C.L., Sherman, C.S., Chin, A. & Simpfendorfer, C.|
|Reviewer(s):||Lawson, J. & Dulvy, N.K.|
|Contributor(s):||Bonfil-Sanders, R., Amorim, A.F., Anderson, C., Arauz, R., Gonzalez, M., Graham, R., Kyne, P.M., Mancini, P.L., Baum, J.K., Ruiz, C. & Smith, W.|
|Facilitator/Compiler(s):||Kyne, P.M. & Walls, R.H.L.|
The Silky Shark (Carcharhinus falciformis) is an oceanic and coastal-pelagic shark with a circumglobal distribution in tropical waters. It is a target or bycatch species in pelagic tuna longline and purse seine fisheries where it is taken in high numbers. The Silky Shark is one of the three most traded shark species in the global shark fin trade. Estimates of trends in abundance over three generations (45 years) from standardized catch rate indices show declines of Silky Shark in the Eastern Central and Southeast Pacific Ocean and the Atlantic Ocean, and an increase of Silky Shark in the Western Central Pacific and Indian Ocean. Across all three ocean regions, there are the major uncertainties in estimates of catch rate and population changes and an inability to conclusively attribute any declines solely to fishing mortality as there is some potential for environmental influences on catchability and sampling artefacts. The weighted global population trend estimated a 16-22% decline over three generations. This reflects the proportionate contribution of each region’s Silky Shark population change. The estimated level of decline warrants a global status of Near Threatened. This assessment should be revisited as further catch data and stock assessments become available.
|Previously published Red List assessments:|
The Silky Shark has a circumglobal distribution in tropical waters (Last and Stevens 2009). In the Western Atlantic it ranges from Massachusetts, USA, to southern Brazil, including Gulf of Mexico, and Caribbean Sea. The species is also found from St. Paul's Rocks in the Central Atlantic. In the Eastern Atlantic it ranges from Madeira, the Atlantic coast of Spain, and from Senegal to northern Angola. In the Indian Ocean the species occurs off Madagascar, Mozambique, Tanzania, Comoros and Aldabra Island, also from Somalia to the Maldives, Oman, Red Sea, and Sri Lanka. In the Western Pacific, Silky Shark occurs off Thailand, Philippines, New Caledonia, New Zealand, China and Taiwan (Province of China). It also occurs around the Caroline Islands, Phoenix and Line Islands westwards. In the Eastern Pacific, the species ranges from Southern Baja California to Peru. It is also present around the Hawaiian Islands, Cocos Island, Revillagigedo Islands, Clipperton and Malpelos Islands (Marín et al. 1998, Last and Stevens 2009, Ebert et al. 2013).
Native:Angola (Angola, Cabinda); Anguilla; Antigua and Barbuda; Australia (Coral Sea Is. Territory, Northern Territory, Queensland, Victoria, Western Australia); Bahamas; Bangladesh; Barbados; Belize; Benin; Brazil (Rio de Janeiro, São Paulo); Brunei Darussalam; Cambodia; Cameroon; China; Colombia (Colombia (mainland), Colombian Caribbean Is.); Comoros; Congo; Congo, The Democratic Republic of the; Costa Rica (Cocos I., Costa Rica (mainland)); Côte d'Ivoire; Cuba; Djibouti; Dominica; Dominican Republic; Ecuador (Ecuador (mainland), Galápagos); Egypt; El Salvador; Equatorial Guinea (Annobón, Equatorial Guinea (mainland)); Eritrea; French Guiana; Gabon; Gambia; Ghana; Grenada; Guadeloupe; Guatemala; Guinea; Guinea-Bissau; Guyana; Honduras (Honduran Caribbean Is., Honduras (mainland)); Hong Kong; India (Andaman Is., Laccadive Is., Nicobar Is.); Indonesia (Bali, Jawa, Sulawesi, Sumatera); Iran, Islamic Republic of; Jamaica; Kenya; Liberia; Madagascar; Malaysia (Sabah, Sarawak); Martinique; Mauritania; Mexico (Baja California, Campeche, Colima, Guadalupe I., Veracruz, Yucatán); Montserrat; Morocco; Mozambique; Myanmar (Coco Is., Myanmar (mainland)); New Caledonia; New Zealand (North Is., South Is.); Nicaragua (Nicaragua (mainland), Nicaraguan Caribbean Is.); Nigeria; Oman; Pakistan; Panama; Papua New Guinea (Bismarck Archipelago, North Solomons, Papua New Guinea (main island group)); Peru; Philippines; Portugal (Madeira, Portugal (mainland)); Puerto Rico (Puerto Rico (main island)); Saint Lucia; Saint Vincent and the Grenadines; Saudi Arabia; Senegal; Sierra Leone; Somalia; Spain (Canary Is., Spain (mainland)); Sri Lanka; Sudan; Suriname; Taiwan, Province of China; Tanzania, United Republic of; Thailand; Togo; Trinidad and Tobago; Turks and Caicos Islands; United States (Florida, Georgia, Hawaiian Is., Louisiana, Massachusetts, New Jersey, New York, North Carolina, Rhode Island, South Carolina, Texas, Virginia); Venezuela, Bolivarian Republic of (Venezuela (mainland)); Viet Nam; Virgin Islands, British; Western Sahara; Yemen (North Yemen, Socotra, South Yemen)
|FAO Marine Fishing Areas:|
Atlantic – southeast; Atlantic – western central; Atlantic – eastern central; Atlantic – northwest; Atlantic – southwest; Indian Ocean – western; Indian Ocean – eastern; Pacific – western central; Pacific – southwest; Pacific – eastern central; Pacific – southeast
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||The Silky Shark population structure is poorly understood. Genetic studies found that in the Pacific, there are potentially three stocks: one stock in the western Pacific and two stocks in the eastern Pacific (north and south) separated by the equator (Aires da Silva et al. 2014). However, the degree of genetic separation is slight and may not be sufficient to consider them separate subpopulations. Population changes have been noted in all ocean regions but no population estimates are available.|
Eastern Central and Southeast Pacific Ocean
Recent attempts have been made to assess the stocks from the eastern Pacific, although only indicators of population change could be used to infer changes in population due to major uncertainties in catch data from the early years of the time series (1994-1998) that hindered calculation of stock status (Aires da Silva et al. 2014, Lennart-Cody 2015). For the north eastern Pacific stock, standardised catch-per-unit effort (CPUE) estimates from the purse seine fishery on floating objects indicated an overall decline from 1994-2015 that included some periods of stability, a sharp decrease from 2010-2012 followed by a sharp increase from 2012-2015 (Aires da Silva et al. 2014, Lennart-Cody 2015). The cause of the changes from 2010 to 2012 and from 2012 to 2015 are unknown and may be partly due to changes in catchability, rather than abundance, because of environmental effects such as the El Niño events in 2010 and 2015 and La Niña events in 2011-2012 (Lennart-Cody 2015). Additionally, in 1994 the spatial distribution of the purse seine fishery was much narrower compared to subsequent years (Aires da Silva 2014). Inclusion or exclusion of this first year of data makes a large difference to the estimate of CPUE decline over time. From 1994-2015 there was a decline of 32% (based on estimates from Figure 2; Lennart-Cody 2015). To estimate this decline over three generations (45 years), the CPUE data was extracted from Figure 2; Lennart-Cody 2015 using http://datathief.org/ and a regression model applied. If 1994 is included, the estimated three generation decline is 60%. If 1994 is excluded, the estimated three generation decline is 17%. For the southeast Pacific stock there was a sharp decline in standardised CPUE during 1994-2004 followed by stability up to 2013, an increase from 2013-2014 and little change in 2015 (Figure 2; Lennart-Cody 2015). The estimate of decline in CPUE over the period 1994-2015 was 60% (based on Figure 2; Lennart-Cody 2015). Inclusion or exclusion of the 1994 does not make a difference to the estimate of three generation decline which is 99% (based on CPUE data extracted from Figure 2; Lennart-Cody 2015 using http://datathief.org/ and a regression model).
Western Central Pacific and Indian Ocean
A stock assessment for the Western Central Pacific Ocean (WCPO) estimated that the 2009 fishing mortality of Silky Shark exceeded the fishing mortality producing maximum sustainable yield by over four times (Rice and Harley 2013). This level of fishing mortality is estimated to have depleted the total stock biomass by 70% from the theoretical virgin stock biomass and is indicative that overfishing has occurred (Rice and Harley 2013). Estimated spawning biomass declined by 33% from 1995-2009 (Rice and Harley 2013). The estimates of biomass declines are driven by the standardised CPUE series which is mostly derived from WCPO observer data that has limitations of low spatial and patchy temporal coverage (Rice and Harley 2013). A more recent assessment of Silky Shark standardised CPUE in the WCPO extended the data series from 1995 to 2014 (Rice et al. 2015) and reported high fluctuations over 1995-2014 and some decline, though it was not a strong trend as there was a large degree of uncertainty in the CPUE estimates (Rice et al. 2015). As the CPUE trend is different to that used in the stock assessment in Rice and Harley 2013, the more recent CPUE data was used to estimate a three generation change. The CPUE data was extracted from Figure 37, Rice et al. 2015 using http://datathief.org/and a regression model applied. A population increase of 18% over three generations was estimated. In Hawaii, the Silky Shark nominal CPUE had remained stable through 2000 to 2010, indicating a possible stable population in that region (Walsh and Clarke 2011). The Hawaii data represents approximately one third of WCPO Observer data but was not included in the 2015 standardised CPUE estimates as it was only available up to 2011 (Rice et al. 2015). A study from the tropical Central Pacific comparing 1950s and 1990s longline data estimated that the Silky Shark population had declined in abundance by 92% (Ward and Myers 2005). The reduction was likely attributed to the effects of fishing, although without a time-series index it was not possible to discount contributions of variations in oceanographic conditions and biases due to the different areas and gear sampled (Ward and Myers 2005).
In the Indian Ocean there is no stock assessment or any fishery indicators of status, therefore the stock status is highly uncertain (IOTC 2013, Lack et al. 2014).
There is no stock assessment of the Silky Shark in the Atlantic Ocean. Analyses of longline research surveys and observer data from the Gulf of Mexico estimated that the abundance of the Silky Shark population had declined by 91% from 1950-1990 (Baum and Myers 2004). United States pelagic longline observer and logbook data (1992-2005) that encompasses both the northwest and western central Atlantic regions was used to estimate a decrease of 46 and 50% respectively in Silky Shark standardised CPUE (Cortés et al. 2007). The observer and logbook standardised CPUE data was extracted from Figures 6B and 6A respectively; Cortés et al. 2007 using http://datathief.org/ and a regression model applied. Population reductions of 95% and 98% respectively were estimated over three generations. However, Cortes et al. (2007) also reported that relative abundance of Silky Shark appeared to be increasing in the area since 2000 and advised caution in interpreting the catch trends due to short-comings in the data and the highly migratory nature of the Silky Shark that requires a more comprehensive analysis of trends throughout their range. Another analyses of the observer data from this same fishery over 1992-2005 combined catches of Dusky Shark (Carcharhinus obscurus), Silky Shark, and Night Shark (Carcharhinus signatus), grouped because of identification problems, and that the standardized catch rates of this species complex were suspected to have declined by 76% (Baum and Blanchard 2010).
To estimate a global population trend, the three generation population trends estimated from standardized CPUE data for each region were weighted according to the relative size of each region’s surveyed area. This assumed that Silky Shark is evenly distributed throughout its surveyed range. For the Eastern Central and Southeast Pacific, the estimated declines in the north (60% and 17%; with and without 1994 data respectively) and the 99% decline for the south were used (based on Lennart-Cody 2015 data); for the Western Central Pacific and Indian Ocean, the 18% increase was used (based on Rice et al. 2015 data); and in the Atlantic Ocean, as the three generation decline estimates from Cortés et al. 2007 data were so similar, the average of the two estimated declines was used, that is, 97%. The estimated weighted global population trend is a 16-22% decline over three generations. This is a best estimate based on the most reliable data currently available from each region and reflects that regions with large declines represent a relatively smaller area of the total population currently surveyed and that the region of increasing population represents a relatively larger area of the surveyed population. There are areas that have no reliable catch data, such as the Indian Ocean, which should be included in the future as data becomes available.
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||The Silky Shark is a circumtropical oceanic and coastal-pelagic species. It is most often found near the edge of continental and insular shelves at depths of 200 m or more in the epipelagic zone although it occurs from the surface to a depth of at least 500 m offshore (Last and Stevens 2009). It is often associated with islands, near insular slopes and over deepwater reefs preferring warmer waters (about 23°C). Smaller sharks are often found in coastal nurseries and adults further offshore over deeper water. In pelagic habitats, the Silky Shark is often associated with drifting materials on the surface (Filmalter et al. 2013). The Silky Shark generally leaves coastal nursery grounds and moves offshore to oceanic waters as sub-adults, frequently joining tuna schools on which they seem to feed (Branstetter 1987). Ontogenetic diet shifts occur from lower trophic position to higher trophic positions as the Silky Shark matures (Rabehagasoa et al. 2012).|
The Silky Shark gives birth to live young, averaging 5-7 pups per litter with a range of 2-18 pups per litter (Clarke et al. 2015a). Size at birth ranges from 65-81 cm total length (TL) and the gestation period ranges from 9-12 months depending on location and study (Clarke et al. 2015a). Fecundity increases with increasing size of females and females give birth every year, every two years, or sometime in between (Clarke et al. 2015a). Life history parameters vary considerably among regions (Clarke et al. 2105a). Maximum size varies from 229-371 cm TL and size at maturity from 180-230 cm TL for males and 180-246 cm TL for females. Age at maturity ranges from 5-13 years for males and 6-15 years for females with maximum ages of 8-28.6 years for males and 11-35.8 years for females. Generation length is estimated to be 15 years based on the average age of maturity of females as 9 years old and average maximum age of 21 years old (excluding outliers). This is in close agreement with generation length of 16 years in Dulvy et al. (2008).
Eastern Central and Southeast Pacific Ocean
Off the west coast of Baja California Sur, Mexico in the Eastern Central Pacific, maximum size was 316 cm TL and size at maturity of males and females was 180-182 cm (Hoyos-Padilla et al. 2012, Galvan-Tirado et al. 2015). Both males and females were found to mature at 7-8 years old with a maximum age of 14 and 16 years observed for the sampled males and females, respectively (Sanchez-de Ita et al. 2011). In the Gulf of Tehuantepec, Mexico, maximum size was 229 cm TL and was smaller than the maximum size found in more open Mexican waters (Galvan-Tirado et al. 2015).
Western Central Pacific and Indian Ocean
Japanese tuna longline and purse seine fisheries data from across the entire Pacific Ocean indicated a maximum size of 288 cm TL. Males were mature at 180-187 cm TL and 5-6 years, and females at 193-200 cm TL and 6-7 years. Maximum age observed was 8 years for males and 13 years for females (Oshitani et al. 2003). In northeast Taiwan, maximum size was 332 cm TL. Males were mature at 213 cm TL and 9.3 years and females at 210-220 cm TL and 9.2-10.2 years. Maximum age was observed at 14 years for males and 11 years for females with a theoretical maximum age estimated as 28.6 year for males and 35.8 years for females (Joung et al. 2008). In eastern Indonesia, Indian Ocean, the maximum size was 242 cm TL for males and 263 cm TL for females (Hall et al. 2012). Males were estimated to mature at 208 cm TL and at an age of 13 years (Hall et al. 2012). Females matured at 216 cm TL and 15 years of age and reached a maximum size of 263 cm TL (Hall et al. 2012,). Males and females were estimated to have a maximum age of 20 and 19 years, respectively (Hall et al. 2012).
Silky Shark grow larger and mature at later sizes in the northwest Atlantic than in the Eastern Pacific and Western Central Pacific (Bonfil 2008). In the Gulf of Mexico, maximum size was recently reported as 371 cm TL (Serafy et al. 2012). In the Gulf of Mexico, males were mature at 215-225 cm TL and 6-10 years, and females at 232-246 cm and 7-12 years (Branstetter 1987, Bonfil et al. 1993, Bonfil 2008). Maximum age was estimated as at least 22 years (Bonfil 1990). In the southeast United States, females have been estimated to mature at 8 years and reach a maximum age of 27 years (Chen and Yuan 2006). Two studies from the equatorial Atlantic report differences in sizes at maturity that varied from 180-200 cm to 210-230 cm TL for males and from 230 cm to 205-210 cm TL for females (Hazin et al. 2007, Lana 2012).
|Generation Length (years):||15|
|Movement patterns:||Full Migrant|
|Use and Trade:||
The meat of Silky Shark is used for human consumption, the fins are taken for the shark fin trade, the skin has been processed for leather and the liver has been used for liver oil (for its high vitamin A content in this species) (Vannuccini 1999).
The Silky Shark is the second most caught species of shark globally, after the Blue Shark (Prionace glauca) (Oliver et al. 2015). The Silky Shark is both targeted or caught as incidental (bycatch) by longline fisheries and purse seine fisheries (especially those using drifting fish aggregating devices [FADs]) as well as by artisanal fisheries. FADs are made of a floating object and nets that lie vertical in the water column to attract schools of fish. The Silky Shark, as well as other species, is easily entangled in the nets; and there have been large increases in the use of FADs since 1996 (Leroy et al. 2013). Whether they are targeted or an incidental catch, the Silky Shark is often either retained for its meat and fins where regulations allow, or released with high mortality rates apparent in the tropical purse seine fisheries (Hutchinson et al. 2015). Total catches of the Silky Shark reported to FAO are mainly from Sri Lanka (Western Indian Ocean) with the FAO catch less than 4,000 tonnes (t) from 2005-2009 before doubling in 2010 and 2011. Catches then decreased to ~5,000 t in 2012 and 2013 (FAO, 2015).
The Silky Shark was found to represent at least 3-4% of the fins auctioned in Hong Kong, the world's largest shark fin trading center - the third highest after Blue Shark and Hammerhead Shark (general) (Clarke et al. 2006a) - and Hong Kong is thought to make up more than half of the global shark fin trade (Clarke et al. 2004, 2006b). Silky Shark fins are valuable to the trade, although they are not one of the highest value fin types (S. Clarke, unpubl. data).
Eastern Central and Southeast Pacific Ocean
The Silky Shark is taken in pelagic commercial fisheries in the eastern central and southeast Pacific, as well as in coastal artisanal fisheries of the region. Fishing pressure from longline and purse seine fisheries targeting tunas and swordfish is high, and it is the main shark species caught in fisheries using FADs as well as longline fisheries in the eastern Pacific (Oliver et al. 2015). In addition to mortality, FADs may be influencing behaviour of the Silky Shark. In the eastern Pacific, Silky Shark diet contained high proportions of FAD-associated prey items, probably due to the increasing likelihood of an encounter with prey items at FADs (Duffy et al. 2015). If this behaviour continues, Silky Shark may be increasingly attracted to FADs and fishing mortality may be increased.
Mexico contributed the most to catch of the Silky Shark in the north and southeast Pacific, catching up to 40% of the total catch (12-16,000 t in the north and 11-16,000 t in the south per year) (IATTC 2013). The IATTC observer database from 1993-2005 showed that the Silky Shark was caught throughout the ocean west of central America, Colombia, Ecuador, and Peru out to about 170°W on purse seines set on FADs. The Silky Shark was the most commonly caught species of shark in the purse seine fishery for tunas in the eastern Pacific Ocean (IATTC 2007). The fishery has largely increased the use of FADs, which the Silky Shark is attracted to, increasing the catch rate of the Silky Shark in the fishery (Aires da Silva et al. 2014). Additionally, it is mainly juveniles caught North of the equator and caught in FAD sets.
Reported catches of the Silky Shark are approximately four times higher north of the equator in the purse seine fleets (IATTC 2013). In the north, 30-45,000 individuals were caught each year (2006-2010), while in the south, <10,000 individuals were caught each year (IATTC 2013). In the longline sector of the IATTC, annual catches of the Silky Shark ranged from 20-60,000 individuals in the north to 10-30,000 individuals in the south. Catches of the Silky Shark in waters around Mexico have been steadily increasing from the late 1970s, when over 3,000 t were caught to the 2000s, when 6-7,000 t of Silky Shark was caught (IATTC 2013). Central American unreported catch was estimated to be similar to the catch in Mexico of 6-7,000 sharks in recent years (IATTC 2013).
The Silky Shark are a major component of the shark catch in many fisheries along the Pacific coast of Central America. Observations made on a longline research vessel that sampled waters in the Exclusive Economic Zones (EEZs) of Panama, El Salvador, and Guatemala found that Silky Shark constituted 31%, 47% and 29% of the total catch respectively, and 80%, 63% and 44% of the shark catch respectively (Porras 1996). Off Chiapas, southern Mexico, Silky Shark (along with Scalloped Hammerhead Sphyrna lewini) form the large bulk of the shark catch (Soriano-Velásquez and Acal Sánchez 2008). In Tres Marías Islands, Mexico, Silky Shark is a major part of the catch constituting 27% of landed sharks in 1996 (Pérez-Jiménez et al. 2005). In San José and Buena Vista, Guatemala, Silky Shark constituted 73% of recorded catches landed, in a sample of 4,211 sharks, from 1996 to 1999 (Ruiz and Ixquiac 2000).
International longline vessels operating in the eastern central Pacific landed and exported over 8,000 t of shark carcasses and 900 t of shark fins from Costa Rica in 2002 (Costa Rica official INCOPESCA Fishery Statistics 2003), the large majority of which were recorded as the Silky Shark. However, these figures must be interpreted with caution as other sharks may have been misidentified as the Silky Shark. International vessels are known to transship shark products on the high seas, meaning these shipments are unreported. In 1991 in Costa Rica, sharks formed 27% of the total catch (Arauz et al. 2004). In 2000, only 8% of the total catch was sharks, 71% of which were Silky Shark. In 2003, the proportion of sharks in the total catch decreased further to 5% of the total catch, the Silky Shark made up 58% of the shark catch (Arauz et al. 2004). In the Costa Rican longline fishery, most of the Silky Shark catch is composed of juveniles. Since 2004, adults made up less than 15% of the Silky Shark catch (Dapp et al. 2013). Additionally, the total length of individuals caught decreased significantly from 2004 and 2007 to 2008-2010, indicating a decrease in spawning stock biomass (Dapp et al. 2013).
In Manta (the largest fishing port in Ecuador), artisanal fisheries data from September 2003 to the end of 2006 showed over 22,000 Silky Shark were caught (J. Martínez pers. comm. Feb. 2007). From 2008-2012, over 137, 000 individuals were caught throughout Ecuador which is six times the amount of the previous four years (Martínez-Ortiz et al. 2015). The landed females ranged in length from 61-309 cm TL, and the males ranged in length from 63-288 cm TL. In terms of landed weight of the Silky Shark, in Ecuador in the early 2000s, less than 400 t were landed annually, and from 2007-2011, 900-1,300 t per year were landed (IATTC 2013). Catch records from 2007-2008 for nine ports in Ecuador show that catches of Silky Shark was highest in the warmer summer months from March-August (IATTC 2011). Individuals landed were mainly 114-246 cm TL for females and 101-233 cm TL for males with a peak at 160 cm TL for both sexes (IATTC 2011). This peak corresponds to immature individuals, therefore, a majority of the Silky Shark landed in Ecuador is immature.
Western Central Pacific and Indian Ocean
The tropical waters of the Western and Central Pacific Ocean (WCPO) currently support the largest industrial tuna fishery in the world with an annual tuna catch approaching one million tonnes. The two main gear types, longline and purse seine, accounted for over 90% of the target tuna catch in this area in 1999 (Williams 1999). The most prevalent bycatch species found in the longline fishery are Silky Shark, Blue Shark, Pelagic Stingray (Pteroplatytrygon violacea), and the Oceanic Whitetip (Carcharhinus longimanus) (Williams 1999). The most prevalent bycatch species in the purse seine fishery is Silky Shark accounting for 95% of the elasmobranch bycatch in the FAD-associated purse seiners (Hutchinson et al. 2015). Due to the high rate of exploitation in the Indo-Pacific, the Silky Shark population is at risk of genetic diversity loss (Clarke et al. 2015b).
In the WCPO, the longline fishery catches the highest number of Silky Shark with the estimated catch (based on observer data) in 2009 of 189,000 individuals compared to 69,790 individual Silky Shark taken in the same year for the purse seine fishery (Lawson 2011). Once brailed from purse seines, Silky Shark have been shown to be deceased 72% of the time when brought on board (Poisson et al. 2014). They have also been shown in the WCPO purse seine fishery to have a high post-release mortality of greater than 84% (Hutchinson et al. 2015). In the WCPO, longlines and purse seines mainly catch juveniles (Rice and Harley 2013). In the western Pacific, a CPUE for Silky Shark of 1-25 kg per set was found on purse seine sets on tuna schools unassociated with FADs from 1998-2010 (Leroy et al. 2013). On purse seine sets on anchored FADs, the CPUE from 1998-2010 was between 5-50 kg per set and on drifting FADs, the CPUE was 25-60 kg per set (Leroy et al. 2013).
In pelagic longlines based off Hawaii, Silky Shark is caught mainly on deep sea sets. Most of the Silky Shark catch (62.5%) was taken above the equator (0° to 10°N), even though only 3% of fishing was observed above the equator (Walsh and Clarke 2011).
In Taiwan, reported catches of Silky Shark increased by 1,058 t from 2003-2011 (Davidson et al. 2016). This may be due to better reporting. Malaysia does not report catches but does list Silky Shark as a commercial species that is used for fins and meat. Silky Shark is sold locally in Malaysia and prices have been increasing (Lack and Sant 2012). Similarly, Vietnam has no data available on the amount of Silky Shark caught, but it is included in catch reports of sharks (Lack and Sant 2012). In Thailand, Silky Shark is noted as being caught at a “normal” amount, however, there is little training for fisheries officers in identification. This may account for an underestimation of Silky Shark catch (Krajangdara 2014).
Papua New Guinea had a coastal longline shark fishery in which Silky Shark made up >50% of the shark catch (Kumoru 2003). The fishery ceased in June 2014 due to the WCPFC requirement to not land or retain Silky Shark (see Conservation). Over 765 t of Silky Shark was caught in 2001. In 2011, 72% of sharks caught on longlines was Silky Shark (Lack and Sant 2012). The proportion of Silky Shark in the shark longline catch was >50% through 2013-mid 2014 (J. Smart pers. comm 2015). In the Solomon Islands, Silky Shark accounted for 23% of all longline bycatch and 84% of purse seine bycatch (in data collated in 2009; Lack and Meere 2009). The meat is kept for local subsistence and fins are exported. As of 2006, all fin exporters had to be licensed which increased the reliability of reported catches (McCoy 2006). In Fiji, Silky Shark accounted for 13% of longline bycatch and was caught in low levels in the purse seine fishery (2009 collated data; Lack and Meere 2009).
The Silky Shark is a very common component of shark and tuna longline and tuna gillnet fisheries in Indonesia (White et al. 2006). Indonesia’s reported shark catch represents 13% of the global total shark catch. Of this, 15% were reported as Silky Shark from 2001-2004. This represents an average annual catch of 15,943 t of Silky Shark (Lack and Sant 2012). Since 2009, a National Plan of Action-Sharks (NPOA) in Indonesia has been implemented and Silky Shark is listed as a commercial species in the Indonesian NPOA (Lack and Sant 2012). Silky Shark is fished over its entire size range with larger individuals caught on longlines at the beginning of the season and smaller individuals caught in gillnets once the catch rate on longlines decreases (Hall et al. 2012).
Pelagic fisheries have operated in the Indian Ocean for more than 50 years; Japanese long-liners in the eastern Indian Ocean since 1952 and in the western region since 1956. Russian, Taiwanese, and South Korean vessels have fished there since 1954-1966 (Gubanov and Paramonov 1993). Sharks are targeted in several areas, including off India where they are captured using hook and line and in large mesh gillnets, which are among the world’s largest (Anderson and Simpfendorfer 2005). Silky Shark was the most commonly caught shark at FADs in the Indian Ocean during a small pilot study of observers onboard French vessels - 61.5% of Silky Shark was discarded dead at sea, 30.5% partially alive kept onboard, ~7% discarded alive (Viera and Pianet 2006). In a more recent study of FADs in the Indian Ocean, it was estimated that 480,000-960,000 Silky Shark become entangled and subsequently die annually in Indian Ocean FADs (Filmalter et al. 2013). The number of Silky Shark that die by becoming entangled in the FADs was estimated as 5-10 times higher than the number estimated to be taken as bycatch in Indian Ocean FAD-associated purse seine sets (Filmalter et al. 2013). Sharks that were caught in FADs died and fell out within two days, therefore, there is the potential for very high levels of cryptic mortality on FADs (Filmalter et al. 2013). In the Indian Ocean, due to the high abundance of FADs, it was estimated a Silky Shark has a 29% chance of surviving to age one, 9% chance of survival to two years, and only a 3% chance of survival to three years old (Filmalter et al. 2013). On the Southwest coast of India, at Cochin, Silky Shark was recorded as a small portion of landed sharks through 2000-2002. In 2010, their contribution had increased to 16.3% of shark landings at Cochin (Kizhakudan et al. 2013); however, this increased contribution may partly be as a result of potential increase in reporting.
In the Maldives, it was estimated that some 85% of oceanic shark catch comprise Silky Shark (Anderson and Jauharee 2009). Although no specific data were available, Maldivian shark fishermen who specialize in longlining for oceanic sharks anecdotally reported declines in abundance and average size of Silky Shark; all fishers commented that Silky Shark numbers had decreased by >50% in the past 20 years (Anderson and Jauharee 2009). A fishery for Silky Shark has operated off Sri Lanka for many years and Sri Lanka are one of the only countries to report species-specific Silky Shark catch to FAO. The Silky Shark reported landings have decreased in Sri Lanka from 2003 to 2011 by 2,798 t (Davidson et al. 2016).
The Silky Shark is caught by Australian longline fisheries operating on both the east and west coasts (AFMA 2014). Due to management restrictions, total retained catch of sharks by these fisheries is low and Silky Shark is not a major of component of those that are retained. Overall effort on the west coast of Australia is low. Due to the spatial distribution of effort on the east coast (greatest between 25-30°S), few Silky Shark are captured. No assessment of Silky Shark has been undertaken by Australia (although the east coast fishery data was included in the WCPO Silky Shark stock assessment) (Rice and Harley 2013). Australia has a range of management measures in place to mitigate impacts on all sharks. These include trip limits of a maximum of 20 sharks, a ban on the use of wire traces and shark finning bans (Gilman et al. 2007).
The first longline fisheries in the Atlantic were begun by the Japanese fleet in 1956 in the western equatorial waters (Uozumi and Nakano 1996). The fleet expanded rapidly in the 1960s, and covered almost the entire Atlantic by the late 1960s (Bonfil 1994), including the areas currently fished by the American fleet. Fishing pressure in the Atlantic Ocean longline fisheries is high and ongoing. In the United States, Silky Shark is caught by the commercial shark bottom longline and the pelagic longline fishery, and in recreational shark fisheries. In the shark bottom longline fishery in Gulf of Mexico and Southern Atlantic, Silky Shark represent a major byproduct species (Enzenauer et al. 2015). Approximately 75% of individuals caught are retained, mainly comprising small individuals (<110 cm TL). The rest are discarded, of which one third are discarded dead (Enzenauer et al. 2015). Silky Shark is one of the five most captured shark species caught in the Cuban longline fishery (Espinosa 2004).
Silky Shark is taken in several longline fisheries in the Southwest Atlantic, including those off Santos, southern Brazil (Arfelli and Amorim 1994, Amorim et al. 1998), Natal, northeastern Brazil (Hazin et al. 1990) and Uruguay (Marín et al. 1998). The species is taken in artisanal gillnet fisheries off southern Paraná State (Costa and Chaves 2006). In Brazil, Silky Shark is generally retained and marketed (Arfelli and Amorim 1994, Marín et al. 1998). Silky Shark catch in the longline tuna fishery off Brazil increased from 2004-2008 and then declined from 2008-2010 (Fredou et al. 2015). The highest catch rates occurred near the coast, with two exceptions in the central Atlantic. Similar to most catch data of Silky Shark, a high proportion of individuals were immature (Fredou et al. 2015). In another part of the Atlantic, in Spain, the landings of Silky Shark plummeted in the early 2000s from 17 t in 2002 to ~1 t in 2011 (Eurostat 2013).
Silky Shark is a member of the family Carcharhinidae, which is listed on Annex I, Highly Migratory Species, of the UN Convention on the Law of the Sea. They were listed on the Convention on Migratory Species (CMS) under Appendix II in 2014. A Memorandum of Understanding (MOU) was signed by 38 countries in 2010 for migratory sharks, and Silky Shark was added to the MOU in February 2016.
The Silky Shark is likely protected in the exclusive economic zones (EEZs) of countries that have banned all targeting of sharks in commercial fisheries operating in their waters. These countries include Palau, Mariana Islands, Marshall Islands, Federated States of Micronesia, French Polynesia, New Caledonia, Cook Islands, the Maldives, Honduras, the Bahamas, the Marshall Islands, and the British Virgin Islands (Pew 2014). The adoption of shark finning bans by fishing states (e.g., USA, Australia, New Zealand), regional entities (EU) and all regional fisheries management organisations (e.g. ICCAT, IOTC, IATTC, WCPFC) may discourage the capture of oceanic sharks for their fins, because most finning bans require that if fins are to be retained, they must remain attached to the carcass until landed.
Silky Shark retention bans are in place for all vessels operating under ICCAT and WCPFC management (ICCAT 2011, WCPFC 2016). Additionally, any Silky Shark that is brought on board must be released in the best condition possible and as quickly as possible. All interactions are recorded and the status upon release is recorded (alive or dead) (ICCAT 2011, WCPFC 2016). ICCAT has exemptions for developing countries that report the catch of Silky Shark, that have no increase in catch of Silky Shark and ensure that Silky Shark will not enter international trade (ICCAT 2011). The implementation of no retention policies for Silky Shark has likely allowed additional Silky Shark to be released, however Silky Shark have a high level of post-release mortality (>84% in tropical tuna purse seines) and high level of hooking mortality (~56% on tropical longlines) (Hutchinson et al. 2015, Coelho et al. 2012). Post-release survival of Silky Shark was markedly improved in the US pelagic longline fishery in the Atlantic and Gulf of Mexico when circle hooks became mandatory after August 2004. Survival increased from 41% in 1992-2004 to 56% in 2004-2010 (Serafy et al. 2012). Regulations were proposed for Silky Shark in the IATTC for 2015 (IATTC 2015), however they were not approved (http://www.projectaware.org/update/eu-scores-pacific-protections-manta-and-devil-rays). No management arrangements are in place for Silky Shark in the IOTC (Tolotti et al. 2015).
Management of Silky Shark should focus on preventing capture and include small scale measures such as temporal and spatial closures as well as large scale regulations (Tolotti et al. 2015). In the Eastern Pacific, significantly more juveniles are taken above the equator, and Watson (et al. 2009) concluded that larger closures above the equator would reduce the bycatch of Silky Shark of up to 33% while having <10% effect on tuna catch. Smaller closures would have smaller effects on both Silky Shark catch and tuna catch, however, would still reduce juvenile fishing mortality (Watson et al. 2009). Management of Silky Shark is made difficult by the limited knowledge of stock structure and migration patterns.
|Citation:||Rigby, C.L., Sherman, C.S., Chin, A. & Simpfendorfer, C. 2016. Carcharhinus falciformis. The IUCN Red List of Threatened Species 2016: e.T39370A2909465.Downloaded on 17 November 2017.|
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