|Scientific Name:||Carcharhinus falciformis|
|Species Authority:||(Müller & Henle, 1839)|
Carcharias falciformis Müller & Henle, 1839
|Red List Category & Criteria:||Near Threatened ver 3.1|
|Assessor(s):||Bonfil, R., Amorim, A., Anderson, C., Arauz, R., Baum, J., Clarke, S.C., Graham, R.T., Gonzalez, M., Jolón, M., Kyne, P.M., Mancini, P., Márquez, F., Ruíz, C. & Smith, W.|
|Reviewer(s):||Stevens, J.D., Dudley, S., Pollard, D., Valenti, S.V., Fowler, S.L. & IUCN SSG Pelagic Shark Red List Workshop participants (Shark Red List Authority)|
This oceanic and coastal-pelagic shark is circumglobal in tropical waters, where it dominates as a target species or bycatch in certain pelagic fisheries, particularly purse seines on drifting FADs (fish aggregating devices). The Silky Shark (Carcharhinus falciformis) has a generation period of 11 years and is significantly less resilient to fisheries than Blue Shark (Prionace glauca). It is vulnerable to a wide variety of pelagic fisheries, and is taken in large numbers, but there are no population estimates and most catches are unreported. It is highly associated with seamounts and is the dominant shark in tuna purse seine fisheries on drifting FADs, where declining catch rates have been recorded in the eastern Pacific. Silky Shark ranks among the three most important sharks in the global shark fin trade, with between half a million and one and a half million Silky Sharks traded annually. Estimates of trends in abundance from standardized catch rate indices for Carcharhinus species combined in the northwest Atlantic range from non-significant trends, to a decline of 85% over 19 years. Species-specific trends for Silky Sharks are difficult to estimate because of difficulties distinguishing it from other Carcharhinid sharks. Declines are also inferred in other areas, and Silky Sharks are known to be particularly important in pelagic fisheries in the Indian Ocean. Globally this species is assessed as Near Threatened, and may prove to meet the criteria for VU A2bd+3bd+4bd in the future.
In addition to the Near Threatened global assessment, a number of regional assessments have also been designated for this species as follows: Vulnerable A2bd+4bd in the eastern central and southeast Pacific; Vulnerable A2bd+4bd in the northwest Atlantic and western central Atlantic; Near Threatened in the southwest Atlantic; and Near Threatened in the Indian Ocean and western central Pacific.
Eastern Central and Southeast Pacific
Silky Sharks are taken in pelagic commercial fisheries and also artisanal fisheries in this region and fishing pressure from longline and purse seine fisheries targeting tunas and swordfish is high. Silky Sharks are the most commonly caught species of shark in the purse seine fishery for tunas in the eastern Pacific Ocean. Preliminary estimates of relative abundance trends for large Silky Sharks derived from purse-seine fisheries in this region show decreasing trends over the period 1993-2004 for each of three types of purse-seine sets (~65%). According to IATTC it is not known whether these decreasing trends are due to fishing, changes in the environment, or other processes, however, these results were also consistent with a separate descriptive study of Silky Shark bycatch rates in dolphin sets. This descriptive analysis showed a decrease in the probability of obtaining sets with bycatches greater than or equal to each of the three threshold levels over the same period. In addition, a study of the tropical central Pacific (which overlapped FAO areas, but mainly included the eastern central Pacific) estimated a decline in abundance of ~90% and in biomass >90%. A comparison of standardized catch rates of pelagic sharks caught off Costa Rica from 1991-2000 (of which Silky Sharks comprised 60-70%) also showed a decreasing trend (~60%). Given the trends described above and continued fishing pressure from pelagic fleets in this region, this species is assessed as Vulnerable A2bd+4bd there.
Northwest and Western Central Atlantic
Silky Sharks are taken as a target or bycatch of both commercial and artisanal pelagic fisheries in this region, including in the US commercial shark bottom longline and the pelagic longline fishery, targeted artisanal longline fisheries off Venezuela and recreational fisheries. This regional assessment is based on several estimates of trends in abundance of both Silky Shark and Carcharhinus species combined from standardized catch rate indices, which estimate declines ranging from 46-91% over different areas and time periods. Given the apparent decline in abundance in the northwest and western central Atlantic, and high fishing pressure from pelagic fleets throughout, this species is assessed as Vulnerable A2bd+4bd this region.
Silky Shark is taken in several longline fisheries in the southwest Atlantic, including those off Santos, southern Brazil, Natal, northeastern Brazil and Uruguay, and is retained. It is also targeted by fisheries in northeastern Brazil. Although there is a lack of catch data, the life-history characteristics of this species make it vulnerable to fisheries. Given the declines observed where this species is heavily fished in the northwest and western central Atlantic, this species is considered Near Threatened as a precautionary measure.
Indian Ocean and Western Central Pacific
This species is taken as a target and bycatch of shark and tuna longline and tuna gillnet fisheries in Indonesia and throughout many areas of its range in the Indian Ocean and western central Pacific. Pelagic fishing pressure is high, with reports of increasing effort in recent years. Sri Lanka is one of the only countries that reports landings of Silky Shark to the FAO. These landings rose from 5,000 t in 1960 to a peak of 25,400 t in 1994, after which they declined to just 1,960 t in 2006 (with landings not exceeding 7,000 t in the last five years of the series), suggesting possible collapse. Maldivian shark fishermen who specialize in longlining for oceanic sharks report declining catch rates of Silky Sharks, although specific data are not available. Whilst Japanese assessment of data from research longline surveys in the Pacific and Indian Oceans suggests that Silky Shark catch per unit effort showed no remarkable change in the 1990s when compared to levels in the 1970s, pelagic fishing effort has increased in recent years. Given the significant declines observed in other areas including the central tropical Pacific, the large decline in reported landings off Sri Lanka, reports of declines in catch rates off the Maldives and high and continuing fishing pressure across this region, an assessment of at least Near Threatened is considered appropriate.
|Range Description:||Silky Shark has a circumglobal distribution in tropical waters (Compagno in prep).
Western Atlantic: ranges from Massachusetts, USA, to southern Brazil, including Gulf of Mexico and Caribbean Sea. Also from St. Paul's Rocks in the Central Atlantic (Marín et al. 1998, Compagno in prep). Eastern Atlantic: ranges from Madeira, the Atlantic coast of Spain, and from Senegal to northern Angola (Compagno in prep). Indian Ocean: occurs off Madagascar, Mozambique, Tanzania, Comoros and Aldabra Island, also from Somalia to the Maldives, Oman, Red Sea, and Sri Lanka (Compagno in prep.). Western Pacific: occurs off Thailand, Philippines, New Caledonia, New Zealand, China and Taiwan (Province of China). Also Caroline Islands, Phoenix and Line Islands westwards (Compagno in prep). Eastern Pacific: ranges from Southern Baja California to Peru. Also Hawaiian Islands, Cocos Island, Revillagigedo Islands, Clipperton and Malpelos Islands (Compagno in prep.).
Native:Angola (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 – eastern central; Atlantic – northwest; Atlantic – southeast; Atlantic – southwest; Atlantic – western central; Indian Ocean – eastern; Indian Ocean – western; Pacific – eastern central; Pacific – southeast; Pacific – southwest; Pacific – western central
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||This oceanic and coastal-pelagic shark is circumglobal in tropical waters, where it dominates as a target species or bycatch in certain pelagic fisheries, particularly purse seines on drifting FADs (fish aggregating devices). Population dynamics and structure are poorly known, although life history parameters seem to vary geographically, perhaps reflecting the existence of distinct stocks for different ocean basins.|
|Habitat and Ecology:||
The Silky Shark is essentially pelagic and 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 to at least 500 m depth offshore (Compagno in prep). It is often associated with islands, near insular slopes and over deepwater reefs. It has also been recorded inshore to as shallow as 18 m depth (Compagno in prep). It is an active swift species, preferring warmer waters (about 23°C). Smaller sharks are often found in coastal nurseries and adults further offshore over deeper water. However, in the central Indian Ocean juveniles are oceanic, associated with drifting objects (C. Anderson pers. comm.).
The Silky Shark is a live bearer, usually having around 6-12 pups every one or two years (2-15 per litter, Last and Stevens 1994). The gestation period is 12 months (Bonfil et al. 1993). Life history parameters of silky sharks seem to vary geographically, perhaps reflecting the existence of distinct stocks for different ocean basins. In the Gulf of Mexico, Silky Sharks grow at a moderate rate and first attain maturity at about 215-225 cm TL for males and 232-246 cm TL for females, or around 6-10 years and 7-12+ years respectively (Branstetter 1987, Bonfil et al. 1993). They can live to at least 22+ years (Bonfil 1990) and attain a length of up to 330 cm TL. In this part of the world, the young are born at about 76 cm TL during the summer, whereas in other oceans the species seems to have no seasonality in reproduction (Bonfil et al. 1993). This species has a moderate to low intrinsic rate of increase; estimated at 0.043 by Smith et al. (1998) (moderate range: 0.04-0.07). Hoyos (2003) provides information on the life-history parameters of silky sharks from Mexico in the Eastern Central Pacific and Ruíz and Ixquiac (2000), Morales (2002) and Villatoro and Rivera (1994) from Guatemala and El Salvador. Both males and females reportedly mature at 180-182 cm TL off Mexico in the Eastern Central Pacific, with females reaching a maximum size of 316 TL and producing 2-9 pups per litter (Hoyos 2003). Oshitani et al. (2003) reported on the age and growth of silky sharks in the Pacific Ocean (using samples collected from Japanese tuna longline and purse seine fisheries). They found that males mature at >186 cm TL and at 5-6 years of age, and females at 193-200 cm TL and 6-7 years of age. Size at birth ranged from 65-81 cm TL. In contrast to earlier studies in the Pacific Ocean, Oshitani et al. (2003) noted that their results were similar to those of studies in the Atlantic Ocean. In the Maldives, Indian Ocean, males are estimated to mature at about 205 cm TL, reaching a maximum size of 242 cm TL; females reaching a maximum size of 263 cm TL (Anderson unpublished data). Size at birth is reported at 56-72 cm TL in this area (Anderson unpublished data).
Recently, intensive demographic modelling of shark populations revealed that the silky shark from southern and northern Gulf of Mexico has moderate levels of intrinsic rate of increase (ca. 0.10 and 0.055) (Cortés 2002).
Silky Sharks generally leave their coastal nursery grounds and move offshore to a more oceanic existence as sub-adults, frequently joining tuna schools on which they seem to feed (Branstetter 1987). The Silky Shark is a piscivorous shark feeding on sea catfish, mullets, mackerel, yellowfin tuna, albacore, porcupine fish and other fish species, as well as on a variety of cephalopods (Compagno 1984, Bonfil 1990).
|Use and Trade:||Meat utilised for human consumption, fins are taken for the shark fin trade to make soup, skin has been processed for leather and the liver has been used for liver oil (for its high vitamin A content in this species) (Compagno in prep).|
The Silky Shark is probably fished either directly or as a bycatch throughout its range. It is taken in coastal longline fisheries, oceanic purse seine fisheries on drifting FADs (fish aggregating devices) (particularly the latter) targeting tuna, swordfish and other billfish around the world, as well as by coastal artisanal fisheries. Whether they are an incidental catch or not, Silky Sharks are often retained for their meat and fins. Catch statistics for this species are under-reported. Total catch reported to FAO has decreased steadily since 2000; 11,680 t were reported in 2000, 9,330 in 2001, 8,384 in 2002, 5,305 in 2003, to 4,358 in 2004 (Maguire et al. 2006).
The number of pelagic sharks landed by fishing fleets in all oceans has become increasingly important in recent years (Mejuto et al. 2006). The Spanish pelagic longline fishery for swordfish and sharks is expanding rapidly in the Pacific, with effort expanding from the traditional grounds in the southeast Pacific into the central south Pacific and areas of the North Pacific in recent years (Mejuto 2005).
Silky Sharks were found to represent at least 3-4% of the fins auctioned in Hong Kong, the world's largest shark fin trading center - the 3rd highest after Blue Sharks (Prionace glauca) and Scalloped Hammerhead Sharks (Sphyrna lewini) (Clarke et al. 2006a) - and Hong Kong is thought to make up more than 50% 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).
Rough estimates of numbers of Silky Sharks taken as bycatch in tuna longline fisheries of the south and central Pacific Ocean (Bonfil 1994) indicate that perhaps up to 900,000 individuals were taken there during 1989. However, there is large uncertainty surrounding these calculations and there are no estimates of numbers discarded alive and numbers actually killed. Large numbers of Silky Shark are caught in Mexico, Yemen and Sri Lanka. Bonfil et al (1993) conclude that local stocks of this species cannot support sustained heavy fishing pressure.
Eastern Central and Southeast Pacific
Silky Sharks are taken in pelagic commercial fisheries in the eastern central and southeast Pacific, as well as in artisanal fisheries. Fishing pressure from longline and purse seine fisheries targeting tunas and swordfish is high, and it is the main shark species caught using fish aggregating devices (FADs). The IATTC observer database from 1993-2005 shows that silky sharks were caught throughout the ocean west of central America, Colombia, Ecuador, and Peru out to about 170°W on purse seines set on FADs. Accurate identification is an issue because of the similarity to other Carcharhinids and it is often grouped with other Carcharhinid species. Silky Sharks are the most commonly caught species of shark in the purse seine fishery for tunas in the eastern Pacific Ocean (IATTC 2007). Preliminary estimates of relative abundance trends for large Silky Sharks derived from purse-seine fisheries in the eastern Pacific Ocean show steadily decreasing trends over the period 1993-2004 for each of three types of purse-seine sets, from a catch per set of 10.4 in 1994 to 3.6 in 2005 (decline of ~65%) (IATTC 2007). According to IATTC it is not known whether these decreasing trends are due to fishing, changes in the environment (e.g., the 1997-98 El Niño event), or other processes. These results were also consistent, however, with a preliminary descriptive study of Silky Shark bycatch rates in dolphin sets. Whilst it is thought that Silky Sharks are attracted to floating objects, Silky Sharks caught in dolphin sets may have been caught simply by chance. Because the distribution of Silky Shark bycatch per set in dolphin sets is extremely right-skewed, the descriptive analysis focused on bycatches above and below the following three thresholds: presence or absence of any sharks, presence or absence of more than five sharks per set, and presence or absence of more than 20 sharks per set. This analysis showed a decrease in the probability of obtaining sets with bycatches greater than or equal to each of the three threshold levels from 1994-2005 (IATTC 2007).
Strasburg (1958 in Castro et al. 1999) noted that Silky Sharks comprised 52% of all sharks caught south of 10°N latitude in the equatorial Pacific, in a sample of 4,157 sharks. Silky Sharks are important in many fisheries along the Pacific coast of Central America. Observations made on a longline research vessel that sampled waters in the EEZs of Panama, El Salvador, and Guatemala found that silky sharks constituted 30.94%, 47.21% and 29.11% of the total catch respectively, and 79.80%, 63.3% and 44.29% of the shark catch respectively (Porras 1996). Off Chiapas, southern Mexico, Silky Sharks along with Scalloped Hammerheads form the large bulk of the shark catch (Soriano-Velásquez and Acal Sánchez 2008). In Tres Marías Islands, Mexico, Silky Sharks are one of the most important species, constituting 27% of landed sharks in 1996 (Pérez-Jiménez et al, 2005). In San José and Buena Vista, Guatemala, Silky Sharks constituted 73% of recorded catches landed, in a sample of 4,211 sharks, from 1996 to 1999 (Ruiz and Ixquiac 2000). In Acajutla, El Salvador, from 1991-1992, Silky Shark landings ranged from 60.3% to 32.6% of the total catch, in a sample of 412 sharks (Villatoro-Vaquiz and Rivera-González 1994).
International longline vessels operating in the eastern central Pacific landed and exported over 8,000 tons of shark carcasses and 900 tons of shark fins from Costa Rica in 2002 (Costa Rica official INCOPESCA Fishery Statistics 2003), the large majority of which are recorded as Silky Sharks. However, these figures are likely underestimates, because landings also occur at private docks where enforcement and monitoring is very difficult. Furthermore, international vessels are known to trans ship shark products on the high seas, going unreported. A comparison of standardized catch rates of pelagic sharks (species-specific information was not available) in the EEZ of Costa Rica from 1991-2000 showed a decrease of 60%, with Silky Sharks comprising 60-70% of recorded shark catch (Arauz et al. 2004). In 1991, sharks formed 27% of the total catch. In 2000, only 7.64% of the total catch was sharks, 71% of which were Silky Sharks. In 2003, the proportion of sharks in the total catch decreased further to 4.9% of the total catch, 58.2% of which were silky sharks (Arauz et al. 2004).
In Manta (the largest fishing port in Ecuador), Southeast Pacific, artisanal fisheries data from September 2003 to the end of 2006 showed the total landings of sharks was 150,321 individuals, of which C. falciformis represented 15% of the total (over 22,000 individuals) (J. Martínez pers. comm. Feb. 2007). The females ranged in length from 61-309cm total length, and the males ranged in length from 63 -288cm total length. In the tuna purse seine fishery, Silky Shark is caught as a bycatch, and is the main shark species caught using FADs) (J. Martínez pers. comm. Feb. 2007.
Recent information from the tropical Central Pacific suggest community biomass reductions to only 11% of historic levels, from the 1950s to the 1990s, with a current community of smaller fish and fewer large predators (Ward and Myers 2005). This study estimated that the Silky Shark population had declined in abundance by about 90% and in biomass by more than 90% (Ward and Myers 2005). These estimates were made by a comparison of pelagic longline research surveys in the 1950s carried out in the tropical Pacific Ocean with recent data (1990s) collected by observers on pelagic longline fishing vessels, which have been standardized to account for differences in depth and soak time (Ward and Myers 2005).
Indian Ocean and Western Central Pacific
This species is a very common component of shark and tuna longline and tuna gillnet fisheries in Indonesia (White et al. 2006) and throughout many areas of its range in the Indo-West Pacific. They are usually retained and marketed. 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). The Spanish swordfish longline fishery, which also targets sharks, is also active across the Indian Ocean (ICCAT 2006) and Mejuto et al. (2006) report that fishing effort has increased since 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). Pelagic fishing effort is high and continuing. Silky Sharks were the most commonly caught shark at FADS in the Indian Ocean during a small pilot study of observers onboard French vessels - 61.5% of silkies were discarded dead at sea, 30.5% partially alive kept onboard, ~7% discarded alive (Viare and Pianet 2006).
In the Maldives, it is estimated that some 85% of oceanic shark catch is C. falciformis (C. Anderson unpublished data). Although no specific data are available, Maldivian shark fishermen who specialize in longlining for oceanic sharks report declining catch rates of Silky Sharks (C. Anderson unpublished data). A fishery for Silky Sharks 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 (although Bonfil noted that only 75% of the reported catch may be attributed to silky sharks). Landings reported to FAO progressively rose from 5,000 t in 1960 to a peak of 25,400 t in 1994, after which they began to decline to 20,875 t in 1998, 8,240 t in 2001, 6,830 t in 2002, and continued to decrease to just 1,960 t in 2006, suggesting that the fishery collapsed.
The tropical waters of the western and central Pacific Ocean currently support the largest industrial tuna fishery in the world with an annual catch approaching one million metric 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 species found in longline sets are Blue Shark, Silky Shark, Pelagic Stingray and the Oceanic Whitetip. The most prevalent species found in purse seine sets are Silky Shark and the Oceanic Whitetip (Williams 1999).
Japanese assessment of data from research longline surveys in the Pacific and Indian Oceans suggests that silky shark catch per unit effort showed no remarkable change in the 1990s when compared to levels in the 1970s. In recent years, based on longline logbook data, recorded Japanese catches of Silky Sharks worldwide ranged from 0-11 mt with an average of 2.2 mt. The resource is considered stable with no management action required other than ongoing monitoring (Japan Fisheries Agency 2006).
Whilst the Japanese longline surveys described above showed no change in CPUE of Silky Sharks between the 1970s and 1990s, pelagic effort began 20 years earlier in the 1950s and has reportedly increased in recent years. Given the significant declines observed in other areas including the central tropical Pacific, the large decline in reported landings off Sri Lanka after 1994, reports of declines in catch rates off the Maldives and high fishing pressure across this region, with reports of increasing effort, continued fishing pressure is thought to be unsustainable.
There are data on Silky Shark catches from the Australian east coast Tuna and Billfish Fishery, however, these have not been analyzed to date, and need to be looked at in the future (J.D. Stevens pers. comm. Feb. 2007).
Northwest and Western Central Atlantic
The first longline fisheries in the Atlantic were begun by the Japanese 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 is high and ongoing. In the US, Silky Sharks are caught by the commercial shark bottom longline and the pelagic longline fishery, and in recreational shark fisheries. The northwest and western central Atlantic assessment is based on several estimates of trends in abundance from standardized catch rate indices. The first analysis is for the Gulf of Mexico, and is based on data from U.S. pelagic longline research surveys in the mid-1950s and U.S. pelagic longline observer data in the late-1990s. This analysis suggests that silky sharks in the Gulf of Mexico declined by 91% (95%CI: 85-95%) over this forty year period (Baum and Myers 2004), which is similar to the length of three generations (~45 years).
The second analysis, which encompasses both the northwest and western central Atlantic regions, is of the US pelagic longline logbook data between 1992 and 2000 (Baum et al. 2003). This analysis is not species-specific (because of problems with identification among Carcharhinid sharks), but instead estimated a decline of 61% (95%CI: 55-66%) over this time period for six species in the genus Carcharhinus combined. Species-specific analysis on the same US logbook data set for 1992-2000 by Brown and Cramer (2002) reveals a non-significant trend, although accurate identification of Silky Sharks in the data set is an issue, as stated above. Another species-specific analysis of the same logbook data for the same time period by Cortés et al. (2007) estimated a decrease of 50% in Silky Shark since 1992. Cortés et al. (2007) also report the results of this logbook analysis restricted to the Gulf of Mexico and Caribbean areas only, for the 1992-2000 period, which showed a decline of 48%. This analysis had little temporal overlap with Baum and Myers (2004) analysis of data from the longline research surveys and observer data in the same area (described above), which covered almost the entire three generation period for this species (1950s-1990s).
The most recent analysis of observer data from the US Atlantic pelagic longline fishery from 1992-2005 (which combined catches of Dusky Sharks, Silky Sharks, and Night Sharks because of identification problems) suggests that this species complex has continued to decline significantly, by 70% (95%CI: 54-81%) during this recent time period (Baum et al. in prep). When the two estimates obtained by (Baum et al. in prep) for Carcharhinus species are combined (i.e., 1986-2005), the estimated decline amounts to 85%. However, it should be noted, again, that this is not a species-specific estimate for silky shark, but it is difficult to obtain species-specific estimates for silky sharks because of difficulties distinguishing it with other carcharhinid sharks. Cortés et al.'s (2007) species-specific analysis of the observer dataset for the same time period, showed a similar decline since 1992 to that they estimated in the logbook data (46% vs 50%), but larger inter-annual variation (r=-0.047, 95% CI: -0.290 to 0.197). The nominal observer series showed a 40% decline from beginning to end.
Silky Shark is one of the five most important shark species caught in the Cuban longline fishery (Espinosa 2004). In Belize, Silky Sharks were frequently caught off the barrier reef and atolls by commercial pelagic longline fisheries during the 1980s and 1990s. Although no landings data are available, anecdotal observations by former shark fishers noted a decline in catches and mean size for all shark species caught and pelagic fishing pressure has declined as a result (R. Carcamo and D. Neal pers. comm.). Landings data recorded at artisanal shark fishing ports in Mexico and Central America also indicate that Silky Sharks constitute one of the main components of the catch. In 1987, the Silky Shark (and Scalloped Hammerhead Shark), represented >80% of the shark by-catch of the winter swordfish/tuna longline fishery of the northwestern Gulf of Mexico (Branstetter 1987).
Silky Sharks are thought to be overexploited as juveniles in the fishery of Yucatan (Bonfil 1990), but due to the lack of estimates of total catches of this species and the size of the population, the status of the stock as a whole is unknown.
In Venezuela, sharks are targeted by both artisanal and industrial longline fisheries, and Tavares (2005) comments on a history of over-exploitation. In artisanal longline fisheries targeting sharks around Venezuelan offshore islands, the Silky Shark is one of the most important species (Tavares 2005). Shark fishing is increasing at these Venezuelan oceanic islands and a monitoring programme has been instigated (Tavares 2005). However, given the life-history of the species, it is unlikely that Silky Sharks will be able to sustain continued, let alone increasing, directed fishing.
This species 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). It is primarily oceanic and epipelagic, although it is taken in artisanal gillnet fisheries off southern Paraná State (Costa and Chaves 2006). Lessa et al. (2000) also noted that Silky Shark was the main species to be targeted by fisheries in northeastern Brazil. They are generally retained and marketed (Arfelli and Amorim 1994, Marín et al. 1998).
Rincon et al. (2000) analysed fins unloaded in Natal, Rio Grande do Norte, Brazil, from April to July, 1998. This species represented 50% of total fins sampled; however the number of shark fins did not correspond to the number of carcasses. In 1997, 233 tons of dry fin corresponded to 15,533 tons of carcasses, while only 2,603 tons of unloaded carcasses were registered. This may indicate extensive finning practices being carried out at sea.
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. States are urged to cooperate over the management of these species. No such management yet exists. Precautionary adaptive collaborative management of target and bycatch fisheries is urgently needed for this highly migratory species. It is also essential to improve and sustain data collection and develop stock assessments for this species. Listing on international resource management agreements, such as the Convention on Migratory Species (CMS) could help to drive improvements in national and regional management and facilitate collaboration between states, for this species and other migratory sharks.
The adoption of shark finning bans by fishing states (e.g., USA, Australia), regional entities (EU) and regional fisheries organisations (e.g. ICCAT, IOTC, IATTC, WCPFC) is accelerating and should increasingly prevent the capture of oceanic sharks for their fins alone.
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|Citation:||Bonfil, R., Amorim, A., Anderson, C., Arauz, R., Baum, J., Clarke, S.C., Graham, R.T., Gonzalez, M., Jolón, M., Kyne, P.M., Mancini, P., Márquez, F., Ruíz, C. & Smith, W. 2009. Carcharhinus falciformis. The IUCN Red List of Threatened Species. Version 2014.3. <www.iucnredlist.org>. Downloaded on 25 April 2015.|