|Scientific Name:||Alopias superciliosus|
|Species Authority:||Lowe, 1841|
Alopecias superciliosus Lowe, 1841
|Taxonomic Source(s):||Eschmeyer, W.N. (ed.). 2015. Catalog of Fishes. Updated 6 April 2015. Available at: http://researcharchive.calacademy.org/research/ichthyology/catalog/fishcatmain.asp. (Accessed: 6 April 2015).|
|Red List Category & Criteria:||Vulnerable A2bd ver 3.1|
|Assessor(s):||Amorim, A., Baum, J., Cailliet, G.M., Clò, S., Clarke, S.C., Fergusson, I., Gonzalez, M., Macias, D., Mancini, P., Mancusi, C., Myers, R., Reardon, M., Trejo, T., Vacchi, M. & Valenti, S.V.|
|Reviewer(s):||Cavanagh, R.D., Stevens, J.D., Kyne, P.M., Soldo, A., Francis, M., Fowler, S.L. & participants of the Pelagic Shark Red List Workshop (Shark Red List Authority)|
All members of genus Alopias, the thresher sharks, are listed as Vulnerable globally because of their declining populations. These downward trends are the result of a combination of slow life history characteristics, hence low capacity to recover from moderate levels of exploitation, and high levels of largely unmanaged and unreported mortality in target and bycatch fisheries.
The Bigeye Thresher Shark (Alopias superciliosus) apparently is a highly migratory, oceanic and coastal species found virtually circumglobally in tropical and temperate seas. It has low fecundity (2-4 pups/litter) and an exceptionally low (0.002) potential annual rate of population increase, compared with other thresher sharks. This species is especially vulnerable to fisheries exploitation (target and bycatch) as its epipelagic habitat occurs within the range of many largely unregulated gillnet and longline fisheries in which it is readily caught, and it has been fished throughout its range. Significant reductions in thresher CPUE have been reported in pelagic longline fisheries in the northwest Atlantic and the eastern tropical Pacific, and declines are also suspected to have occurred in other areas. Although data are lacking for many parts of its range, it is evident that this Vulnerable species, with such low productivity, faces major threats throughout most of its range, where fishing pressure is unlikely to cease or decrease anytime in the immediate future. However, this may underestimate the extent of global decline and there is an urgent need for global review of all available data throughout its range.
In addition to the Vulnerable global assessment, a number of regional assessments have also been designated for the Bigeye Thresher Shark as follows: Vulnerable (VU A2bd) in the eastern central Pacific; Endangered (EN A2bd) in the northwest Atlantic and western central Atlantic; Near Threatened in the southwest Atlantic; Data Deficient in the Mediterranean Sea; and Vulnerable (VU A2d) in the Indo-west Pacific.
Northwest Atlantic and Western Central Atlantic
Estimates of trends in abundance from standardized catch rate indices of the USA pelagic longline fishery suggest that this species has likely undergone a decline in abundance in this region. Thresher sharks are generally recorded by genus by observers as well as in logbooks, which includes both Common Thresher Shark (Alopias vulpinus) and Bigeye Thresher Shark in this region, of which the Bigeye Thresher Shark is the more common. The area covered by the analyses, ranging from the equator to about 50°N, encompasses the confirmed range of threshers in this region. Estimates of the decline based on logbook and observer records of combined thresher sharks from 1986-2005 range between 50-80%. Fishing pressure on thresher sharks began over two decades prior to the start of this time series, thus the estimated declines are not from virgin biomass. Bigeye Thresher Shark is more biologically vulnerable and has a lower rate of intrinsic population increase than the Common Thresher Shark, with which it is grouped in the logbook and observer datasets. Given this species' intrinsic vulnerability and that fishing pressure on thresher sharks began over two decades prior to the start of these longline time series, the combined analyses may underestimate the decline in A. superciliosus and it is assessed as Endangered (EN A2bd) in this region.
The Bigeye Thresher Shark is a bycatch of the semi-industrial fisheries (swordfish and other pelagic fisheries) of southern Spain, Morocco, Algeria, Sicily and Malta, and of artisanal trammel and gillnet fisheries elsewhere in the Mediterranean Sea. However, Bigeye Thresher Shark has been poorly documented in the Mediterranean and is considered scarce or rare. No data are available on catch trends in this region, although pelagic effort is high. Despite the apparent threat posed by bycatch, the lack of records and further information on the population of Bigeye Thresher Shark in the Mediterranean precludes an assessment beyond Data Deficient at this time.
The Bigeye Thresher Shark is caught in tuna and swordfish longline fisheries, and to a lesser extent gillnets, with other wide-ranging oceanic shark species in this region. Increased targeting of pelagic sharks due to the increasing demand and the commercial value of their fins has also been reported. Data are sparse in the South Atlantic and pelagic fishing pressure high. Both Bigeye Thresher Shark and Cxommon Thresher Shark are caught in the Brazil santos longline fishery, with Bigeye Thresher Shark comprising the large majority of the catch. The landed catch and CPUE of Bigeye Thresher Shark in this fishery increased from 1971 to 1989, and then gradually decreased from 1990 to 2001. This does not necessarily reflect stock abundance because changes in the depth of fishing operations also occurred, which may have affected the time series. Given the declines reported in other areas for which data are available and high fishing pressure from fleets throughout the Southwest Atlantic, the species is assessed as Near Threatened in this region based on the limited information currently available. There is a need to collect further data from throughout the South Atlantic.
Eastern Central Pacific
Pelagic fleets operating in this region are known to take Bigeye Thresher Shark. It is a known bycatch of the purse seine fishery operating in the Eastern Pacific Ocean. Trends in abundance and biomass of thresher sharks combined in the eastern tropical Pacific Ocean were estimated by comparison of pelagic longline research surveys in the 1950s with recent data (1990s) collected by observers on pelagic longline fishing vessels, standardized to account for differences in depth and soak time. This analysis was not species specific but estimated a decline in abundance of 83% and a decline in biomass to approximately 5% of virgin levels. Given the apparent decline in combined thresher abundance in this region, continued fishing pressure from pelagic fleets and this species' vulnerable life-history characteristics it is assessed as Vulnerable (VU A2bd) in the eastern central Pacific.
This species is taken by a variety of pelagic fisheries throughout many areas of its range in the Indo-West Pacific, including the Spanish surface longline fleet swordfish and sharks, the Korean tuna longline fleet, South Africa's longline fisheries for tuna and tuna fisheries in the eastern Indian Ocean and Andaman Sea. Pelagic fisheries have operated in the Indian Ocean for more than 50 years. Pelagic fishing pressure is high and the amounts of sharks caught by longliners targeting swordfish in the Indian Ocean have been constantly increasing since the mid-1990s. Sharks are also targeted in several areas. A recent review of fisheries in the Indian Ocean reported that sharks in this region are considered fully to over-exploited. Most artisanal and industrial marine fisheries in the Indian Ocean are multispecies and the state of most resources is poorly documented. Catch data are incomplete and cannot be used to estimate the real magnitude of catches or trends. Although complete data on are not available for evaluation from this region, this species is a known catch of many fisheries operating throughout much of its range in this region, as described above. Given that the Bigeye Thresher Shark has high biological vulnerability and a low intrinsic rate of increase, coupled with the declines observed in other areas of its range, declines are inferred based on continuing high levels of exploitation and it is assessed as Vulnerable (VU A2d) in the Indo-West Pacific.
|Range Description:||This oceanic and coastal shark is virtually circumglobal in tropical and temperate seas (Compagno 2001). The below distribution is taken directly from (Compagno 2001).|
Western Atlantic: ranges from New York to Florida, Mississippi and Texas, USA, Mexico (Veracruz to Yucatan), Bahamas, Cuba, Venezuela, Brazil (from Praia do Forte, Bahia to Rio Grande de Sul), Uruguay, and almost certainly Argentina.
Eastern Atlantic: from eastern Portugal and Spain, Madeira, near Azores, Morocco, Canary Islands, Senegal, Guinea to Sierra Leone, Angola, South Africa (Western Cape), including the Mediterranean Sea.
Indian Ocean: South Africa (Eastern Cape and Kwazulu-Natal), Madagascar, Arabian Sea (Somalia), Gulf of Aden, Maldives, Sri Lanka.
Western Pacific: Southern Japan (including Okinawa), Taiwan, Viet Nam, between northern Mariana Islands and Wake Island, Northwestern Submarine Rise, New Caledonia, Australia (northwestern coast), New Zealand. Also towards the Central Pacific in the area between Wake, Marshall, Howland and Baker, Palmyra, Johnston Islands.
Eastern Pacific: California, USA, Mexico (Gulf of California) to approximately 15°S latitude off Peru, including west of Galapagos Islands, Ecuador; possibly off northern Chile (Compagno 2001). Also Hawaiian Islands, USA, north and south of Hawaiian Islands, off east of Line Islands, and between Marquesas and Galapagos Islands.
The longest straight line movement of a conventionally tagged Bigeye Thresher Shark is reported at 2,767 km, from waters off New York to the Eastern Gulf of Mexico (Weng and Block 2004, Kohler and Turner 2001). A study of two bigeye thresher sharks using pop-up satellite archival tags in the Gulf of Mexico and Hawaiian Archipelago suggest a pattern of diel vertical migration (Weng and Block 2001). Nakano et al. (2003) also report distinct diel vertical migrations from acoustic telemetry studies in the Eastern Central Pacific, with the studied sharks staying at 200 to 500 m depth during the day and at 80 to 130 m at night.
Native:Angola (Angola); Argentina; Australia; Bahamas; Brazil; Costa Rica; Cuba; Ecuador (Ecuador (mainland), Galápagos); El Salvador; French Polynesia (Marquesas); Greece; Guatemala; Guinea; Israel; Italy; Japan; Madagascar; Maldives; Mexico; Morocco; New Caledonia; New Zealand; Nicaragua; Panama; Portugal (Azores, Madeira, Portugal (mainland)); Senegal; Sierra Leone; Somalia; South Africa; Spain (Canary Is., Spain (mainland)); Sri Lanka; Taiwan, Province of China; Turkey; United States (Alabama, California, Florida, Georgia, Hawaiian Is., Louisiana, Mississippi, New York, North Carolina, South Carolina, Texas, Virginia); United States Minor Outlying Islands (Howland-Baker Is., Johnston I., Wake Is.); Uruguay; Venezuela, Bolivarian Republic of; Viet Nam
|FAO Marine Fishing Areas:|
Atlantic – northwest; Atlantic – western central; Atlantic – eastern central; Atlantic – southwest; Atlantic – southeast; Atlantic – northeast; Indian Ocean – western; Mediterranean and Black Sea; Pacific – southeast; Pacific – northwest; Pacific – southwest; Pacific – eastern central; Pacific – western central
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Atlantic and Indo-Pacific populations are presumably largely isolated. An ongoing genetic study of global population structure in A. superciliosus using DNA sequences from the mitochondrial control region indicates no structuring of populations within the Pacific Ocean, but significant genetic structure between Bigeye Threshers in the northwest Atlantic and those in the Pacific Ocean (Trejo 2004). The existence of separate Indian Ocean and Pacific Ocean stocks is unconfirmed.|
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||Found in coastal waters over the continental shelves, sometimes close inshore in shallow waters, and on the high seas in the epipelagic zone far from land; also caught near the bottom in deep water on the continental slopes (Compagno 2001). Ranges from the surface and in the intertidal to at least 500 m deep and has been recorded to 723 m deep (Nakano et al. 2003), mostly below 100 m depth (Compagno 2001). Mediterranean captures are typically from offshore continental shelf waters.|
Estimated age at maturity is years 12-13 (females), 9-10 years (males) (Liu et al. 1998) (data from Taiwan waters, Northwest Pacific). Longevity is estimated at 20 years for females (19 years for males) (Liu et al. 1998). Males mature at 270-300 cm total length (TL) and females at 332-355 cm TL (Chen et al. 1997, Liu et al. 1998, Moreno and Moron 1992, Stillwelli and Casey 1976). The maximum recorded size of A. superciliosus is 461 cm TL (Compagno 2001). Size at birth ranges from 64-140 cm TL (Golani 1996, Chen et al. 1997, Bauchot 1987). The gestation period is 12 months with average litter sizes 2-4 pups/litter, usually two (Compagno 2001). Of the thresher sharks, the Bigeye Thresher has the lowest rate of annual increase, estimated at 1.6% under sustainable exploitation (Smith et al. 2008), or 0.002-0.009 (Cortés 2008, Dulvy et al. 2008). There have been several studies on the biology of A. superciliosus from Brazilian waters in the southwest Atlantic; see (Amorim et al. 1998, Amorim et al. in press, Gonzalez and Magenta-da-Cunha in press).
A recent tagging study of two A. superciliosus (one from Hawaii and the other from the Gulf of Mexico) indicates strong diel vertical migration (Weng and Block 2004). These sharks spent most of the nightime in waters warmer than 20°C and commonly spent eight or more hours during the daytime in waters cooler than 10°C, requiring them to be eurythermal. Nakano et al. (2003) undertook acoustic tracking of this species in the eastern Pacific Ocean.
Reported diet consists of pelagic bony fishes including scombroids, clupeoids and small billfishes; hake and cephalopods (Compagno 2001). Uses its tail to stun the pelagic fishes on which it feeds (Compagno et al. 2005). Anecdotal reports from Maltese fishermen suggest a strong association between these sharks and highly mobile scombroids such as Bullet Tunas Auxis rochei in the Mediterranean. Observations from Sardinia show that the Bigeye Thresher sometimes interacts with swordfish, receiving fatal wounds (Vacchi and Serena 2000).
Members of the genus Alopias, thresher sharks, are threatened from a combination of slow life history characteristics, hence low capacity to recover from moderate levels of exploitation, and high levels of largely unmanaged and unreported mortality in target (for fins and their valuable meat) and bycatch fisheries. The Bigeye Thresher Shark has the lowest intrinsic rebound potential and least resistance to fisheries of the genus.
Thresher shark species (including A. superciliosus) were found to represent at least 2-3% of the fins auctioned in Hong Kong, the world's largest shark fin trading center (Clarke et al. 2006a). Thresher shark fins are generally low value compared to other species because of their low fin ray count (S. Clarke unpubl. data). It is estimated that between 350,000 and 3.9 million thresher sharks (Alopias species) are represented in the shark fin trade each year or, in biomass, 12,000 to 85,000 mt (Clarke et al. 2006b). These estimates are 1-2 orders of magnitude higher than catches of Alopiidae reported to FAO, which since the early-1980s have generally been less than 1,600 tonnes, and around 1,000 mt since 1998 (Maguire et al. 2006). Catches of thresher sharks are clearly hugely under reported globally. Although trend data are as a result largely lacking, these fisheries are unlikely to be sustainable. A recent FAO analysis states, "unless demonstrated otherwise, it is prudent to consider these species as being fully exploited or overexploited globally" (Maguire et al. 2006).
The life history of this species, including a late age at maturity (12-13 years) and very low fecundity (average two pups per litter) (Chen et al. 1997), render it highly vulnerable to overexploitation. Alopias superciliosus has the lowest annual rate of population increase of all thresher sharks and is therefore particularly at risk from depletion in fisheries. Its epipelagic habitat occurs within the range of commercial longline fisheries in which it is readily caught. Caught or formerly caught in the oceanic longline fisheries operated by the former USSR, Japan, Taiwan (Province of China), Spain, the USA, Brazil, Uruguay, Mexico, and probably other countries. Especially important areas for these fisheries are the northwestern Indian Ocean, western and Central Pacific, eastern North Pacific, and North Atlantic (Compagno 2001). This species is also taken as incidental bycatch in fixed bottom and pelagic gill nets, in trawls, and as a rare catch of anti-shark nets off KwaZulu-Natal, South Africa. It has been caught by anglers with sportsfishing gear (rod-and-reel) in the USA, South Africa, and New Zealand, in some instances by anglers targeting swordfish at night (Compagno 2001).
The Bigeye Thresher was formerly a very important component of the Cuban longline fishery, and more recently has been taken in considerable numbers by longliners off the northeastern USA and by gill net vessels off southern California (USA) and the eastern Atlantic (by Spanish vessels), and by longliners off Taiwan (Province of China; where about 220 t per year are landed) (Compagno 2001).
Northwest Atlantic and Western Central Atlantic
A. superciliosus has been caught incidentally in the northwest and western central Atlantic for about four decades. 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. Fisheries monitoring of bigeye thresher shark catch is extremely limited because they are an incidentally caught species. In Canada and the U.S., less than 5% of the pelagic longlining fleets are monitored by observers, making it difficult to elucidate reliable trends in abundance from these data. Fisheries monitoring in international waters is even more limited. The pelagic longline fishing grounds for the US fleet extend from the Grand Banks (about 45°N) in the northwest Atlantic to 5-10°S off the South American coast, within which geographical areas of longline fishing are defined for classification (Cortés et al. 2007).
The northwest and western central Atlantic regional assessment is based on several estimates of trends in abundance from standardized catch rate indices of the U.S. pelagic longline fishery, between 1986-2000, for thresher sharks (Baum et al. 2003). The analysis is not species-specific but, based on the location of the catches, the majority of threshers in the dataset are most likely bigeye thresher sharks (K. Goldman pers. comm.), and the sample size of threshers in these data is over 20,000. The estimated decline of thresher sharks combined (A. superciliosus and A. vulpinus) is 80% during this fifteen-year time period, which is just over the length of one generation for bigeye threshers (Baum et al. 2003). An alternative analysis of the same logbook dataset for 1986-2005 that also combined A. vulpinus and A. superciliosus, resulted in an overall decline of 63% (Cortés et al. 2007). Fishing pressure on thresher sharks began over two decades prior to the start of this time series, thus the estimated declines are not from virgin biomass.
A more recent analysis of Alopias spp. trends from scientific observer data between 1992 and 2005 in the same U.S. pelagic longline fishery found an almost identical instantaneous rate of decline (-0.12 up to the year 2000) as in the logbook analysis (Baum et al. unpublished manuscript). For this nine year period (1992-2000), the decline amounts to 68%, therefore the decline back to when the fishery started in the 1960s (less than three generation period of 51 years) would be much greater. However, because of recent increases in the catch rates in 2004 and 2005, the overall trend from 1992-2005 of -0.024 was non-significant, and would amount to only a 26% decline (Baum et al. unpublished manuscript). Cortés et al. (2007) also conducted an alternative analysis of this same observer dataset for the same time period that also combined A. vulpinus and A. superciliosus. This analysis of the observer dataset showed a trend opposite to that of the logbook analysis, with a 28% increase since 1992. In contrast, the nominal observer series showed a 39% decline and the logbook index for the same time period showed a decrease of 50%. Furthermore the sample size in the observer analysis was much smaller (n=14-84) than that in the logbook analysis (n=112-1292) and thus the trend estimated should be regarded with caution. Cortés et al.'s (2007) observer analysis was restricted to four out of the 11 geographical areas covered by the pelagic longline fishing fleet to keep a balanced statistical design (Cortés et al. 2007). Their full logbook analysis, which showed an overall decline of 63%, had much larger sample sizes and is thus better to estimate trends with more certainty (Cortés et al. 2007).
Bigeye Thresher is more biologically vulnerable than the Common Thresher (A. vulpinus), with which it is grouped in the logbook and observer datasets. Given this species' intrinsic rate of population increase is much lower and that fishing pressure on thresher sharks began over two decades prior to the start of these longline time series, the combined analyses may underestimate the decline in A. superciliosus. The area covered by the dataset analyzed, ranging from the equator to about 50°N, encompasses the confirmed range of bigeye threshers in these two regions (Compagno 2001).
This species is a bycatch of the semi-industrial fisheries (swordfish and other pelagic fisheries) of southern Spain, Morocco, Algeria, Sicily and Malta, and of artisanal trammel and gillnet fisheries elsewhere in the Mediterranean Sea (Bauchot 1987). However, A. superciliosus has been poorly documented in the Mediterranean and is considered scarce or rare (Barrull and Mate 2002). As a result, no data are available on catch trends for this species in the region. In recent years, increasing numbers of new records from the eastern Mediterranean (sometimes multiple captures) demonstrate that this species also penetrates widely to the east of Malta, occurring in the waters off Israel (Levantine basin), in the Aegean Sea off Turkey and southern Greece, and off southern Crete (I. Fergusson pers. obs., Golani 1996, Clò et al. 2005, Clò et al. 2008). Evidence from offshore pelagic fisheries in southern Sicily and Malta indicate that A. superciliosus is caught in unknown numbers each year, but routinely discarded at sea (hence the vernacular name "False Thresher", because of a perceived low local value). Despite the apparent threat posed by bycatch and the species' vulnerable life-history characteristics, there is a lack of records and further information on the population of A. superciliosus in the Mediterranean Sea is required.
Eastern Central Pacific
The California drift gill net fishery for common thresher, A. vulpinus provided strong evidence that thresher sharks are highly vulnerable to rapid population decline when heavily fished, when landings of A. vulpinus declined to 28% of peak landings in 1982 by the late 1980s (Goldman 2005, Maguire et al. 2006). Pelagic fleets operating in the eastern central Pacific are known to take A. superciliosus, for example it is a bycatch of the purse seine fishery operating in the Eastern Pacific Ocean (Román-Verdesoto and Orozco-Zöller 2005). Ward and Myers (2005) estimated the biomass of thresher sharks to be approximately 5% of the virgin biomass and estimated a decline in abundance of 83% in the eastern tropical Pacific. 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).
This species is caught in tuna and swordfish longline fisheries, and to a lesser extent gillnets, with other wide-ranging oceanic shark species in this region. In the tuna and swordfish longline fisheries some vessels now also target sharks due to the increasing demand and commercial value of fins, particularly in Brazil and Uruguay (Domingo 2000). The landed catch and CPUE of Bigeye Thresher in the Santos (São Paulo) tuna longline fishery increased from 1971 to 1989, and then gradually decreased from 1990 to 1996 (Arfelli and Amorim 1994, Amorim et al. 1998). In later years a switch to shallower set longlines in the fishery (to target swordfish instead of tuna) reduced the catch of most sharks and may have affected the CPUE time-series which continued to show a decrease to 2001 (A. Amorim pers. obs.). Both A. superciliosus and A. vulpinus are caught in the Brazil Santos longline fishery, with A. superciliosus comprising the large majority of the catch and A. vulpinus only a small proportion of captures (Sadowsky and Amorim 1977). A. superciliosus has also been documented as bycatch in the industrial tuna longline fishery operating out of Natal, Rio Grande do Norte, northeastern Brazil (Hazin et al. 1990). Off Uruguay, CPUE values obtained from the pelagic longline fishery observer program for 2001-2005 are very low and show no clear trend (Berrondo et al. 2006). It is also taken by the Taiwanese tuna longline fishery operating in the South Atlantic (Joung et al. 2005).
This species is taken by a variety of pelagic fisheries throughout many areas of its range in the Indo-West Pacific. 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 amounts of sharks caught by longliners targeting swordfish in the Indian Ocean have been constantly increasing since the mid-1990s and some have switched to targeting sharks in recent years (IOTC 2006). Large gillnet fisheries operating off India and Sri Lanka have also been taking important catches of pelagic sharks since the 1980s. Finning and discarding of carcasses has also been reported, especially in offshore and high seas fisheries (Anderson and Simpfendorfer 2005, IOTC 2006). A recent review of fisheries in the Indian Ocean (Young et al. 2006) reported that sharks in this region are considered fully to over-exploited.
A. superciliosus is known to be taken by the Spanish surface longline fleet for swordfish and sharks, Korean tuna longline fleet and others operating in the Indian Ocean (García-Cortés and Mejuto 2000, Yang et al. 2005). Mejuto et al. (2006) report that fishing effort in this Spanish fleet has increased since 1993. It is also documented in catches of Spanish experimental cruises in the Southwest Indian Ocean (Ariz et al. 2006). South Africa's longline fisheries for tuna set a total of 21 million hooks between 2000 and 2005. Observer coverage (14%) indicated that the shark bycatch of this fishery comprised 4% thresher sharks, after Blue Shark, Shortfin Mako and Crocodile Shark (IOTC 2006). It is a known catch of tuna and shark longline and driftnet fisheries operating in the Indian Ocean and areas of the western Pacific (Rajruchithong et al. 2005, White et al. 2006). IOTC collects catch information for sharks, but this is considered very incomplete and cannot be used to estimate total catches of sharks in the Indian Ocean, even for species for which partial data are available (IOTC) (IOTC 2006). Both A. superciliosus and A. pelagicus have been reported in the catches but as noted above, no information is available to estimate the magnitude of catches or trends.
Japanese assessment of data from research longline surveys in the Pacific and Indian Oceans suggests that thresher shark (Alopias spp.) catch per unit effort increased in the 1990s (to near one shark per 1,000 hooks) over levels in the 1970s (near zero sharks per 1,000 hooks). However, this result is thought to be possibly attributable to an increase in hook depths in the latter period. In recent years, based on logbook data, recorded Japanese catches of thresher sharks worldwide ranged from 252 to 596 mt with an average of 347 mt. The resource is considered stable with no management action required other than ongoing monitoring (Japan Fisheries Agency 2006). Coastal longline fishermen off the coast of Japan report that they retain thresher sharks preferentially over other sharks because of their lower urea content. One fishermen cited values of 250 USD per shark for thresher shark carcasses (Gilman et al. 2007).
Although complete data on are not available for evaluation from this region, this species is a known catch of many fisheries operating throughout much of its range in this region, as described above. Given that this species has high biological vulnerability and a low intrinsic rate of increase, coupled with the declines observed in other areas of its range, declines are inferred based on continuing high levels of exploitation.
In Northwest Atlantic US waters, the Bigeye Thresher Shark (Alopias superciliosus), has been listed as a Prohibited Species under the Fishery Management Plan of the Atlantic tunas, swordfish and sharks (NMFS 2003) since 2000. The U.S Fishery Management Plan for Highly Migratory Species by the NMFS provides a biennial review of the status of Bigeye Thresher with management contingent on the health of the stock (PFMC 2004). No catch guidelines are recommended at this time on the West coast of the USA.
Family Alopiidae is listed on Annex I, Highly Migratory Species, of the UN Convention on the Law of the Sea, which urges States 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 biologically vulnerable shark. It is also essential to improve 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 fishing of thresher sharks for their fins alone.
|Citation:||Amorim, A., Baum, J., Cailliet, G.M., Clò, S., Clarke, S.C., Fergusson, I., Gonzalez, M., Macias, D., Mancini, P., Mancusi, C., Myers, R., Reardon, M., Trejo, T., Vacchi, M. & Valenti, S.V. 2009. Alopias superciliosus. The IUCN Red List of Threatened Species 2009: e.T161696A5482468.Downloaded on 27 April 2017.|
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