|Scientific Name:||Carcharhinus obscurus|
|Species Authority:||(Lesueur, 1818)|
|Infra-specific Taxa Assessed:|
Squalus obscurus Lesueur, 1818
|Taxonomic Source(s):||Compagno, L.J.V. 1973. Carcharhinidae. In: J.-C. Hureau and T. Monod (eds), Check-list of the fishes of the north-eastern Atlantic and of the Mediterranean (CLOFNAM). Volume 1, pp. 23-31. Unesco, Paris.|
|Red List Category & Criteria:||Vulnerable A2bd ver 3.1|
|Assessor(s):||Musick, J.A., Grubbs, R.D., Baum, J. & Cortés, E.|
|Reviewer(s):||Stevens, J.D., Dudley, S.D. Pollard, D. & Soldo, A. (Shark Red List Authority)|
The Dusky Shark (Carcharhinus obscurus) is a large wide-ranging coastal and pelagic warm water species, which is among the slowest-growing, latest-maturing of known sharks, bearing small litters after a long gestation period. Its very low intrinsic rate of increase renders this species among the most vulnerable of vertebrates (including the great whales and sea turtles) to depletion by fisheries. Unfortunately the dusky shark is difficult to manage or protect because it is taken with other more productive sharks in mixed species fisheries, and has a high mortality rate when taken as bycatch. This species' fins are highly valued. Time series data are available from the Northwest and Western Central Atlantic, where catch rates have declined. Management requiring all individuals captured in the US longline fishery to be released was introduced in 2000, however, while this may have led to an increase in the numbers of juvenile sharks, adults still appear to be declining. A recent stock assessment of the fishery off southwestern Australia estimated that CPUE of this species declined by >75% from the early 1970s-2004. Additional management measures were then introduced to this fishery in 2006. Given the very high intrinsic vulnerability of this species to depletion, significant estimated declines in several areas of its range and inferred declines in highly fished areas from which data are not available, C. obscurus is assessed as Vulnerable globally.
Northwest and Western Central Atlantic
The initial decline in C. obscurus in this area was caused by a targeted recreational fishery that developed in the late-1970s and by bycatch in the pelagic swordfish longline fishery. This was followed by rapid expansion on the US directed commercial shark fishery in the late 1980s. The species was protected in US Atlantic waters in 2000 as a result of declines in abundance. Although this management action may have led to an increase in the numbers of juvenile Dusky Sharks, adults still appear to be declining. A stock assessment which analysed catch data and multiple fisheries-independent and fisheries-dependent time series data sets led to estimated declines in dusky shark abundance of 62-92% between 1974 and 2003. Other analyses based on long term survey data from off North Carolina, observer data from the US Atlantic pelagic longline fishery and data from US pelagic longline research surveys and observer data from the Gulf of Mexico estimated declines of between 70 and 98.8% over periods of 13-40 years. Given the decline in abundance in this region, C. obscurus is assessed as Endangered in the Northwest and Western Central Atlantic.
The first record of the presence of this species in the Mediterranean was in 1928 off Spain, and since then, records from other areas (e.g., Sicily Straits) have been sporadic but possibly under-represented because of misidentification with similar "grey sharks" requiem sharks. It is caught sporadically in a variety of fishing gears, principally off North Africa, but also in the Sicilian Channel, and marketed. Significant population declines have been estimated in other large shark species in the Mediterranean Sea as a result of intensive coastal and pelagic fishing pressure. However, there is currently insufficient information on occurrences of this species there to make an assessment beyond Data Deficient in this region.
This species is taken both incidentally and as a target species in longline and intensive artisanal fisheries in the South Atlantic. A number of countries operate longline fleets targeting tuna and swordfish in the Southwest Atlantic. Sharks are now known to be targeted due to increasing demand for shark products and the high value of their fins. Significant declines have been estimated in other areas of this species' range for which population trend data are available. Given the species' highly restricted life-history characteristics and continued fishing pressure in this area, C. obscurus is considered to qualify for at least Near Threatened in the Southwest Atlantic and may be found to meet the criteria for Vulnerable A2bd.
This species occurs throughout Australian waters. A demersal gillnet fishery off southwestern Australia targets neonates of this species on their nursery grounds. This fishery developed in the 1940s and rapidly increased in the late 1970s to produce annual catches of 500-600 t. A recent stock assessment found that the stock was less productive than previously thought, and that mortality of older dusky sharks in wetline fisheries outside the target fisheries was leading to a decline in recruitment. This assessment also estimated that the catch per unit effort (CPUE) of Dusky Sharks declined by more than 75% between the early 1970s and 2004 and that the decline was continuing. In 2006 additional management measures were introduced to the fishery, including a maximum size limit for dusky shark. These management measures should arrest further declines, but continued monitoring and assessment will be essential to monitor the stock, and the effectiveness of these measures. All this considered, the species is assessed as Near Threatened throughout Australian waters, close to meeting the criteria for Vulnerable A1bd. Continued monitoring and regular reassessment is recommended.
|Previously published Red List assessments:|
|Range Description:||The Dusky Shark has a cosmopolitan but patchy distribution in tropical and warm temperate seas (Last and Stevens 1994). |
Western Atlantic: Southern Massachusetts and Georges Bank to Florida, Bahamas, Cuba, northern Gulf of Mexico, and Nicaragua, southern Brazil (Compagno in prep., Last and Stevens 1994).
Eastern Atlantic: This shark's distribution is uncertain in the northeast and eastern central Atlantic and these records, and others from tropical insular areas, may be misidentifications of C. galapagensis (J. Musick pers. comm.). It has been recorded from Canary and Cape Verde Islands, Senegal and Sierra Leone. Uncertain records exist from elsewhere, including Portugal, Spain, Morocco and Madeira (Compagno in prep., Last and Stevens 1994).
Mediterranean Sea: Most records are from the western and central-southern regions, along the North African coasts and Sicily Straits. It is likely that this species ranges further east in the Ionian Sea and Levantine Basin (Fergusson and Compagno 2000). Around 20 specimens have been recorded in the Mediterranean Sea to date: after Lozano Rey's first record for the Mediterranean (1928), Moreno (1982) cites three specimens from the Alboran Sea and Hemida and Labidi (2002) reported another three from eastern Algeria. A single Tunisian report is from Capapé et al. (1979) and similarly, one from Malta (Fergusson and Compagno 2000). At Mazara del Vallo, Sicily, a further three specimens were observed (Cigala-Fulgosi pers. comm. in Fergusson and Compagno 2000) and Vacchi and Serena (1997) observed one. A single specimen was caught at Capo Testa, Sardinia (Fergusson and Compagno 2000) and at least five were seen in Tripoli, Libya, by Trevor Meyer (pers. comm. 2002).
Indian Ocean: South Africa, Mozambique, Madagascar, and possibly in the Red Sea. Also, patchy records scattered in the Arabian Sea and recorded from the Andaman Islands in the Bay of Bengal (Compagno in prep., Last and Stevens 1994).
Western Pacific: Japan, China, Viet Nam, New Caledonia and throughout Australian waters (Compagno in prep., Last and Stevens 1994).
Eastern Pacific: Southern California to Gulf of California, Revillagigedo Islands, and possibly Chile (Compagno in prep., Last and Stevens 1994).
The Dusky Shark undertakes long temperature-related migrations. On both coasts of the U.S., Dusky Sharks migrate northward in summer as the waters warm and retreat southward in fall as water temperatures drop (Musick et al. 1993). In western Australia, adolescents and adults move inshore during the summer and fall, with neonates occupying separate inshore areas (Last and Stevens 1994). Seasonal migrations (north in winter and south in summer) also occur off South Africa (Bass et al. 1973). In the Indian Ocean, the young are known to aggregate in dense assemblages when feeding (Compagno 1984). Nursery areas occur in shallow waters (Compagno in prep.). Off Brazil, Mazzoleni (2000) suggested that there was possibly a nursery area in the north of Santa Catarina State, when 79 neonates were caught by the artisanal fishery from 1994 to 2000. Neonates were abundant in the end of summer to the middle of autumn (February-May). In the winter season virtually no specimens were caught. Nursery areas also occur off the southern coast of Natal, South Africa and off South Carolina, USA (Compagno in prep.).
Native:Algeria; Australia; Bahamas; Belize; Brazil; Cape Verde; China; Colombia; Costa Rica; Cuba; El Salvador; French Guiana; Guatemala; Guyana; Honduras; Italy (Italy (mainland), Sardegna); Japan; Libya; Madagascar; Mexico (Revillagigedo Is.); Mozambique; New Caledonia; Nicaragua; Panama; Senegal; Sierra Leone; South Africa; Suriname; United States (Alabama, California, Connecticut, Delaware, Florida, Georgia, Maryland, Massachusetts, Mississippi, New Jersey, North Carolina, Rhode Island, South Carolina, Texas, Virginia); Venezuela, Bolivarian Republic of; Viet Nam
|FAO Marine Fishing Areas:|
Atlantic – northeast; Atlantic – eastern central; Atlantic – southwest; Atlantic – southeast; Atlantic – northwest; Atlantic – western central; Indian Ocean – eastern; Indian Ocean – western; Mediterranean and Black Sea; Pacific – western central; Pacific – eastern central; Pacific – northwest; Pacific – southwest; Pacific – southeast
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||This species is estimated to have undergone population decline in several areas of its range, as summarized in the threats section, below.|
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||The majority of the information below is taken from Camhi et al. (2005).|
This shark is coastal and pelagic in its distribution, where it occurs from the surf zone to well offshore, and from the surface to depths of 400 m (Compagno 1984). Because it is poorly adapted to osmoregulate at lower salinities, it is not commonly found in estuaries (Compagno 1984, Musick et al. 1993). A study off Brazil by Motta et al. (1997) recorded the species at depths of 8-15 m off southern Sao Paulo State. Tagging studies in the southwestern Indian Ocean (Davies and Joubert 1967, Bass et al. 1973), the Northwest Atlantic, Gulf of Mexico (Kohler 1996), and the southeastern Indian Ocean (Simpfendorfer unpublished data) have all shown that C. obscurus is a highly migratory species. The longest distance between tagging and recapture is 2,052 nautical miles, and the longest period at liberty 15.8 years. Movements normally show seasonal patterns, with adults moving into more temperate areas as temperatures rise in summer. Movements of adults are longer than those of neonates and juveniles, although juveniles of approximately a year old have been recorded moving as much as 742 nautical miles off South Africa (Dudley et al. 2005). The juveniles are known to migrate down as far as the southern and western Cape when the waters warm up during the summer months. They retreat back to the east coast as it cools (D. Ebert pers. comm. 2004). Major nursery areas for C. obscurus have been identified off the KwaZulu-Natal coast of South Africa (Bass et al. 1973), the New Jersey to South Carolina coast of the United States (Musick and Colvocoresses 1988, Castro 1993), and the southwest coast of Australia (Last and Stevens 1994, Simpfendorfer 1999). The neonates occur in nearshore waters in all of these nursery areas, but do not enter lower salinity areas.
Carcharhinus obscurus is a large shark that reaches 360 cm in length and 180 kg (Castro 1983). Off KwaZulu-Natal a female of 383 cm precaudal length (PL) and 450 kg has been captured in the protective shark nets (Dudley et al. 2005). Maximum sizes recorded in the Mediterranean Sea were a male of 311 cm and a female of 349 cm (Fergusson and Compagno 2000). Size and age data are available from several areas. In the northwest Atlantic, males mature at 231 cm fork length (FL) and at 19 years of age, and females at 235 cm FL and at 21 years of age (Natanson et al. 1995). In the southwest Indian Ocean, off South Africa, Dudley et al. (2005) report that males mature at 210 cm PL and 19.2 years of age and females at 214 cm PL and 20 years of age. The oldest Dusky Shark reported from vertebral ageing studies is 37 years, although they are believed to live to a maximum of 40-50 years (Natanson et al. 1995, Sminkey 1996).
The Dusky Shark is placentally viviparous, with litters normally ranging in size from 3-16 pups, of 70-100 cm (Last and Stevens 1994, Dudley et al. 2005). Recent work has suggested that gestation may be as long as 22 months (Branstetter and Burgess 1996, Dudley et al. 2005, Romine 2004). The lack of large yolky ova in the ovary of late-term pregnant C. obscurus indicates that there is one year resting period between birth and mating, making the reproductive cycle at least three years long (Musick 1995, Branstetter and Burgess 1996, Romine 2004).
Recent demographic analyses of C. obscurus in the western Atlantic have generated estimates of the annual rate of population increase of 2.8% (Cortés 1998) and 5.57% (Sminkey 1996). Both of these estimates are for the population without fishing mortality and assume a two-year reproductive cycle. Given that it is now thought that the reproductive cycle lasts three years these population increase rates are probably lower. Romine (2004) estimated the annual rate of population increase only to be around 1.9% and the population doubling time was estimated to be 36 years. Simpfendorfer (1999), using a three year reproductive cycle, estimated the annual rate of population increase for the Australian population was 4.3%. The low rates of population increase highlight the need for conservative management of fisheries that capture C. obscurus (Cortés 1998).
Diet: Carcharhinus obscurus has a varied diet that includes teleosts, elasmobranchs and cephalopods. Neonates and juveniles mostly consume small pelagic teleosts (e.g., sardines and anchovies) and squid (Smale 1991, Stevens 1990, Simpfendorfer pers. data). With increasing size larger teleosts (e.g., groupers, jacks) and elasmobranchs (e.g., dasyatids Raja spp., Rhinobatus spp., squatinids, carcharhinids, mustelids and squalids) become more important in the diet (Bass et al. 1973, van der Elst 1979, Castro 1983, Smale 1991, Gelshleichter et al. 1998). As a common apex predator C. obscurus plays an important (but poorly studied) role in the marine ecosystem. In the western Atlantic, the dusky has always been less abundant than some other species of carcharhinid sharks with which it is sympatric, such as the sandbar shark (Musick et al. 1993). This seems to be in keeping with its larger size and higher trophic position.
|Movement patterns:||Full Migrant|
|Use and Trade:||Both the fins and meat are considered among the most valued in their respective categories. The flesh is utilized fresh, dried salted, frozen and smoked for human consumption; hides used for leather; fins for shark-fin soup base; and liver oil extracted for vitamins (Compagno in prep.).|
Carcharhinus obscurus is taken as both bycatch and target in commercial shark fisheries using, set nets, longlines, hook and line and trawls (Cramer 1995, Compagno in prep.) in many areas of its range. The species has among the most sought after fins for shark fin soup because of their large size and high fin needle content (ceratotrichia) (TRAFFIC 1996, R. Hudson pers. comm.). Because of its high-value fin, dusky sharks caught incidentally in tuna and swordfish fisheries are now regularly landed rather than released. Carcharhinus obscurus was found to represent at least 1.2-1.7% of the fins auctioned in Hong Kong, the world's largest shark fin trading center (Clarke et al. 2006a). It is estimated that between 144,000 and 767,000 Dusky Sharks are represented in the shark fin trade each year or, in biomass, 6,000 to 30,000 mt (Clarke et al. 2006b). The very low intrinsic rate of increase of the Dusky Shark renders this species among the most vulnerable of all vertebrates (including great whales and sea turtles) to man-induced mortality (Compagno et al. 2005, Musick 1999). Furthermore, Compagno et al. (2005) report that the species is difficult to manage or protect because it is taken in mixed species fisheries, and has a high mortality rate when taken as bycatch.
Further threats to C. obscurus are from beach meshing programs in Australia and South Africa and from recreational fishing. Beach meshing in Australia (Queensland and News South Wales) undoubtedly catches C. obscurus, however, species-specific data are not available. Between 1972 and 1990, the New South Wales programme caught a total of 765 whaler sharks (Reid and Krough 1992), of which Stevens (1984) reported the capture of larger juvenile and adult C. obscurus by recreational fishers off the east coast of Australia. Dusky sharks were one of the most important species in the trophy shark tournaments held in Florida, USA, until the stock collapsed (Heuter 1994).
Northwest and Western Central Atlantic
The initial decline of C. obscurus in the Northwest and Western Central Atlantic was caused by a targeted recreational fishery that developed in the late-1970s and by bycatch in the pelagic swordfish longline fishery (Musick et al. 1993). A rapid expansion of the directed commercial shark fishery in the US in the late 1980s was fuelled in large part by the demand for shark fins in the markets of Asia (Cook 1990). Although Dusky Shark meat is used domestically in the US, the very high value of the fins suggests that the decline in this dusky shark population over past decades has been, and continues to be, driven by international trade in shark fins. There is little reason to believe that the demand for dusky shark products will lessen, especially as other fishery resources become increasingly depleted. In the Gulf of Mexico during the late 1980s, the dusky shark was the fourth most abundant species in the tuna longline bycatch, where medium to large dusky sharks were often shot, finned and discarded (Russell 1993).
Declining catch rates for Dusky Sharks in this area have been a cause for concern. Off North America the proportion of C. obscurus in the catch decreased, while fishing for more abundant species continued (Musick et al. 1993). Carcharhinus obscurus was put on the protected list in 2000, requiring all individuals captured in the longline fishery to be released. Even though the mortality of small juveniles on the longline was as high as 50% (lower for larger juveniles and adults) (Romine 2004), the juveniles have shown an increase of about 30% from the lowest point in the time series to 2005 in the Virginia Institute of Marine Science (VIMS) survey, and continue to rise. This increase was not apparent in the stock assessment, which analysed catch data and multiple fisheries-independent and fisheries-dependent time series data sets in the western Atlantic (NMFS 2006). This assessment led to estimated declines of 62-92% between 1974 and 2003. These declines are of the same magnitude as those found in analyses of the VIMS survey data (Musick et al. 1993, Romine 2004, Ha 2006), but in these studies the low point in Dusky Shark abundance occurred in the early-1990s. The NMFS (2006) assessment, which did not include the 2004-2006 VIMS survey data (which shows a further increase in juveniles) may not have detected the recent increase of juveniles because it was heavily influenced by pelagic data sets collected in deep water where juveniles are absent. The other long-term shark targeted survey in this region, which has been conducted off Cape Lookout, North Carolina through the University of North Carolina annually between 1972 and 2005 (data available to 2003) has captured 1,036 Dusky Sharks. This survey series shows a large, statistically significant decline of 98.8% (95%CI: 97.5-99.6%) and found evidence of no increase in recent years (Myers et al. in prep.).
A new analysis of observer data from the U.S. Atlantic pelagic longline fishery from 1992-2005 (which combined catches of Dusky, Silky, and Night Sharks because of identification problems) suggests that this species complex has declined significantly, by 70% (95% CI: 54-81%) during this recent time period (Baum et al. in prep.). The observer data shows a steeper decline when Dusky Shark is analysed alone, but this analysis ignores species identification problems, and hence is not considered reliable (Baum et al. in prep.). For the Gulf of Mexico, an analysis of data from U.S. pelagic longline research surveys conducted in the mid-1950s and U.S. pelagic longline observer data from the late-1990s estimated that dusky sharks declined by 79% over this forty-year period (Baum and Myers 2004), which is less than the three generation period for this species. Thus, recent management actions in the US may have led to an increase in the numbers of juvenile Dusky Sharks, however adults still appear to be declining.
This species is taken both incidentally and as a target species in longline and artisanal fisheries throughout Brazilian waters and elsewhere in the South Atlantic. It is taken by longline off Santos where it is retained as "other sharks" (Arfelli and Amorim 1994). The species has also been recorded from artisanal fisheries off Rio de Janeiro State (Sant'Anna and Siqueira 2000) and from southern Sao Paulo State (Motta et al. 1997, Bertozzi et al. 2000, Gadig et al. 2000). From July 1996 to February 1997 the species made up 1.12% of the catch (Motta et al. 1997) and 5.61% of the total catch from 1996 to 1999, which consisted mostly of juveniles, most frequently caught in June and August (Gadig et al. 2000). A number of countries operate longline fleets targeting tuna and swordfish in the high seas areas of the Southwest Atlantic region. In addition to the coastal nations of the Southwest Atlantic, nations including Taiwan, Korea, Japan, Spain, Bolivia, Cape Verde, United Kingdom, China and Barbados also operate vessels here. Tuna and swordfish longline fisheries now also target sharks due to increasing demand for shark products and the value of their fins (Bonfil et al. 2005, Mejuto et al. 2005).
The main threats in the Mediterranean Sea are commercial fisheries. This species is caught sporadically in longline, setline, gillnet and sometimes by tuna trap ("Tonnara - Tonnarella") fisheries, principally off North African and rather less frequently by surface longlines, artisanal setlines and possibly trawlers in the Sicilian Channel. Carcharhinid sharks have been caught as target or bycatch in historical fisheries in this region, where fishing pressure is high, however there are few species-specific records from which to elucidate population trends. Carcharhinus obscurus is rarely observed on fishmarkets in the Mediterranean, but is easily mistaken for other "grey" requiem shark carcasses, such as C. plumbeus. This species is utilised for human consumption in several areas of the Mediterranean, including Sicily, Malta and Libya, although the meat is considered of low commercial value.
The fishery for C. obscurus off southwestern Australia developed in the 1940s, but rapidly increased in the late 1970s to produce annual catches of 500-600 t. The fishery uses demersal gillnets (16.5 to 17.8 cm stretched mesh) to target neonates in the nursery area and the selectivity of the nets results in very few individuals over three years of age being captured. The flesh of the young C. obscurus is highly regarded and fetches a good price on local markets. Fins are also sold. Current estimates are that 18-28% of neonates are caught in their first year. An assessment using demographic models indicated that the fishery was sustainable at the then level of catch provided the fishing mortality of animals larger than two metres was less than 4% (Simpfendorfer 1999). However, a more recent assessment (McAuley et al. 2005) found that the stock was less productive than previously thought, and that mortality of older dusky sharks in wetline fisheries outside the target fisheries was leading to a decline in recruitment. This assessment also estimated that the catch per unit effort (CPUE) of Dusky Sharks declined by more than 75% between the early 1970s and 2004, and that the decline was continuing. In 2006 additional management measures were introduced to the fishery (see below), including a maximum size limit for Dusky Shark. These management measures should arrest further declines, but continued monitoring and assessment will be essential to monitor the stock, and the effectiveness of these measures.
Off South Africa, there is a pelagic tuna longline fishery that may take some as bycatch but this is unconfirmed. There is a small commercial line fishery taking juvenile C. obscurus, and there is a recreational line fishery that targets juvenile C. obscurus but most are released (they were not released a decade ago) and there is the beach meshing operation that catches juveniles, adolescents and adults. Carcharhinus obscurus would have been a significant component in the beach meshing program off KwaZulu-Natal, South Africa, between 1978 and 1999, when the mean annual catch of was 256 individuals (range 129-571, Dudley et al. 2005). There was no trend in either catch or catch rate. A subsequent update by Dudley and Simpfendorfer (2006) showed no trend in either catch rate or size over the period 1978-2003. The large mesh size of the nets used in these programs means that many of the C. obscurus taken are sub-adults and adults, but juveniles are also caught. The occurrence of sub-adult and adult C. obscurus off KwaZulu-Natal is affected by an annual winter influx of sardines Sardinops sagax and shark catches are influenced by attempts to remove the nets in advance of the arrival of the sardine shoals (Dudley et al. 2005). Reports of C. obscurus in recreational fisheries are limited. Van der Elst (1979) reported that large numbers of juvenile C. obscurus were taken by recreational shore anglers in South Africa and Govender and Birnie (1997) have expressed concerns about the high rate of instantaneous fishing mortality in this fishery, although there is an increasing tendency to release the sharks.
Precautionary adaptive collaborative management of target and bycatch fisheries is needed for this biologically vulnerable shark. It is also essential to improve data collection and develop stock assessments for this species. Family Carcharhinidae is listed as highly migratory under the 1995 UN Agreement on the Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks (UNFSA). The Agreement specifically requires coastal and fishing States to cooperate and adopt measures to ensure the conservation of listed species. To date, there has been little progress (see United Nations Convention on the Law of the Sea for further details). Also of relevance is the FAO International Plan of Action for the Conservation and Management of Sharks (IPOA-Sharks) which recommends that Regional Fisheries Organisations (RFOs) carry out regular shark population assessments and that member States cooperate on joint and regional shark management plans. This is of particular importance for species such as dusky shark whose stocks are exploited by many States on the high seas. Steps are being taken by some RFOs, such as ICCAT, to collect species-specific data on pelagic sharks. To date two RFOs, ICCAT and IATTC, have adopted finning bans, as have several range states (e.g., Canada, USA, EU, Australia, Brazil etc.). More are likely to follow suit.
Canada and the USA have shark management plans (NMFS 1993, Joyce 1999). In US Atlantic waters dusky sharks are a prohibited species (outside of the shark research fishery) on the Fishery Management Plan for Atlantic tunas, swordfish, and sharks. Prohibited species must be released immediately with minimum injury and without removing them from the water. The population appears to be responding to this measures, with substantial recruitment over the last six years (Ha 2006, Musick unpublished, Romine 2004), but it remains to be seen how many of these recruits will survive to maturity given the high bycatch mortality.
Management of the Australian fishery is through input controls implemented as time-gear units. In 2006, the Western Australian Government introduced a number of changes in all commercial fisheries to reduce mortality, particularly of dusky and sandbar sharks, including: a maximum size limit for Dusky Shark; additional controls on the use of longline; and the conversion of monthly gear units to daily gear units (McLoughlin 2008, McAuley et al. 2005). The main management objective is to achieve target biomass levels of 40% of the initial biomass by 2040 for Dusky Shark (McLoughlin 2008).
|Citation:||Musick, J.A., Grubbs, R.D., Baum, J. & Cortés, E. 2009. Carcharhinus obscurus. The IUCN Red List of Threatened Species 2009: e.T3852A10127245.Downloaded on 30 April 2017.|
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