Sphyrna lewini (Western Indian Ocean subpopulation)
|Scientific Name:||Sphyrna lewini (Western Indian Ocean subpopulation)|
See Sphyrna lewini
|Red List Category & Criteria:||Endangered A4bd ver 3.1|
|Assessor(s):||Baum, J., Clarke, S., Domingo, A., Ducrocq, M., Lamónaca, A.F., Gaibor, N., Graham, R., Jorgensen, S., Kotas, J.E., Medina, E., Martinez-Ortiz, J., Monzini Taccone di Sitizano, J., Morales, M.R., Navarro, S.S., Pérez, J.C., Ruiz, C., Smith, W., Valenti, S.V. & Vooren, C.M.|
|Reviewer(s):||Musick, J.A. & Fowler, S.L. (Shark Red List Authority)|
Catch per unit effort of S. lewini declined significantly from 1978-2003 in shark nets off the beaches of Kwa-Zulu Natal, South Africa, suggesting a 64% decline over this period. Sphyrna lewini is captured throughout much of its range in the Indian Ocean, including illegal targeting of the species in several areas. Landings reported to FAO in Oman, surveys of landings sites in Oman and interviews with fishermen there also suggest that catches of S. lewini have declined. The species faces heavy fishing pressure in this region, and similar declines in abundance are also inferred in other areas of its range in this region. Given continued high fishing pressure, observed and inferred declines, the species is assessed as Endangered in the Western Indian Ocean.
|Range Description:||In the Indian Ocean, the Scalloped Hammerhead Shark occurs from South Africa (Western Cape to KwaZulu-Natal), Maldives, and Red Sea to Pakistan, India, Myanmar (Compagno in prep).|
Native:Egypt; Eritrea; India; Iran, Islamic Republic of; Kenya; Mozambique; Oman; Pakistan; Saudi Arabia; Somalia; South Africa; Sudan; Tanzania, United Republic of; United Arab Emirates; Yemen
|FAO Marine Fishing Areas:|
Indian Ocean – western
|Range Map:||Click here to open the map viewer and explore range.|
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||This is a coastal and semi-oceanic pelagic shark, found over continental and insular shelves and in deep water near to them, ranging from the intertidal and surface to at least 275 m depth (Compagno in prep.). The pups of this species tend to stay in coastal zones, near the bottom, occurring at high concentrations during summer in estuaries and bays (Clarke 1971, Bass et al. 1975, Castro 1983). They have been observed to be highly faithful to particular diurnal core areas (Holland et al. 1993) and sometimes form large schools which migrate to higher latitudes in summer (Stevens and Lyle 1989).|
Horizontal migration is observed from inshore bays to a pelagic habitat as the sharks grow. This species segregates by sex, with females migrating offshore earlier and at smaller sizes than males. In the Gulf of Mexico and northern Australia, it was observed that males less than 1 m long were more abundant over the continental shelf, but females bigger than 1.5 m dominated areas near the edge of the shelf. Adults spend most of the time offshore in midwater and females migrate to the coastal areas to have their pups (Clarke 1971, Bass et al. 1975, Klimley and Nelson 1984, Branstetter 1987, Klimley 1987, Chen et al. 1988, Stevens and Lyle 1989). Nursery areas are found in shallow inshore waters, while the adults are found offshore (Compagno 1984, Holland et al. 1993, Kotas et al. 1995, Lessa et al. 1998). Neonates and juveniles are known to shoal in confined coastal pupping areas for up to two years before moving out to adult habitat (Holland et al. 1993). In the Northwest and Western Central Atlantic, the coastal area between South Carolina and central Florida is believed to be an important nursery area (Castro 1993). In southern Brazil, near-term gravid females migrate inshore to nursery grounds (at 2–10 m depth; bottom water temperature of 20–24°C) and give birth in spring (November–February) (Dono et al. in prep., Vooren and Lamónaca 2003). Juveniles then remain between the shore and 100 m depth (Vooren 1997, Kotas et al. 1998). In northern Brazil (latitude 3°S), this species appears to breed at a smaller size and have lower fecundity than reported elsewhere (Lessa et al. 1998).
Throughout the species’ range in the Eastern Pacific, parturition is thought to occur between May and July in shallow nursery areas (Ruiz et al. 2000, Torres-Huerta 1999). The northern Gulf of California and Bahía Almejas on the Pacific coast of Baja California Sur appear to be important pupping and possible nursery grounds.
The species is viviparous with a yolk-sac placenta. Only the right ovary is functional. In Taiwanese (POC) waters, ovum development takes approximately 10 months and ova reach a maximum diameter of 40–45 mm. The number of oocytes in the ovarium can be as many as 40–50 per female (Chen et al. 1988). The gestation period is around 9–12 months, with birth in spring and summer. The average number of embryos in the uterus ranges from 12–41 and females pup every year. Newborn size ranges from 31–57 cm (Castro 1983; Compagno 1984; Branstetter 1987; Chen et al. 1988; Stevens and Lyle 1989; Chen et al. 1990; Oliveira et al. 1991, 1997; Amorim et al. 1994; White et al. 2008). Predation on pups and juveniles is high, mainly by other carcharhinids and even by adults of the same species. This is probably the most significant source of natural mortality on the population (Clarke 1971, Branstetter 1987, Branstetter 1990, Holland et al. 1993), and may explain, in evolutionary terms, the higher fecundity of this species compared to some other sharks.
Maximum size reported by different studies, ranged from 219–340 cm TL for males and 296–346 cm for females (Clarke 1971, Bass et al. 1975b, Schwartz 1983, Klimley and Nelson 1984, Stevens 1984, Branstetter 1987, Chen et al. 1988, Stevens and Lyle 1989, Chen et al. 1990). Males mature between 140–198 cm TL and females at around 210–250 cm TL (Compagno 1984b, Branstetter 1987, Chen et al. 1990, Carrera and Martinez in prep., White et al. 2008). Branstetter’s (1987) growth study in the Gulf of Mexico found asymptotic length for both sexes of 329 cm TL and 253 cm fork length (FL), with an index of growth rate of k = 0.073 y-1. Piercy et al.’s (2007) more recent study used Fork Length (FL) rather than total length (TL) and suggested faster growth, with asymptotic length of 214.8 cm FL for males and 233.1 cm FL for females, with an index growth rate of k=0.13 year-1 for males and k=0.09 year-1 for females. It is unclear whether these differences are related to sample size, methodology or changes resulting from a density-dependent compensatory response to population depletion. In Ecuadorian waters, Carrera-Fernández and Martínez-Ortíz (2007) found that females matured at 225 cm TL, reaching a maximum size of 302 cm TL, and males matured at 190 cm TL, reaching a maximum size of 282 cm TL.
The age and size of first maturity has been studied in several different areas; the Gulf of Mexico, Western Central Atlantic, Taiwanese (Province of China) waters, Northwest Pacific and Mexican waters, Eastern Central Pacific. Branstetter (1987) estimated that males mature at 10 years, 180 cm TL and females at 15 years, 250 cm TL in the Gulf of Mexico. During a recent study by Piercy et al. (2007) on the age and growth of S. lewini in the Gulf of Mexico the oldest age estimate obtained was 30.5 years for both males and females. Whereas, Chen et al. (1990) estimated that males mature at 3.8 years, 198 cm TL and females at 4.1 years, 210 cm in Taiwanese Pacific waters and Anislado-Tolentino and Robinson-Mendoza (2001) estimated that males mature at 4.3 years and females at 5.8 years in the Mexican Pacific waters. Both studies in the Gulf of Mexico show that this species appears to grow more slowly and have smaller asymptotic sizes than reported in the Pacific Ocean. The vast differences in age and growth reported between Taiwanese Pacific waters/Mexican Pacific waters and other oceanic regions may arise from different interpretation of vertebral band formation rather than true geographic variation (W. Smith pers. comm.). Current published age estimates of S. lewini from the Mexican Pacific and Taiwanese Pacific are based on growth estimates that assume the deposition of two centrum annuli per year (Chen et al. 1990, Ansilado-Tolentino and Robinson-Mendoza 2001), whereas studies in the Gulf of Mexico assume the deposition of one growth band per year (Branstetter 1987, Piercy et al. 2007). The Pacific estimates have not been validated and the deposition of two centrum annuli has not been confirmed in any other shark species to date (W. Smith pers. comm.), therefore these estimates should be viewed with caution. Previous evidence of the deposition of two annual bands in the Shortfin Mako Shark (Isurus oxyrinchus), has not proven to be valid and this may be the case for S. lewini (Campana et al. 2002). If growth data presented by Chen et al. (1990) were converted to reflect a one growth band per year hypothesis, then the results of these studies would agree more closely. Validation of the periodicity of growth-band deposition is required for both the Pacific and Atlantic populations to resolve this issue (Piercy et al. 2007).
Comparing different estimates for the values of k on S. lewini (0.054–0.160 yr-1), by different authors, suggests that this is a ‘medium growth species’ (Branstetter 1987). Smith et al. (1998) estimated the intrinsic rate of increase at MSY of 0.028.
Adult S. lewini feed on mesopelagic fish and squids. In certain areas stingrays of the (Dasyatis spp.) are the preferred food. Pups and juveniles feed mainly on benthic reef fishes (e.g., scarids and gobiids), demersal fish and crustaceans. (Bigelow and Schroeder 1948, Clarke 1971, Bass et al. 1975, Compagno 1984, Branstetter 1987, Stevens and Lyle 1989).
Reliable species-specific catch information is available for shark nets set off the beaches of Kwa-Zulu Natal, South Africa, in the southewestern Indian Ocean, from 1978–2003 (Dudley and Simpfendorfer 2006). Catch per unit effort of S. lewini declined significantly during this period from approximately 5.5/km net/year to approximately 2/km net/year (Dudley and Simpfendorfer 2006). This fishery independent data indicates a decline of approximately 64% over a 25 year period. About 120 longline vessels were reportedly operating illegally in coastal waters of the western Indian Ocean prior to 2005, and this number was expected to increase (IOTC 2005). These vessels are primarily targeting hammerhead sharks and Giant Guitarfish (Rhynchobatus djiddensis) for their fins (Dudley and Simpfendorfer 2006). Illegal fishing by industrial vessels and shark finning are reported in other areas of the Indian Ocean also (Young et al. 2006). Dudley and Simpfendorfer (2006) also report large catches of newborn Sphyrna lewini by prawn trawlers on the Tudela Bank, South Africa, ranging from an estimated 3,288 in 1989 to 1,742 in 1992, with almost 98% mortality. An inshore, artisanal fishery that uses multiple gear types (including seine nets and gillnets) along the coast of Mozambique and takes sharks as bycatch also potentially affects S. lewini (Dudley and Simpfendorfer 2006).
Sphyrna lewini is captured in various other fisheries throughout the rest of its range in the Indian Ocean. Few species-specific data are available from other areas, however, declines are also likely to have occurred in other areas where this species is heavily fished. Other countries with major fisheries for sharks include the Maldives, Kenya, Mauritius, Seychelles and United Republic of Tanzania (Young et al. 2006). Sharks are considered fully to over-exploited in these waters (Young et al. 2006). Landings data are available from FAO for Oman since 1985. Sphyrna lewini is one of five dominant species in the catches of Oman. Landings of sharks for Oman varied between 2,800– 8,300 t, since 1985, with peaks noted from 1986–1988 and 1995–1997. After 1997 landings continued to decline to under 4,000 t in 2000 (FAO 2008). Oman has a long-established traditional shark fishery (Henderson et al. 2007). Henderson et al. (2007) surveyed landings sites in Oman between 2002 and 2003 and report a notable decline in catches of S. lewini in 2003, although the trend varied between areas. Henderson et al. (2007) note that large pelagic sharks such as S. lewini were displaced during 2003 by smaller shark species. Although it is possible that this is due to sampling bias, informal interviews with fishermen revealed a general trend of declining shark catches over the last number of years, particularly large pelagic species (Henderson et al. 2007). Artisanal gillnet and longline fisheries also target sharks off Madagascar for their fins, which are exported in the international shark fin trade. A study of directed shark fisheries at two sites in southwest Madagascar from 2001–2002 showed that hammerhead sharks represented 29% of sharks caught and 24% of the total wet weight, but species-specific data are not available because fishermen do not differentiate between S. lewini and S. zygaena (McVean et al. 2006).
Fishing pressure is also high in other areas of the Indian Ocean and Western Pacific, with many countries in this region among the largest shark fishing nations in terms of global catch in the world (Clarke and Rose 2005, SEAFDEC 2006). Indonesia has the largest chondrichthyan fishery in the world, with a reported 105,000 and 118,000 tonnes landed in 2002 and 2003 respectively (White et al. 2006). This species is a target and bycatch of shark longline, tuna gillnet fisheries and trawls in several areas of this region (White et al. 2006, SEAFDEC 2006). The species is utilised for its fins (high value in adults), meat, skin and cartilage (White et al. 2006, SEAFDEC 2006). White et al. (2008) suggest that this species is prone to overfishing in Indonesian waters, where substantial catches of S. lewini are taken in gillnet and longline fisheries. They found that almost all of the S. lewini caught by gillnetting, and the majority caught by longlining, were immature, and were therefore removed from the population before they had the opportunity to breed. Inshore fishing pressure is intense throughout Southeast Asia and juveniles and neonates are very heavily exploited, with large numbers of immature sharks in catches in other areas also (SEAFDEC 2006). Foreign vessels are also reported to target sharks in eastern Indonesian waters (Clarke and Rose 2005). Given the marked declines in this species’ abundance in areas for which data are available, there is every reason to suspect that declines have also occurred in other areas of the Indian Ocean and Western Pacific, where fishing pressure is high.
Japanese data on hammerhead species are limited, but reported landings in Japan’s coastal ports totaled 11–34 mt annually between 2000 and 2004 with an average of 24 mt per annum. No CPUE trends are available (Japan Fisheries Agency 2006).
|Citation:||Baum, J., Clarke, S., Domingo, A., Ducrocq, M., Lamónaca, A.F., Gaibor, N., Graham, R., Jorgensen, S., Kotas, J.E., Medina, E., Martinez-Ortiz, J., Monzini Taccone di Sitizano, J., Morales, M.R., Navarro, S.S., Pérez, J.C., Ruiz, C., Smith, W., Valenti, S.V. & Vooren, C.M. 2007. Sphyrna lewini (Western Indian Ocean subpopulation). The IUCN Red List of Threatened Species 2007: e.T165294A6001370.Downloaded on 19 February 2018.|