Sphyrna lewini (Eastern Central Atlantic subpopulation)
|Scientific Name:||Sphyrna lewini (Eastern Central Atlantic subpopulation)|
See Sphyrna lewini
|Red List Category & Criteria:||Vulnerable 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)|
Although there are no data on species-specific trends in abundance for S. lewini in the Eastern Central Atlantic, fishing pressure from pelagic longline fleets in this area is high and potentially comparable to that in the Northwest and Western Central Atlantic, where significant declines in abundance of S. lewini have been documented. The larger hammerhead shark, Sphyrna mokarran, is assessed as Critically Endangered in this region, from which it has apparently virtually disappeared. There is also concern for S. lewini in this area and although it is still present in the catches, catches are comprised entirely of juveniles in some areas. Given continued high fishing pressure throughout this species’ shelf habitat off Western Africa and the declining trends observed in other areas of this species’ range where it is fished, it is considered to meet the criteria for at least Vulnerable in this region.
|Range Description:||In the eastern Atlantic, the Scalloped Hammerhead possibly occurs in the Mediterranean Sea and around the Azores. The species is probably present all along the western Africa coast, confirmed from Mauritania, Senegal, Gambia, Ivory Coast, Guinea, Guinea Bissau, Sierra Leone, Gabon, and Congo (Compagno in prep, M. Ducrocq pers. obs. 2006).|
Native:Congo; Côte d'Ivoire; Gabon; Gambia; Guinea; Guinea-Bissau; Mauritania; Portugal (Azores); Senegal; Sierra Leone
|FAO Marine Fishing Areas:|
Atlantic – eastern central
|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).
Data to indicate trends in abundance are generally not available for the Eastern Central Atlantic. Zeeberg et al. (2006) suggest that similar population trends for hammerheads (grouped) to those documented in the Northwest Atlantic can be expected in the Northeast and Eastern Central Atlantic because longline fleets in this area exert comparable fishing effort, and effort is seen to shift from western to eastern Atlantic waters (Buencuerpo et al. 1998, Serafy et al. 2004, Zeeberg et al. 2006). European industrial freeze trawlers targeting small pelagic fish (Sardinella, sardine, and horsemackerel) operate on the northwestern African shelf nearly year-round with five to ten large vessels (9,000-18,000 horse power). A study of bycatch rates in more than 1,400 trawl sets off Mauritania from 2001–2005, showed that Sphyrna species combined represented 42% of total bycatch during this period (Zeeberg et al. 2006).
Hammerheads are caught by both inshore artisanal fisheries and offshore European fisheries operating along the coast of western Africa. The Subregional workshop for sustainable management of sharks and rays in West Africa, 26–28 April 2000 in St Louis, Senegal (Anonymous 2000) noted the high threat to sharks in the West African region and a noticeable decline in the CPUE of total sharks and rays. Walker et al. (2005) also noted that there is concern for Sphyrna lewini off Mauritania, with catches comprised exclusively of juveniles, often newborn. Increased targeting of sharks began in the 1970s, when a Ghanaian fishing community settled in the Gambia and established a commercial network throughout the region, encouraging local fishermen to target sharks for exportation to Ghana. By the 1980s many fishermen were specialising in catching sharks, resulting in a decline in overall shark populations (Walker et al. 2005). There has been rapid growth in the shark fin market in this region, for export to the Far East, and yearly production of dried fins exported from Guinea-Bissau alone is estimated at 250 t (dry weight) (Walker et al. 2005).
This species is frequently caught along the western African coast and is heavily targeted by driftnets and fixed gillnets from Mauritania to Sierra Leone (M. Ducrocq pers. comm. 2006). There is anecdotal evidence for some declines in catches off Senegal and Gambia (M. Ducrocq pers. comm. 2006). Juveniles are very susceptible to coastal fisheries using drift or fixed gill nets such as sole, sciaenid and Sepia spp fisheries (M. Ducrocq pers. comm. 2006). They were taken as bycatch in the milk shark fishery and in the Banc d'Aguin national park, Mauritania, until the fishery was stopped in 2003 and they are still caught in large quantities in the Sciaenid fishery. A specialized artisanal fishery for carcharhinid and sphyrnid species was introduced in Sierra Leone in 1975, and since then fishing pressure has been continuous (M. Seisay pers. obs. 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 (Eastern Central Atlantic subpopulation). The IUCN Red List of Threatened Species 2007: e.T165297A6001730.Downloaded on 21 June 2018.|