Sphyrna lewini (Southwest Atlantic subpopulation)
|Scientific Name:||Sphyrna lewini (Southwest 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)|
Sphyrna lewini faces two main threats related to fisheries in the Southwest Atlantic: 1) fishing of juveniles and neonates on the continental shelf by gillnets and trawl nets and 2) fishing of adults by gillnets (only in Brazil) and longlines on the continental shelf and oceanic waters, mostly for fins. Catches are inadequately recorded and landings data do not reflect the numbers finned and discarded at sea. The species is taken by fisheries throughout all parts of its life-cycle and greater demand for shark fins and flesh has resulted in a substantial increase in retention rates and targeting of sharks. In view of the intensive fisheries in the coastal and offshore areas where S. lewini occurs in this region and documented declining trends where the species has been heavily fished in other areas of its range, the species is assessed as Vulnerable in the Southwest Atlantic.
|Range Description:||In the western Atlantic, this shark occurs from New Jersey (United States) to Uruguay, including Gulf of Mexico and Caribbean Sea (A. Domingo pers. obs. 2007, Compagno in prep).|
|FAO Marine Fishing Areas:|
Atlantic – southwest
|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).
In the southwest Atlantic, the Scalloped Hammerhead faces two main threats related to fisheries: 1) fishing of juveniles and neonates on the continental shelf by gillnets and trawl nets (Vooren and Lamónaca 2003, Kotas and Petrere 2002, Doño 2008); and 2) fishing of adults by gillnets (only in Brazil) and longlines on the continental shelf and oceanic waters, mostly for fins (Kotas et al. 2001, Kotas and Petrere 2002, Kotas and Petrere 2003, Zerbini and Kotas 1998). The species therefore faces intensive fishing pressure throughout its range in this area and at all points in its life cycle. Because all Brazilian fisheries statistics for hammerhead sharks are grouped under the headings “shark” or “hammerhead shark”, it is not possible to determine species-specific trends. Annual landings of hammerhead sharks (six species of hammerhead sharks occur off Brazil) in the ports of Rio Grande and Itajaí (Brazil) combined increased rapidly from ~30 t in 1992 to 700 t in 1994, after which catches decreased, fluctuating between 100–300 t from 1995–2002. The majority of this catch was taken by surface gillnet fisheries that targeted hammerhead sharks on the outer shelf and slope between 27° and 35°S (Kotas 2004, Vooren et al. 2005). Neonates and small juveniles are caught in coastal waters by directed gillnet fishing and as bycatch by bottom trawls (Vooren and Klippel 2005). In the inshore nursery area (depths down to 10 m), neonates are fished intensively by coastal gillnets and are also caught as bycatch by shrimp trawl, pair trawl and intensive recreational fisheries. Their abundance in coastal waters has decreased markedly as a result (Haimovici and Mendonça 1996, Kotas et al. 1995, 1998, Kotas and Petrere 2002, Vooren and Lamónaca unpublished data). Finning of hammerhead sharks, with discarding of the carcasses at sea, is often practised (Kotas 2004, Vooren and Klippel 2005). Fisheries statistics only refer to the landed carcasses and therefore the true extent of catches is unknown.
In southern Brazil and northern Uruguay, adult hammerhead catches (S. lewini and S. zygaena) by monofilament longliners are highest in winter and spring at the shelf edge and the continental slope between 30° and 35°S (Kotas and Petrere 2002). The Brazilian pelagic fishery based in Santos catches significant numbers of sharks, including S. lewini (Amorim et al. 1998). Until 1997, most of this shark catch was discarded but greater demand for fins and flesh has resulted in a substantial increase in retention rates and targeting of sharks (Bonfil et al. 2005). Because hammerhead shark fins are highly valued for their high fin-ray count, this species is unlikely to be released alive. The artisanal fishing fleet in São Paulo has operated since 1996 and also takes sharks. The majority of the hammerheads caught by this fishery were newborns or juveniles (Bonfil et al. 2005). In Uruguay (oceanic coast) some neonates are also captured (together with S. zygaena) in artisanal gill nets, in summer (between December and February) (A. Domingo pers. obs. 2007). In view of the intensive fisheries in the coastal and offshore areas where S. lewini occurs in this region and documented declining trends where the species has been heavily fished in other areas of its range, the species is assessed as Vulnerable in the Southwest Atlantic.
In Brazil, there are laws restricting the length of pelagic gillnets and banning trawl fishing at a distance of less than three nautical miles from shore (equivalent depths of less than about 10 m), however enforcement of these laws has been difficult. Therefore trawling in inshore nursery grounds has continued and gillnetting within nursery areas is not regulated. Some fisheries along the coast are poorly documented and the multi-species nature of many of the fisheries makes species-specific regulation very difficult. Therefore, it is recommended that coastal protected sea areas are established, in which fishing is banned, to protect nursery grounds.
In 1998, the Brazilian Government’s Environmental Agency (IBAMA - Brazilian Institute for the Environment and Natural Renewable Resources) made a first effort to control "finning" by issuing a federal regulation (Portaria IBAMA nnordm; 121 of August 24th, 1998), prohibiting shark finning by all vessels licensed to fish in Brazilian waters (Kotas et al. 2002). The enforcement of this law has been proven difficult and probably will require international financial aid, trained personnel for sampling work along the main fishing harbours and the establishment of a national observer program (Kotas et al. 2002).
|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 (Southwest Atlantic subpopulation). The IUCN Red List of Threatened Species 2007: e.T165296A6001591.Downloaded on 14 December 2017.|