Sphyrna lewini (Eastern Central and Southeast Pacific subpopulation)
|Scientific Name:||Sphyrna lewini (Eastern Central and Southeast Pacific subpopulation)|
|Species Authority:||(Griffith & Smith, 1834)|
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)|
This species is heavily exploited through its range in the Eastern Pacific. Of particular concern is increasing fishing pressure at adult aggregating sites such as Cocos Island (Costa Rica) and the Galapagos Islands (Ecuador), and along the slopes of the continental shelf where high catch rates of juveniles can be obtained. The number of adult individuals at a well-known S. lewini aggregation site in the Gulf of California (Espiritu Santo seamount) has declined sharply since 1980. Large hammerheads were also formerly abundant in coastal waters off Central America, but were reportedly depleted in the 1970s. A comparison of standardized catch rates of pelagic sharks (species-specific information was not available) in the EEZ of Costa Rica from 1991-2000 showed a decrease of 60%. In Ecuador, landings (grouped for the family Sphyrnidae) peaked in 1996 and declined until 2001. Illegal fishing for shark fins is occurring around the Galapagos. There are no species specific data for these fisheries, but S. lewini is one of the most common species around the Galapagos and given the high value of its fins, it is very likely being targeted. Divers and dive guides in the Galapagos have noted a severe decrease in shark numbers and schools of hammerhead sharks. Given continued high fishing pressure, observed and inferred declines, the species is assessed as Endangered in this region.
|Range Description:||In the eastern Pacific, this species occurs in southern California and the Gulf of California to Panama, Ecuador and possibly to northern Peru (Compagno in prep).|
Native:Colombia; Costa Rica; Ecuador; El Salvador; Guatemala; Mexico; Panama; Peru; United States (California)
|FAO Marine Fishing Areas:|
Pacific – eastern central; Pacific – southeast
|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).
Throughout this species’ range in the Eastern Pacific, juveniles and neonates are heavily exploited in directed fisheries, and are also taken as bycatch of shrimp trawlers and coastal fisheries targeting teleost fish. Fishing pressure directed at juveniles also appears to have increased in parts of the Gulf of California and in Costa Rica, and is likely to be increasing elsewhere as other, more valuable fishery stocks are depleted. Patchy distribution resulting from aggregating behavior of adults and the use of historic nurseries, where neonates shoal with spatially confined movements, make this species particularly easy to target. As in other areas, the large fins of this species are highly prized for their value in the international shark fin trade. Increased fishing pressure from international longline fleets in the Eastern Central Pacific and Southeast Pacific, driven by increasing demand for fins, is of concern. Furthermore, as traditional and coastal fisheries in Central America are depleted, domestic fleets have increased pressure at adult aggregating sites such as Cocos Island (Costa Rica) and the Galapagos Islands (Ecuador), or along the slopes of the continental shelf where high catch rates of juveniles can be obtained (Vargas and Arauz 2001).
In the Gulf of California, Sphyrna lewini is a common catch in the directed artisanal elasmobranch fisheries of Sonora, Sinaloa, Baja California, and Baja California Sur, Mexico. Juveniles, including neonates, dominate the overall landings of this species; most are less than 100 cm total length (Bizzarro et al. in press). Bottom set gillnets and longlines produce the majority of the catch. Adults are landed in artisanal pelagic longline and gillnet fisheries, but represented less than 20% of the total S. lewini observed in artisanal catches during 1998 and 1999 fisheries surveys (Bizzarro et al. In Press). The indirect take by trawlers and artisanal teleost and shrimp fishermen is unknown. Landings data for 1996–1998 from the Gulf of Tehauntepec, Mexico, indicates that Scalloped Hammerheads were the second most important shark caught in the artisanal shark fishery, representing 36% of the total catch from a sample of 8,659 individuals (Soriano-Velassquez et al. 2002). The size of the individuals in this sample is unknown. Marquez (2000) reports that this species represented only 4.61% of the total catch of the artisanal fishery in the Gulf of California, contrasting with reports for Sinaloa, Mexico in 1994, 1995 and 1996, in which scalloped hammerheads represented 80.3%, 52.54% and 85.68% of the shark catch respectively (Marquez 2000). Off Pacific Guatemala, the importance of this species in the fishery landings appears to vary across areas, from 6% (n=339) to 74% (n=800) of the total catch from 1996–1999 (Ruiz and Ixquiac 2000). Data from El Salvador collected from July of 1991 to June of 1992, indicate this species represented 11.9% of the landed catch in a sample of 412 (Villatoro and Rivera 1994).
The number of adult individuals at a well-known S. lewini aggregation site in the Gulf of California (Espiritu Santo seamount) has declined sharply since 1980. In 1981 Klimley and Nelson estimated the size of a school at 525 individuals using Lincon Index mark recapture methods. Between 1998 and 2004 at least 20 attempts have been made to recreate this study, however in most cases fewer than 8 individuals have been observed at one time (Klimley 1999, Klimley and Jorgensen unpublished data).
Large hammerheads were formerly abundant in coastal waters off Central America, but were reportedly depleted in the 1970s (Cooke 1990). Industrial longlining initiated in the early 1980s, and again large hammerheads provided valuable fins for this market. A comparison of standardized catch rates of pelagic sharks (species-specific information was not available) in the EEZ of Costa Rica from 1991–2000 showed a decrease of 60% (Arauz et al. 2004). In 1991, sharks formed 27% of the total catch. In 2000, only 7.64% of the total catch was sharks, and in 2003 this decreased further to 4.9% of the total catch, 58.2% (Arauz et al. 2004). In 2001 and 2003, scalloped hammerheads only constituted 0.14% and .09% of the total catch by individuals, respectively.
In Ecuador, catch records (grouped for the entire family Sphyrnidae) indicate a peak of approximately 1,000 tons in 1996, followed by a steady reduction until 2001 (Herrera et al. 2003). Landings in the port of Manta (accounting for 80% of shark landing in Ecuador) of S. lewini, caught by artisanal longline and drift net fleet were about 160 t in 2004, 96 t in 2005 and 82 t (2006). Artisanal fishery landings into the port of Manta for Sphyrna spp declined by 51% between 2004 and 2006 (Martínez-Ortíz et al. 2007). According to Carrera-Fernández and Martínez-Ortíz (2007) the percentage of juveniles in landings is 83% for females and 71% for males. Most of the landings for this species (74%) take place between January and June.
Divers and dive guides in the Galapagos have noted a severe decrease in shark numbers and schools of hammerhead sharks (P. Zarate pers. comm.). Illegal fishing around the Galapagos is not only practiced by fishermen from the Galapagos, but also by the industrial and artisanal fleet from continental Ecuador and international fleets (Coello 2005). These illegal fisheries target sharks for their fins. There are no species specific data for these fisheries, but S. lewini is one of the most common species around the Galapagos (J. Martinez pers.obs.), and given the high value of fins of this species, it is very likely that it is targeted in illegal finning activities. In an effort to help stopping the illegal finning occurring in the Galapagos, the Ecuadorian Government issued Decree 2130 in 2004 prohibiting fin export from Ecuador. Unfortunately, the Decree had the reverse effect of establishing illegal trade routes, with fins being exported mainly via Peru and Colombia where there is no finning ban in place. Interviews with fishermen and traders in both Ecuador and Peru suggested that illegal trade routes operated for fins transported both from Ecuador and directly from Galapagos to Peru (Saenz 2005, WildAid 2005). Ecuador then abolished Decree 2130 and issued two new Decrees (482 and 902) in 2007 which establish better controls; traceability of the exported products; re-confirm the prohibition of finning established in 1993; a database on trade and establish as State policy the National Action Plan for the Conservation and Management of Ecuadorian Sharks (PAT - Ec).
|Conservation Actions:||There are no species-specific measures in place for S. lewini in the Eastern Pacific, although steps are being taken towards the management of elasmobranch fisheries. In Ecuador the current regulations prohibit shark fishing in the core zone of the Galápagos marine reserve, however extensive poaching has been reported. Ecuador issued two new Decrees (482 and 902) in 2007 which establish better controls; traceability of the exported products; re-confirm the prohibition of finning established in 1993; a database on trade and establish as State policy the National Action Plan for the Conservation and Management of Ecuadorian Sharks (PAT - Ec). In Mexico, some known adult aggregating sites are protected within the Revillagigedo Island archipelago, however enforcement is lacking and there are many reports of poaching. Protection of known nursery adult aggregating sites is recommended. Estimates of acceptable catch rates should be viewed with precaution until there is more certainty in age and growth parameters.|
|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 and Southeast Pacific subpopulation). The IUCN Red List of Threatened Species 2007: e.T165291A6000761.Downloaded on 23 October 2016.|
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