|Scientific Name:||Sphyrna tiburo|
|Species Authority:||(Linnaeus, 1758)|
|Red List Category & Criteria:||Least Concern ver 3.1|
|Reviewer(s):||Musick, J.A. & Fowler, S.L. (Shark Red List Authority)|
This assessment is based on the information published in the 2005 shark status survey (Fowler et al. 2005).
The Bonnethead Shark (Sphyrna tiburo) is a very abundant small hammerhead of shallow estuaries and bays on the Atlantic and Pacific coasts of the Americas. Despite pressure from both directed and incidental fisheries, this is an abundant species with some of the highest population growth rates calculated for sharks, making it much less susceptible to removals than most other species of sharks.
|Range Description:||This species occurs off the American continent only. In the western Atlantic it has been reported from southern Brazil to North Carolina, USA, and occasionally further north. It is also common in the Gulf of Mexico and part of the Caribbean. In the eastern Pacific it is reported from southern California to Ecuador (Compagno 1984b).
The bonnethead shark is an abundant, small coastal shark commonly found in shallow estuaries and bays over grass, mud and sandy bottoms. Off Florida's west coast it is very abundant in shallow estuaries during the summer months and moves to deeper water off the beaches in winter (Hueter and Manire 1994). This species shows sexual segregation.
Native:Aruba; Bahamas; Belize; Bermuda; Bonaire, Sint Eustatius and Saba; Brazil; Colombia; Costa Rica; Curaçao; Ecuador; El Salvador; Guatemala; Guyana; Honduras; Mexico; Nicaragua; Panama; Suriname; Trinidad and Tobago; United States (Alabama, California, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, Texas); Venezuela, Bolivarian Republic of
|FAO Marine Fishing Areas:||
Atlantic – northwest; Atlantic – southwest; Atlantic – western central; Pacific – eastern central; Pacific – southeast
|Range Map:||Click here to open the map viewer and explore range.|
|Habitat and Ecology:||
The Bonnethead Shark is a small coastal sphyrnid that reaches about 150 cm total length (TL). The life history of this species in the Gulf of Mexico has received considerable attention. In the eastern Gulf of Mexico, female Bonnethead Sharks seldom exceed 130 cm TL, whereas males rarely surpass 110 cm TL. Females generally mature between 80-95 cm TL (or 2-3 years of age) and males mature between 68-85 cm TL (two years of age). Maximum observed ages are 6-7 years or more for females and 5-6 years or more for males, whereas theoretical longevities derived from von Bertalanffy growth curves range from 5-6 years for males and from 10-12 years for females (Parsons 1993a, Carlson and Parsons 1997). Empirical data for populations of this species in the eastern Gulf of Mexico reveal a latitudinal increase in maximum size, size at maturity and offspring size (Parsons 1993, Carlson and Parsons 1997, C.A. Manire pers. comm.).
The Bonnethead Shark is a placental viviparous species that reproduces annually. Its gestation period is one of the shortest known in sharks, lasting approximately 4.5-5 months; litter size averages nine. The periodicity of parturition also varies latitudinally, taking place in mid to late August in Florida Bay (southernmost location), early September in Tampa Bay (middle location) and mid to late September off north-west Florida (northernmost location) (Manire et al. 1995, J. Carlson pers. comm.). Size at birth ranges from an average of 27 cm TL in Florida Bay to 35 cm TL in Tampa Bay (Parsons 1983). Parsons (1993) and Manire et al. (1995) found that mating occurs in November and sperm is stored until ovulation/fertilisation the following March or April. Ongoing tagging studies along the west coast of Florida (R.E. Hueter and C.A. Manire pers. Comm.) indicate that individuals of this species are highly site-attached, with little evidence for long-distance migrations and mixing of populations.
The shallow grass bottoms off Florida's west coast are documented nursery grounds for this species, which probably utilises similar habitats as nursery areas throughout its range (Hueter and Manire 1994). The Bonnethead Shark's diet off south-west Florida is very homogeneous, dominated by crustaceans, consisting mostly of portunid crabs (Cortés et al. 1996). Stomach contents also show a high incidence of angiosperms, which are likely ingested incidentally to prey capture and denote the benthic feeding habits of this species (Cortés et al. 1996). This species also feeds on cephalopods and fish, but to a much lesser extent. Bonnethead sharks are specialist hunters (Cortés et al. 1996) that appear to have higher daily rations than other species of sharks for which quantitative food consumption data exist (Cortés unpubl.).
Cortés and Parsons (1996) compared the demography of two populations off Florida's west coast and found short generation times (4-5 years) and high population growth rates (1-28% per year). Recent demographic studies of this species by Cortés (in press) incorporating uncertainty in estimates of vital rates indicate that the bonnethead has very high population growth rates (l) (mean=1.304 yr-1; 95% confidence interval=1.150-1.165 yr-1) and short generation times (A) (mean=3.9 years, 95% CI=2.6-4.5 years). Elasticity analysis (which examines the proportional sensitivity of l to a proportional change in a vital rate) also showed that l is most sensitive to juvenile survival and adult survival than to fertility (which includes survival to age-1). Annual survivorship values used in Cortés (in press) were estimated through five indirect life history methods and ranged from 55-81%. The high l values and elasticity patterns for this species are a result of its "fast" life history characteristics.
In the USA, Bonnethead Sharks are caught in commercial and recreational fisheries and also as bycatch. Recent commercial landings of this species indicate that it accounted for over 50% of all landings of small coastal sharks in the south-eastern USA in 1995, but was the least important small coastal species of shark represented in commercial landings from 1996-1999. Commercial landings of bonnetheads in numbers averaged about 22,000 individuals from 1995-1999 (Cortés 2000b).
Recreational catch estimates from several surveys indicate that about 29,000 bonnetheads were caught annually from 1981-1998, ranging from a minimum of about 13,000 sharks in 1991 to a peak of about 53,000 sharks caught in 1986 (Cortés 2000b). Additionally, bycatch estimates from the shrimp trawl fishery operating in the Gulf of Mexico indicate that about 410,000 individuals were caught annually from 1972-1999 (Cortés unpubl.).
Bonnetheads are also exploited in Mexico. In Mexican coastal waters of the Gulf of Mexico, S. tiburo is the second most important species in the artisanal fisheries, accounting for 15% of the landings numerically (Castillo et al. 1998). Targeted fisheries for this species have also been documented for Trinidad and Tobago (Shing 1999) and Ecuador (Martinez 1999). Bycatch in other fisheries, mainly from shrimp trawling, is probably also significant in other fishing nations of the American continent.
Nursery areas for this species are located inshore and adults frequent inshore waters, making this species vulnerable to exploitation and human-induced habitat degradation. Preliminary results of an ongoing study on the reproductive endocrinology of this species off Florida's west coast show that high levels of organochlorine contaminants are present in tissues of sampled individuals (C.A. Manire pers. comm.).
In the USA, the Bonnethead Shark is classified as a small coastal species in the Federal Management Plan (FMP) for Sharks of the Atlantic Ocean, Gulf of Mexico and Caribbean, together with the Blacknose Shark (Carcarhinus acronotus), the Finetooth Shark (C. isodon), the Smalltail Shark (C. porosus), the Atlantic Angel (Squatina dumeril), the Atlantic Sharpnose (Rhizoprionodon terranovae) and the Caribbean Sharpnose (R. porosus) sharks (NMFS 1993). The small coastal shark complex is not currently considered to be overfished, but there are fishing regulations in effect. A more recent FMP (NMFS 1999) called for more stringent measures, including a reduction of the annual commercial quota for small coastal sharks to 359 t.
The Bonnethead Shark is a very abundant species, with early age at maturity, short lifespan and generation time, and high litter size and population growth rates, capable of withstanding much higher removal levels than many other species of sharks. It is thus considered to be of lesser risk because of its life history and population characteristics.
|Citation:||Cortés, E. 2005. Sphyrna tiburo. The IUCN Red List of Threatened Species. Version 2015.2. <www.iucnredlist.org>. Downloaded on 01 August 2015.|
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