Easily identified by it’s flattened, narrow, shovel-shaped hammer with no medial or central indentations. Dorsal coloration grey-brown sometimes with small dark spots. Ventral surface pale.
Maximum length approximately 150cm. Average size at birth is 35-40cm.
Turbid or clear water in sandy and muddy bays, sea grass beds, and occasionally coral reefs and mangroves. From the surface to 80m but generally found in shallow water.
The bonnethead is a tropical species present on both coasts of Central and South America. In the Atlantic, the bonnethead occurs from Rhode Island (in the Gulf Stream) to southern Brazil.
In the Eastern Pacific it occurs from Southern California to Ecuador.
Despite pressure from both target and bycatch 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. Given its high rate of population increase and short generation length relative to other shark species, and the fact that it is considered not overfished and with overfishing not occurring in regions where stock assessments are available, this species is currently assessed as Least Concern.
In the USA, Bonnethead Sharks are caught in commercial (gillnet and hook and line) and recreational fisheries and also as bycatch, especially in shrimp fisheries.
The Bonnethead Shark is also exploited in Mexico. In Mexican coastal waters, off the Gulf of Mexico, this is the second most important species in the artisanal fisheries accounting for 15% of the landings numerically (Castillo-Géniz 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 within this species range.
Nursery areas for this species are located inshore and adults frequent inshore waters, making this species vulnerable to exploitation and human-induced habitat degradation. Results of ongoing work 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; however, there have been no documented indications of resulting physiological impact (Gelsleichter et al. 2005).
Citations and References
Cortés, E., Lowry, D., Bethea, D. & Lowe, C.G. 2016. Sphyrna tiburo. The IUCN Red List of Threatened Species 2016: e.T39387A2921446. https://dx.doi.org/10.2305/IUCN.UK.2016-2.RLTS.T39387A2921446.en. Downloaded on 21 September 2020.
Geographic variation in reproduction between two populations of the bonnethead shark, Sphyrna tiburo Glenn R. Parsons Department of Marine Science, University of South Florida, St. Petersburg, FL 33705, Us.A. Present address: Department of Biology, University of Mississippi, University, MS 38677, Us.A.
A viviparous species with yolk-sac placenta. 4-16 pups per litter.
The bonnethead has an interesting and well-documented reproductive cycle. Researchers working in Tampa Bay noted that mating took place in late October. All captured females displayed mating scars, whereas no captured females had mating scars a week earlier. However ovulation does not occur until 4-5 months later, indicating that the female stores the male’s sperm over the winter. The gestation period has been estimated at 4-5 months, with births (in Florida) occurring in September.
Diet consists mainly of crustaceans: crabs, shrimps, mantis shrimps, acorn barnicles. Also to a lesser degree, bony fishes; probably demersal species.
Bonnets in the eastern US are known to prey heavily on Atlantic blue crabs – Callinectes sapidus.
Bonnetheads are migratory, moving north within the Gulf Stream during the warmer months. They are a social species that are usually found in small groups of 3-15. There are reports of extremely large aggregations moving together along the coast and within mangroves in John Pennekamp State Park, in Key Largo.
Reaction to divers
Very shy around divers and difficult to approach.
Bonnethead sharks are occasionally seen in the upper Florida Keys by divers and snorkelers. From Key Largo to Isla Morada seems to be the best area to attempt to see them.
As the sound of scuba would be an issue, most likely, the best chances for close encounters would be by quietly snorkeling in shallow bays or mangroves and free-diving down when the sharks come into view.
I have had some success by chumming from a small boat but the bonnethead that showed up, bumped the boat and bolted instantly. With a little perseverance, I suspect chumming would work well to photograph this species.
Scoophead Shark distinguished by its more mallet-shaped hammer.
Scalloped Bonnethead As above, distinguished by its wider more mallet-shaped hammer.
Smalleye/Golden Hammerhead As above, distinguished by its wider more mallet-shaped hammer.