Understanding Great Hammerhead Migration

By Tanya Houppermans

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Although great hammerhead sharks (Sphyrna mokarran) appear powerful and robust, recent research has shown that they are in fact one of the most fragile shark species, being particularly vulnerable to the stress of capture. Even those that are released after being hooked have a nearly 50% chance of succumbing after their ordeal.1 To better protect these sharks, a greater understanding of their movements is needed. The results of a new study conducted by scientists at the Bimini Biological Field Station in Bimini, Bahamas have provided a major step forward by showing the migration patterns and regional connectivity of great hammerheads between the Bahamas and the United States.

The research team, led by Dr. Tristan Guttridge, used acoustic and satellite tagging, conventional tagging, laser-photogrammetry, and photo identification to track great hammerheads in both Bimini, Bahamas and Jupiter, Florida. They were especially interested in the amount of time the great hammerheads were spending at each location (known as ‘site fidelity’), and the extent to which the sharks were moving between the Bahamas and Florida.

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The results indicated that great hammerheads exhibit philopatric behavior, meaning that they make return migrations after departing a particular area in search of mates or food. Additionally, they seasonally reside in specific locations and they show site fidelity in returning to the same areas repeatedly even over a number of years. A few of the sharks in the study even returned to the Jupiter, Florida and Bimini, Bahamas areas after traveling as far north as Virginia, U.S, a return trip of approximately 3,000km. The sampled sharks that did not embark on northerly migrations still showed movement to and from Jupiter and Key Largo, Florida, and the Bahamian islands of Andros, Bimini, and Grand Bahama. The highest concentration of great hammerheads in Jupiter and Bimini occurred during the winter months, with regional and northerly migrations commencing in the early spring. A smaller number of great hammerheads were still found in Jupiter and Bimini throughout the summer and into the fall, indicating that great hammerheads may exhibit partial-migration, where some members of the population take part in lengthy migrations, whereas others remain in a localized region. More research is needed to determine the motivating factors behind the type of migration an individual hammerhead partakes in and why.

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Knowledge of migration patterns and site fidelity is critical in determining the best conservation and management plans for the great hammerhead shark, a species categorized as endangered on the IUCN Red List. Along with their susceptibility to stress-related mortality during capture, great hammerheads are often caught as bycatch and targeted for their fins. Although protected inside of state waters off the coast of Florida (defined as three nautical miles from shore on the Atlantic side, and nine nautical miles off of the Gulf side), it is still legal to catch and release great hammerheads within those boundaries. Florida Fish and Wildlife Conservation Commission regulations state:

            “…it is now illegal to possess, sell, or exchange tiger sharks and great, scalloped, and smooth hammerhead sharks harvested from state waters. It is still legal to catch and release these sharks, and they can still be harvested in adjacent federal waters.”

Since great hammerheads frequently move across U.S. and Bahamian boundaries, cooperation between these two nations is essential in forming any future plans for species protection, including fisheries management and environmental agreements, especially considering that these sharks are often caught in waters between the Bahamas and U.S. Understanding regional preferences and migration routes of the great hammerhead shark will contribute significantly to future protection efforts for this beautiful but fragile creature.  

Link to featured study: http://journal.frontiersin.org/article/10.3389/fmars.2017.00003/full

Reference:

1. Gallagher AJ, Serafy JE, Cooke SJ, Hammerschlag N (2014). Physiological stress response, reflex impairment, and survival of five sympatric shark species following experimental capture and release. Mar. Ecol. Prog. Ser. 496, 207-218. doi: 10.3354/meps10490 

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