Migration and residency of the Great hammerhead
Insert: Acoustic telemetry and tracks of great hammerhead #12 (black dashed line) and greathammerhead #13 (green dashed line) and multiple individuals (blue dashed line). Numbers below symbols denote #directed movements between locations. Adapted from Guttridge et al. (2017)
In my Blog Hammer is Hot of March 30th 2016 I described the rising popularity of the Great Hammerhead shark (GH) among UW photographers. The 'hammies' have truly become a hot target for wide angle shooters, like the Pygmy sea horse or Picasso shrimp for macro shooters.Two places were you have a good chance to meet these magnificent creatures are Bimini in the Bahamas and Jupiter near the Palm Beach coastline of South East Florida (see picture left). The west Atlantic Gulf Stream is probably the reason why big sharks prefer these locations. Squeezed between the Bahamas Banks and the coast of Florida, it brings in masses of clear and nutritious water. Near Jupiter the land mass underneath the water called continental shelf is relatively shallow and only a few kilometers wide. Which allows the Gulf Stream to sweep in closer to the coast than anywhere else in North America (see Blog of 24 Sept 2016). The GH feeds on smaller sharks like the blacktip shark, stingrays and eagle rays, species that are abundant in the more shallow waters of Bimini. GHs have been reliably encountered off the west of Bimini, in shallow sand bottom waters during the winter months, the places that have become most popular among UW photographers.
Where, when and why Knowledge of the behavior of large pelagic predators is increasing steadily, but their migration patterns are still a bit of a mystery: where, when, and why these species moves. Large marine vertebrates often move over long distances and live in habitats that are difficult to track down by marine biologists. The GH it not a typical pelagic open water shark like the Oceanic shark. It is described as a highly mobile, semi-pelagic species that inhabits deep waters as well as shallow lagoons and coral reefs. In particular there is need for further information on the GHs spatial hotspots and migratory corridors. This is not only important for science, but also for the hammerheads protection. Great hammerheads are still treathened by big fisheries -often as bycatch- which may have led to a substantial population decline in many areas. Thus, knowing which habitats they use for feeding and breeding may help to protect these areas from fishing activities that harm the survival or breeding and growth of the species.
A recent comprehensive study by marine biologists from the Bimini Biological Field station and Florida International University (*) has addressed these important issues. The study supervised by Guttridge et al. used a variety of techniques to follow the GHs, in particular biotelemetric acoustic and sattelite tagging in the years between 2010 and 2015 in the two areas I mentioned earlier, Bimini and Jupiter. Pictures of scars, deformations, pigment spots etc. were also used to identify individual sharks.
Site fidelity and seasonal residency Results revealed large scale annual returns of the GHs to their habitats, seasonal residence lasting 5-6 months and numerous long distance interstate movements. Some findings confirm earlier more limited studies on migration patterns of sharks by other investigators (**). Most long distance movements were in the northern direction, as far as Charleston in North West Florida (see insert). Maximum distance travelled amounted to 3030 km, to northernly destinations in South Carolina and Virginia. The long-term site fidelity and seasonal residency of GHs at Bimini as well as the Jupiter area indicates that GHs are ‘philopatric’: they ‘love their homeland’ in the terminology of marine biologists Overall, great hammerheads, most of them females, were detected in Bimini throughout the year, with the exception of September and showing the greatest numbers in the winter months Jan-March. Most GHs seemed to leave their residences when water temperatures get above 26◦C. Another interesting finding was that the sites of Bimini do not serve for mating. There were no mating wounds on female great hammerheads, or swollen claspers in males, which suggests that the Bimini sites mainly serve for feeding. One reason might be that the Bimini islands are the only mangrove habitat on the northwestern edge of the Great Bahama Bank. They provide key nursery areas for diverse species, including various elasmobranches that represent important dietary items for the great hammerheads. Eagle ray and stingray, their favourite dish, are abundant. The northern migrations of female GHs from Bimini in spring might be triggered by their intention to visit mating sites in the western North Atlantic. It is thought that female GHs also give birth in the late spring or early summer in the same regions, and also in estuaries of South Carolina where neonates have been spotted.
*Guttridge TL, Van Zinnicq Bergmann MPM, Bolte C, Howey LA, Finger JS, Kessel ST, Brooks JL, Winram W, Bond ME, Jordan LKB, Cashman RC, Tolentino ER, Grubbs RD and Gruber SH (2017) Philopatry and Regional Connectivity of the Great Hammerhead Shark, Sphyrna mokarran in the U.S. and Bahamas. Front. Mar. Sci. 4:3.
**Hammerschlag, N., Gallagher, A.J.,Lazarre, D.M. and Slonim,C.(2011). Range extension of the endangered great hammerhead shark Sphyrna mokarran in the Northwest Atlantic: preliminary data and significance for conservation. Endanger.Species Res. 13,111–116.
**Graham,F.,Rynne,P.,Estevanez,M.,Luo,J.,Ault,J.S.,N. (2016).Use of marine protected areas and exclusive economic zones in the subtropical western North Atlantic Ocean by large highly mobile sharks. Divers Distributions 22,534–546.
Chapman,D.D.,Feldheim,K.A.,Papastamatiou,Y.P.,and Hueter,R. E.(2015). There and back: a review of residency and return of migrations in sharks. Ann.Rev.Mar. Sci. 7,547–570.