By Sarah Francis, Graduate Student, University of Miami & Shark Angels Intern
Scientific Study: Philopatry and Regional Connectivity of the Great Hammerhead Shark, Sphyrna mokarran in the U.S. and Bahamas by Tristin Guttridge et al., 2017
Background
The great hammerhead, Sphyrna mokarran, is a large shark that can be up to 20 feet long! It is a highly mobile species, meaning it travels great distances of up to 3000 kilometers. It is a long way from Jupiter, Florida up to Virginia and back. They can be found in coastal-pelagic waters, the open ocean closer to shore, throughout the tropics. Along the eastern U.S. coast, great hammerheads can be found cruising from the Florida Keys all the way up to New Jersey. Though the northern Atlantic waters are pretty frigid, the great hammerhead most likely finds relief in the warm waters of the Gulf Stream originating from the Caribbean. The main diet of these predatory fish includes a variety of fishes, like rays, as well as squids and crustaceans.
The International Union for the Conservation of Nature (IUCN) Red List recognizes great hammerheads as Endangered. As of 2013, great hammerheads are listed under Appendix II of the Convention on International Trade in Endangered Species (CITES), which lists species that are not currently threatened with extinction but could if trade continues uncontrolled. They are also listed under Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), which includes migratory species that require international agreements due to their conservation status, as well as those that would significantly benefit from international cooperation for their conservation and management. Still, great hammerheads are a target or bycatch, accidentally/unintentionally caught, species in fisheries around the world.
The Goal
To investigate (a) how long great hammerhead sharks spend in the coastal areas of The Bahamas and the U.S. and (b) when and where they migrate to between the U.S and The Bahamas.
For sixteen years, great hammerhead sharks were tracked to see if they have a home between the Bahamas and Florida.
Bimini,Bahamas (left) and Jupiter, Florida (right)
Where?
The focus was on two study sites: Bimini, The Bahamas and Jupiter, Florida, U.S. On the western side of Bimini, the sandy shallows slope into the deep pelagic zone towards the Gulf Stream. This western side of the islands is where they commonly find the great hammerheads. The waters of Jupiter comprise of a narrow continental shelf which are located on the southeast Atlantic coast of Florida.
Why?
It is important to understand where sharks travel so that we can protect them in those areas. Bimini is thought to be a feeding destination for great hammerheads because of the high levels of productivity (plenty of food available). South Carolina and Georgia are thought to be nursery grounds for baby hammerheads because some results suggest that great hammerheads give birth there. Long term monitoring can be a good predictor of movement patterns and residency areas for the great hammerhead. This study can be helpful in identifying great hammerhead essential fish habitat, which can minimize effects of target or non-target fishing of the great hammerheads by creating protected areas. These essential fish habitats could be their nursey grounds, mating grounds or feeding grounds.
When? 2000-2016 Jupiter (44) and Bimini (64)
Between the various tagging events, the monitoring periods ranged between 2000-2016 in Bimini where they caught or pole tagged a total of 64 great hammerheads (48 female, 14 male, and 2 unconfirmed). The sizes ranged from 230 to 360cm total length tip of the cephalophoil (the “hammer” head) measured down the middle of the shark to the tip of the tail. From 2006-2016 in Jupiter, a total of 44 great hammerheads were tagged (11 female, 27 male, 6 unconfirmed) with sizes ranging from 195 to 400cm total length. (Unconfirmed sex identification could be due to forgetting to note the sex or unidentifiable sex organs.)
How? Tagged 108 Great Hammerheads total
The researchers captured the great hammerhead sharks and equipped them with either acoustic, satellite, and/or conventional tags (explained in the next section). Methods varied at each site with the people and equipment available. Bimini’s great water conditions, such as clear water, allowed for a wider range of data collection such as photo-identification, laser photogrammetry, and a breath-hold technique to attach tags.
What tags/methods did they use and what can they tell us?
Acoustic telemetry- there are two types of acoustic telemetry, active and passive. For this study, passive tracking was used. An acoustic transmitter was either externally attached with a speargun, or it was surgically implanted in the shark. The acoustic receivers were deployed in various habitats in Bimini, The Bahamas, as well as along the eastern coast of Florida. When a shark with an acoustic transmitter passed within detection range (usually 650-1640 feet), the tag number, date, and time were recorded and stored on the receiver. The researchers in this study downloaded the data every six months.
Archival satellite telemetry– external tags that automatically release the shark after a specific time. The tag uses satellites to triangulate the location of the shark, making it easier for scientists to track the shark’s movements.
Conventional mark and recapture tags– This is the simplest tag and there are different types. For this study, a National Marine Fisheries Service (NMFS) dart tag was used which has an identification number in a tube attached near the first dorsal fin of the shark.
Photo identification– photographs were taken and used to identify individuals using natural marks, fin morphology, wounds, and pigmentation. This is important for re-identification.
Laser photogrammetry– a method to estimate the lengths of large marine animals in the water.
Results
This study is the first evidence of a home base, or philopatric behavior, in the highly mobile great hammerhead.Great hammerheads in Bimini showed that they would routinely return to Bimini and even stay there during a particular season (October-April) based on tag detections from consecutive seasons. This was also the case in Jupiter, Florida (October-March).The six great hammerheads tagged in Bimini moved between Florida and The Bahamas, while four made long-distance movements departing April and May to South Carolina and even Virginia!
Discussion
Since the sharks around Bimini and Florida did not have any physical indications to suggest they travelled there to mate or give birth (pup), and the area has high productivity (lots of food), they most likely spend time in these locations for feeding opportunities. It is also possible that some long-distance movements align with the changing water temperature since great hammerheads prefer to live in waters close to 22.54°C (72.57°F). This theory is enforced by three great hammerheads tagged in Bimini who left around April/May when water temperatures began to rise.
There is some evidence of partial migration, which means that some of the population of a species migrates, while others stay in the same area. This understanding is important in implementing the essential fish habitats to ensure the protection of the species in these areas.
What does this mean for management?
Animals that have predictable movements and patterns are susceptible to exploitation from fishing pressures. The great hammerhead is highly migratory and is found from the Florida Keys up the coast to Virginia. During this migration, they are not fully protected. These sharks are under protection off the coast of Florida and in the Bahamas. The cooperation between these jurisdictions is also important and key in successful management. The Bahamas serves as a shark sanctuary and, while they are protected within that area, it is important to understand that they make movements up and down the eastern U.S. coast. It is also important to remember that there is not enough knowledge about the great hammerhead shark and more research needs to be done to fully understand the stressors of this species and their global population numbers.