Hammer Time

Dropping anchor in 5 meters of water we begin the process of attracting sharks.

James, a manager at the Bimini Shark Lab, pushes a pungent crate of fish heads off the side of the boat. Keeping the fishy crate mid-water column to prevent the nurse sharks from vacuuming up all the bait.

Handing me a jack steak, I begin ripping off bits and pieces of flesh to season the water with. Nurse sharks are the first to arrive. Five or six brown shapes sifting vehemently through the sand. Generally hunting at night, nurse sharks rest during the day in large groups. The promise of a free meal draws them from the safety of the ledges they hide beneath. Gliding across the bottom, these sharks generally feed on bottom-dwelling fish, crabs, lobster, sea urchins, and even coral, which is why they have one of the strongest bite forces. While the nurse sharks are unfazed by the presence of divers, bull sharks are not as welcoming. Choosing to remain in the shadows, these hulking silhouettes monitor the periphery of the feed. I keep ripping pieces of bait, hoping we will attract the shark that has made Bimini famous for shark diving.

We have been baiting for almost an hour and all we have encountered are ravenous nurse sharks and the occasional jack. Occupied by their foraging below, my back is turned away from the boat. “Kara! Turn around!” shouted Kate from above. Swiveling around I come face to face with a ten-foot great hammerhead.

Globally Threatened

Originally found in both tropical and temperate oceans, a globally distributed species like the great hammerhead should be relatively easy to spot. This is no longer the case. The great hammerhead (Sphyrna mokarran) is listed as endangered by the International Union for Conservation of Nature and Natural Resources (IUCN). A highly migratory species, no conservation management plan for this species currently exists. It is essential to improve data collection of this species. Due to its vulnerability to depletion, low survival at capture and high value for the fin trade, this species is globally  endangered, with evidence for declines greater than 50% according to the IUCN’s assessment of the great hammerhead.   

For shark enthusiasts and scientists, the IUCN’s verdict made it seem as if there would never be a consistent place to swim with these iconic animals.

Location, Location, Location

With the gulf stream on its doorstep, Dr. Samuel Gruber understood the potential for a shark research laboratory on Bimini. The warm water attracts an abundance of wildlife, with pelagic species thriving in its nutrient rich waters. As the stream slopes north between the Bahamas and Florida, depths can reach 790 meters (2600 feet). Water depth drastically deepens as the stream follows the continental slope beyond the edge of the continental shelf, bringing up nutrients for many pelagic species to feed on.

While it was well known that great hammerheads could be seen around the banks off Bimini, to further protect the local population researchers at the Shark Lab had to determine whether the hammers they were seeing were seasonal residents or aimless wanderers. Another key question was where did the great hammerheads go after they disappeared from Bimini in late spring each year?

Shark Lab managers Grant Johnson, Katie Grudecki, and Sean Williams began an initial tagging project in 2008 to identify individual hammers.

Current Research

The preliminary results of the tagging project showed that there are individuals who return to Bimini annually. In 2012, with enough data to back up their initial findings, the Shark Lab partnered with the Save Our Seas Foundation to establish a more sophisticated tagging program.

Dr. Tristan Guttridge, lab director and senior scientist at the Bimini Shark Lab, is leading the charge in 2017 with his great hammerhead research published in Frontiers in Marine Science. Guttridge’s paper discusses the extent of movements and connectivity of great hammerheads between the U.S. and Bahamas. A landmark study, the tracking results revealed that the great hammerheads make large scale migrations, with some covering up to 3030km round trip! Found as far north as Virginia, this study provides incredibly important information for fisheries management. Protected in Bahamian waters, the great hammerhead becomes vulnerable once it leaves and is subject to different fishing regulations when it enters federal waters.

Setting up a wide periphery of acoustic receivers along the boundaries of Bimini, the Shark Lab began to implant acoustic tags in great hammerheads. The lab has successfully inserted 11 acoustic tags in great hammerheads thus far. Whenever a shark with an acoustic tag passes one of the receivers, an individual code is generated, cataloguing the daily movements of that particular animal.

The acoustic receivers around Bimini are part of an array of receivers that were put in place to study the different species found in the North Atlantic Ocean. By accessing this larger network of receivers, researchers at the Shark Lab are beginning to determine the parameters of the great hammerhead’s range. After several years of generating data, researchers are starting to see a pattern where great hammerheads travel to Bimini during the winter, and migrate up the east coast almost 782km (485mi) in the spring and summer. Gaia, a seasonal hammer resident to Bimini, has followed the same migratory pattern for the past five years, returning back to Bimini each winter.

“During winter we are seeing that the hammers are more contracted in their range, and in the summer they widen their patterns,” said Guttridge. “If we find data that suggests they come to Bimini specifically to pup in the winter this could have serious implications for how we choose to manage them in the future,” he said.

With only 11 acoustic tags surgically implanted in the Bimini hammers, the Shark Lab relies primarily on photo identification of individuals. With photo archives going back five years, researchers are able to identify individuals through unique notches in the dorsal fin, pigmentation or shading anywhere on the body, and any significant markings. Through this method the Shark Lab has been able to identify three female hammers that have returned to Bimini five years in a row.

Back in the water

The sharks observed and photographed around the waters of Bimini have been nicknamed for Greek goddesses. I was face to face with Medusa. A characteristically “cheeky” shark, she routinely moves through the water column. While I do not feel threatened by Medusa, it can be unnerving when she barrels past you in search of a piece of bait.

While Medusa swims off in the distance, patrolling the wider periphery, I make my way to the sandy bottom where Harry has just buried a scrap a couple meters away. I watch as dark brown, innocuous little nurse sharks bob and weave on top of each other, swarming the fishy crate. With a limited air supply I hope Medusa notices the scrap before I run out of breath. Swimming right over the bait I can see her visibly register there is something there. With one quick flick of her tail she makes a hair-pin turn and circles quickly back. While her hammer is great for pinning southern stingrays, her eyes are too far apart to notice the small chunk of bait located just below her. Returning for another pass, she uses her extremely sensitive hammer as a metal detector. Finally locating the scrap, she opens her jaw quickly, sucking in the scrap as well as the adjacent sand. Roving past me I can see her casey tag beside her dorsal fin. Much larger than me in the water I am in awe of the way she moves. Every time I get into the water with the sharks of Bimini I try to remind myself of how lucky I am to get to observe them in their natural habitat.

Realizing I am completely out of air, yet feeling inspired by her presence, I try to wait until she has turned her hammer away from my direction so I do not scare her or worse have her mistake the fluttering of my fins above her as a potential prey. Trying to keep my motions as fluid as possible, I move towards the surface. Feeling the oxygen return to my lungs I look around and smile thinking about the experience.

On our way back to the lab I could not help but wonder if the influx of divers was potentially changing the behavior of hammerheads. Even though the area we were diving is part of the hammers’ hunting grounds, what if the presence of so many divers in the water alters their natural foraging habits? Future research is being conducted by the Shark Lab on multi-species interactions and ecological movements around Bimini, with a focus on hammerhead provisioning in particular. We know so little about these animals that the data the Shark Lab is collecting is truly exciting to be a part of.

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