We finished checking lines A, B, C, and D just as the sun begins to rise over the east side of the island. As the boat races south towards the tiger grounds, the golden orb peaks over the horizon, lighting up the turquoise waters of Bimini with crystal clarity. The fifth and last line to check is wildcard. You can see where the three tiger sharks had been caught the night before because of how far the line billows in certain sections. Hooks 3, 6, and 13 are all missing. This has been a very productive 24-hour long line survey. Finding sharks on every check the night before, three of the five tiger sharks were females over 3 meters, some of the biggest recorded this year.
The Shark Lab uses long line surveys to record data on larger sharks, using baited long-lines because the sharks would become easily stressed if netted. They also provide a standardized survey method for the surrounding waters, showing the habitat usage, migration movement, and growth rates of multiple species of sharks. The team sets five, 500-meter long lines, with 15 baited hooks on each line at different GPS points. The lines are checked every four hours throughout the day and night.
In the distance a reflective orange float signifies the end of the wildcard line. Approaching on the starboard side of the line, the team begins to count the baited gangions. “15…14….” we begin in unison just as Tristan Guttridge, Lab Director and Senior Scientist, revs down the engine to inspect where hook 13 should have been located. Just as we gaff the line, a powerful splash breaks the surface further down the line.
Eye of the tiger
Excited by the idea of finding a shark on the line, Tristan points the boat towards where the splash came from. “12..11..10..9..8..” I say under my breath as we move down the line. The two floats around hook 7 are being pulled under by something heavy. Ches, Lab Manager, hooks the line and begins hauling to where the gangion has been pulled under. Moving along the sunken line, a huge shadow becomes visible in the morning sun. Resting on the sandy sea bed is a massive female tiger shark.
“Ches—did you bring a mask and fins?” Tristan asks expectantly. “If she is resting calmly like this it’s worth trying to get a tail-rope on her. I’m going to try and sneak up behind her and really gently try to put a tail rope on, if she starts to move at all, I’ll back off and we can try bringing her in alongside the boat,” Tristan says.
Tail rope in hand, Tristan slips into the water a few meters behind the shark. Spooked by the sudden presence of someone in the water, the tiger shark begins to swim away from Tristan. As she moves up and away from the bottom, Tristan backs off and gradually swims up towards the boat. “I was this close, if I had had a weight belt I would have gotten it on her,” he exclaims as he pulls himself back onto the boat.
Found in both tropical and temperate oceans, tiger sharks are the largest apex predators in their ecosystem and keep the food chain in check. Population declines of this top predator are the result of widespread ecosystem changes and overfishing of prey items. Highly mobile, this species is capable of migrating 7,500 km in a year across open ocean, but returns to distinct areas in the Caribbean. With the ability to migrate between oceans, tiger sharks regulate several different ecosystems. The downside to these large scale migrations occurs when sharks cross jurisdictional boundaries which are subject to varying fishing regulations. The Bahamas has banned shark fishing within its waters and prohibited the sale, import and export of shark products. However when sharks migrate elsewhere, they are not afforded the same environmental privileges.
In the past decade or so, the tiger shark has become the most commonly caught species. The high frequency of mature females as well as juveniles led the Shark Lab’s Principle Investigator Matt Smukall to examine if Bimini is a nursery ground for tiger sharks. With juveniles preferring the shallow water habitats to the south, east and north of Bimini. As they mature, tiger sharks progressively move west to deeper oceanic waters.
Re-gaffing the line, Aviad, a marine researcher from Israel, is at the bow guiding her towards the starboard side of the boat. Seeing her at the surface she is even bigger than she looked on the bottom. She is also missing a section of her upper caudal fin. Appearing larger than the male tiger sharks we have seen on previous long line checks, I think this is the biggest tiger shark I have seen in my three months on Bimini. Cleating off the line at the bow of the boat, Tristan and Ches wrangle her dorsal while myself and Kate another volunteer manage to put a tail rope over what remains of her tail and pull tight. With most sharks we catch on the long line, the mono fishing line, someone securing the dorsal, and a tail rope are generally enough to keep the shark stable, and next to the boat. However, she is larger than the average tiger we catch. She thrashes with such force that the 20′ bay boat begins to keel on the shark side. Tristan makes the call to put pectoral ropes around her fins to increase stability. Using two more tail ropes, Tristan leans over her while Ches holds her dorsal steady. Cinching the ropes beneath the shark’s “armpit” she is no longer able to thrash against the boat.
“I like the way she is responding, she has a lot of fight, we must have just caught her,” Tristan said. Leaving Kate on the caudal end, I pick up the data book and start to fill in time of capture, species, sex, and GPS coordinates. Beginning the work up at 7:35, it took us 15 minutes to get a tail rope and pec ropes on her. “This is an incredibly hardy species, they handle being on the long line well, if we had caught a hammerhead or lemon shark that gets so easily stressed I would cut the line immediately,” Tristan said. “Our first priority is the health of the animal, and its welfare,” he added.
The first measurement we take in the field is the pre-caudal length. Measuring from the nose of the shark to the beginning of the tail. “PCL is 2.78 meters,” Kate yells from the side of the boat. Next she moves the tape to the fork in what is left of the tiger’s tail. “I have 3.05 meters for FL,” Kate said. She is missing the upper portion of her tail, so we are unable to take total length measurements. However, Tristan estimates that if she did have it he thinks she would be between 3.40 to 3.50 meters.
Moving on to girth measurements, Kate wraps the measuring tape around the widest part of her caudal region. “PCG is 32 cm,” Kate said. Keeping the tape underneath the shark, Kate slides the tape up to take dorsal girth. “DG is 1.5 meters!” Kate yells. Widest around her pectoral fins, Kate takes the final measurement. “PG is 1.8 meters, she is huge.” Wider than I am tall, this adult female is easily the biggest shark I have ever seen.
Reaching into the work up kit, Ches sets aside a scalpel, sutures, rat tooth tweezers, and a hemostat. Beneath the tools we keep the acoustic tags, once inserted they have a battery life of up to 10 years within a particular shark.
“Acoustic number is just there, and beneath that the serial number,” said Ches to me as I scrawl the numbers into the data book.
“Okay we are going to start moving her into tonic,” Tristan said as he begins to flip one of her pectoral fins and Aviad assists with her dorsal. During surgical procedures, sharks are rolled onto their backs using a maneuver called ‘tonic immobility’ which puts the sharks in a trance-like state and promotes safe handling, little resistance, and minimal stress for the animal.
Ches takes the scalpel and makes steady straight cuts to form an incision in front of the pelvic fin. Cutting through the thick muscle wall, she takes the small tube-like transmitter and inserts it into the body cavity. All the while waves are rocking the boat and sending wavelets over the incision site. Using careful sutures to close the wound, Ches performed a successful surgery on a 3-meter tiger shark. When we release her, the transmitter Ches inserted will emit coded ultrasonic pulses at a known frequency. Signals are processed and the data stored in bottom-mounted receivers.
The incision is no more than 5 cm, a superficial cut compared to the kind of mating scars these sharks have on them. Just as Ches finishes suturing up the incision, I can see another boat flying towards us. Gently turning her back towards the boat, I grab her dorsal to flip her right side up. Upon coming out of tonic she begins to slap the side of the boat a bit with her caudal fin, but not enough to tangle herself. A little bit of thrashing is good because it shows she still has a lot of energy and is able to stay on the line for a bit longer.
The first thing we do in a release is relieve the shark of some of the hooks in its mouth. Using a pair of large pliers, Tristan begins to wriggle the hooks from the tiger’s mouth. Once Tristan gives me the go ahead I can start loosening the tail rope and cinching it until he tells me to remove it fully from the shark. I try to start loosening the rope, but it is cinched on so tight. Ches gives it a try and it still will not budge. “We do not have tail rope!” Ches yells up to Tristan. James, assistant manager, steps in to try to work on the knot. After three tries to get the rope to give way, we make the call to cut the rope. Aviad is securing her dorsal, but we need to get this tail rope off. Taking a bait knife, James begins to whittle down the dock line. “I’ve got it,” James yells just as she begins to start to thrash again. Giving her tail a pump and pushing her towards open water, we watch as her massive shadow swims off, equipped with a brand new acoustic tag.
Hopefully the next time we hear from her, it will be on on one of our acoustic receivers around the island. By understanding the dispersal patterns of tiger sharks, it will allow for improved regional conservation efforts and future fisheries management.