Return to Bermuda – Grabbing Tigers By the Tail, Part II

With the dramatic declines in shark populations caused by shark-finning and other forms of commercial fishing, the need for protection of shark species worldwide has reached a critical point. To this point, the Guy Harvey Ocean Foundation (GHOF) is aggressively supporting several fisheries management studies that are gathering crucial data that can be used to design and implement protective measures.

One of these studies is an ongoing project to track tiger shark migratory patterns in the western Atlantic. For the past two summers, representatives from the GHOF and the Guy Harvey Research Institute (GHRI) have helped tag and track almost twenty tiger sharks off the coast of Bermuda. The expeditions have produced previously unknown data about the tiger sharks in that region, information which may very well lead to new fisheries management practices in the western Atlantic and Caribbean.

Guy Harvey Ocean Foundation Expedition: Bermuda – Part I

In August of 2009, the staff of the GHOF and the GHRI collaborated with the Bermuda Shark Project on an expedition to tag and track adult tiger sharks a few miles southwest of Bermuda. We concentrated our efforts in a location known as Challenger Bank, which is as a hot spot for tiger shark activity. The area certainly lived up to its reputation – by the end of the week, we had caught, tagged and safely released seven adult tiger sharks with PSAT and SPOT electronic tags.

The tagging project was being led by two Bermudians, Dr. Neil Burnie and Choy Aming, with the representatives of the GHOF and GHRI providing assistance in the form of tag provision and deployment, as well as follow-up analysis of the sharks’ migration patterns through the western region of the north Atlantic. Dr. Mahmood Shivji, Director of the GHRI, and Dr. Brad Wetherbee of the University of Rhode Island – both of whom have extensive experience working with tiger sharks in the Bahamas and the USVI – were on hand to calibrate the tags and assist with deployment. And, my children Jessica and Alex – both of whom are world-class fishermen with several IGFA records between them – rounded out the team as our expert tiger shark wranglers.

We used Neil’s 34’ Prowler, Bones, as the expedition’s work boat while Captain James Robinson’s Wound Up served as the catch boat. The sharks were caught on 20/0 circle hooks with no barb and130# gear, then tail roped and restrained by a harness that kept them snug to the boat while Neil drilled small holes in the shark’s dorsal fin to attach the SPOT tags. On average, the process was usually completed in about 15 minutes, during which time the shark’s head remained submerged in the water so it could ventilate normally.

Once the tag had been securely fastened, the tail rope and harness were released and the sharks would swim away at a rapid clip. I was in the water to film the hook removal and rope/harness release, while my professional camera team of Rick Westphal and Dee Gele filmed all the action for a tiger shark documentary I am producing.

The results of last year’s expedition were successful beyond our expectations. Using the tags, we were able to track the sharks’ migration as they moved away from Bermuda when seawater temperatures dropped in October, during which time they migrated south towards the Bahamas, Turk and Caicos Islands, and the Virgin Islands. The tracks showed the sharks were not wandering aimlessly but were actually headed in a more-or-less straight line, as if they knew where they were going. Based on their amazingly direct movements, it’s highly likely the individual animals have taken this migratory path before.

For the rest of the winter months, the tiger sharks behaved like reef sharks, tracking the edges of deep island drop-offs. Presumably, they were feeding opportunistically along the way. As they searched for food at or near the surface, their dorsal fins would be exposed above the water line, which would allow the Argos satellites to pick up the signal from the tags and pinpoint the shark’s location (NOTE: The SPOT tag technology is more suited for attachment on air-breathing mammals and reptiles that constantly interact with the surface. Only a few ocean-going sharks exhibit the necessary type of behavior needed to use the SPOT tag for tracking. For example, my friend Dr. Michael Domeier uses the same equipment in his ongoing research of white shark populations in the Pacific).

As the seawater temperatures started to rise in April and May, the tiger sharks began a northward migration, with most aiming directly for Bermuda. As they approached the island, many began to veer off on an easterly track that led them well north of Bermuda and into the north Atlantic, where some have stayed for most of the summer (Katrin, the only female tagged last year, is currently on a latitude adjacent to New York!).

This pattern of migration away from the island and in to open water raises some big questions: “What are they doing out in the middle of the Atlantic Ocean?”, “Are they feeding, and if so, on what – turtles, dead sea birds, squid?”, “Or, are they perhaps breeding?” These are serious questions that need to be answered in order for our research to have any practical or meaningful conclusions. So, we decided to once again mount an expedition to Bermuda to see exactly what is going on with these perplexing tiger sharks.

Guy Harvey Ocean Foundation Expedition: Bermuda – Part II

The 2010 expedition to Challenger Bank began on July 24, just a couple of weeks earlier in the year than last year’s trip. Much of the crew from the 2009 expedition were on hand again: Neil and Choy – the “local boys” from Bermuda; Mahmood and Brad, our resident scientists; my children, Jessica and Alex; and Capt. James Robinson, whose boat Wound Up once again served the dual role of catch boat and support vessel.

For this year’s trip, the GHRI provided thirteen SPOT tags and Neil purchased four three-year SPOTs with assistance from Bermudian sponsors, some of whom rode along with Capt. James on the Wound Up.  Neil and Choy were doing a great job in Bermuda in getting local businesses involved in the tagging project and the production of a documentary that was created to educate the public about the success of their work.

The expedition’s plan called for Neil and Choy to take us to Challenger Bank to tag as many tiger sharks as possible over the course of six days. The week started fairly slow, with just one shark caught on each of the first three days. However, things heated up in the second half of the week as we caught and tagged 9 more sharks over the three remaining days.

Chumming was the key to catching the sharks. Luckily, we had ample supplies of fish heads, and we added to the chum mix by catching bonitos, ocean robins (local name for an abundant mackerel scad), blackfins, wahoos and barracudas while we were on the Bank. While the sharks were definitely attracted to all of the fish we served up, they seemed to have a preference for one in particular – fresh barracuda, which proved to be irresistible to the tigers.

We fished for the tigers primarily from the Wound Up. When a shark was hooked, Capt. James would transfer it to the Bones and then return to the mooring to continue fishing. Meanwhile, Neil and crew would safely secure the caught shark, apply a tag to the its dorsal fin, and then release the shark unharmed. Our crew was very experienced at tagging sharks and had gained a lot of knowledge during last year’s expedition, so the entire process – from the initial hook up to the final step of releasing the shark – was well planned and executed, which resulted in all of the sharks being released without harm.

While James was fishing with 130s we put out a quarter inch rope line cable leader and 20/0 circle hook, which was baited with barracuda and suspended from a large buoy. We caught four sharks using this method. One of these was pulled in by Alex, and at 8 feet long, it was the smallest shark we had caught so far.

Brad and Neil decided this shark was small enough to pull into the boat for tagging, so the crew hauled the shark on board, then covered its eyes with a wet towel and ran two deck hoses through its gills for ventilation. With the smaller shark secured in the boat, Neil was able to deploy a 3-year SPOT tag on this young male in just a few minutes.

Interestingly, while on board, this small tiger shark regurgitated several squid beaks, and the horn of the foot from a benthic gastropod (like a conch). This indicates opportunistic bottom feeding and mid-water feeding on pelagic squid (one of the big 12 footers regurgitated the remains of a seabird and lots of feathers).

Unfortunately, we didn’t catch any of the sharks that we tagged last year, nor did we catch any tigers that had been tagged previously by Mahmood and Brad in the USVI over the past two years. A somewhat disappointing result, but it suggests that the tiger shark population around Bermuda is comparatively healthy. Of course, we do not know what the population numbers were before the commercial fishing industry exploited this and other species over the last three decades, so it’s difficult to determine just how stable the population has been over time.

Impact of the Guy Harvey Ocean Foundation Expedition: Bermuda

Earlier in July, Brad presented our most up-to-date results at the annual conference of the American Elasmobranch Society, and the results of our comprehensive study amazed the scientific community. The tags applied in 2009 have lasted and stayed attached much longer than expected, and the regular reporting by the sharks (over a year now) is shedding new light on their behavior and migration in the Atlantic. Perhaps the biggest finding so far is that tigers are not the coastal dwellers that they were believed to be. Instead, they appear to make extensive oceanic journeys, and have an oceanic existence for much of the year.

It appears that the majestic tiger shark, which can grow to eighteen feet long, seems just as content in six feet of water chasing stingrays on the Bahamian sand flats as it is lurking near an oceanic bank 2000 miles offshore, hoping to detect and zero in on a dead floating sea bird or loggerhead turtle. This knowledge has serious management implications: since the sharks have been shown to make extensive migrations – passing through the 200 mile Exclusive Economic Zones of several countries in a given year – no country can consider these animals “their resource”.

The GHRI left several SPOT tags in Bermuda with Neil and Choy in the hope that some female tiger sharks would show up later in the year. Oddly, of all the animals tagged so far, only one has been female. This leads us to another question: “Why are there so many males at Challenger Bank at this time of year?” A question perhaps best answered by making another expedition.

Expedition Photo Albums:

August 2009 Expedition

July 2010 Expedition

Acknowledgements

I wish to thank Rehanna Palumbo and the staff at the Fairmont Hamilton Princess Hotel in Hamilton, Bermuda for her assistance with accommodation. This is a beautifully appointed 5-star hotel in a wonderful setting on the Hamilton waterfront close to great shopping and restaurants. Well worth the visit.

Thanks to Neil and Choy for getting us together in the collaborative research effort, and for the chance to swim with these magnificent animals. Thanks to James Robinson and his family for his commitment to the project.

It is our collective responsibility to conserve the marine environment and maintain the biodiversity of the planet. Fish responsibly, dive safely.

Cheers…Guy Harvey PhD.

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