About Guy:
Guy Harvey

Over the past year we have learned a great deal about tiger shark migrations in the western Atlantic and Caribbean. In a combined effort by the Save Our Seas Foundation and the Guy Harvey Research Institute, shark populations around the Cayman Islands are currently being investigated. Tiger sharks did not show up here in May despite considerable effort by research teams to find them. We are embarking on another effort this winter, as we now know from studies in Bermuda that tiger sharks prefer the warmer waters of the Caribbean during the northern winter. We are renewing our efforts to catch and tag tiger sharks and great hammerhead sharks around Grand Cayman in November and December.

In early August of 2009 GHOF staff collaborated with the Bermuda Shark Tagging Project in tagging seven adult tiger sharks with PSAT and SPOT electronic tags. The location was Challenger Bank, a few miles southwest of Bermuda. Local Bermudians, vet Dr. Neil Burnie and Choy Aming are heading up this project on behalf of Bermuda with assistance and collaboration from the GHRI/GHOF staff in terms of tag provision and deployment along with follow-up analysis of these shark’s movements in the western north Atlantic.

This year, Dr. Mahmood Shivji of NSU(GHRI director) and Dr. Brad Wetherbee of the University of Rhode Island were on hand to calibrate the tags and assist with deployment. Both Dr. Shivji and Dr. Wetherbee have worked extensively on tiger sharks in the Bahamas and in the USVI.

Using Neil’s 34’ Prowler “Bones”as a work boat and assisted by Capt James Robinson on the “Wound Up” as a catching boat, the sharks were caught on 20/0 circle hooks with no barb, 130 pound gear, 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 tag. This process takes about 15 minutes during which time the shark’s head is in the water and is ventilating normally. The sharks are then released and swim off at a rapid clip.

We were able to track the sharks’ movements away from Bermuda when sea water temperatures dropped in October as they migrated south towards the Bahamas, Turks and Caicos Islands, and to the Virgin Islands. The tracks showed the sharks were not wandering aimlessly but headed in a straight line, which means they knew where they were going. For the rest of the winter months they behaved like a reef shark, tracking the edges of deep island drop-offs. Presumably they were feeding opportunistically along the way. Their behaviour in searching for food at or near the surface means they expose their dorsal fin and so communicate with the Argos satellite on a regular basis. Only a few ocean going sharks exhibit this behaviour which allows us to use the SPOT (single position only tag) to track them.

Come April and May 2010, as the sea water temperatures rose, all the sharks began their northward migration, some aiming straight for Bermuda. Not far away from the island they started to wander away and out to the east of the island where some stayed for most of the summer. Katrin, the only female tagged last year, is currently on a latitude adjacent to New York. What are they doing out in the middle of the Atlantic Ocean? Feeding perhaps, but on what? Or perhaps they are breeding. We don’t know, and need to get a boat out there to see what is going on.

This year’s tagging expedition started on August 24 as we headed out to Challenger Bank for six days of tagging. Starting out with one shark caught for each of the first three days, we got hot on the last three days, tagging three, four and two.

Chumming still was the key, and we had ample supplies of fish heads and caught bonitos, ocean robins (local name for an abundant mackerel scad), blackfins, wahoos and barracudas on the bank to add to the mix. Half of a fresh barracuda got instant results. James would hook up and catch a tiger, transfer it to Neil’s boat “Bones”, and then return to the mooring and the chum and continue fishing while we operated and released that shark. The GHRI provided 12 SPOTs and Neil purchased four of the three-year SPOTs with assistance from Bermudian sponsors.

Neil and Choy were doing a great job in Bermuda in getting local businesses involved and in producing a documentary to educate the public about their efforts and success with the project.

While James was fishing with 130s we put out a quarter inch rope line cable leader and 20/0 circle hook baited with barracuda and suspended from a large buoy. We caught three sharks using this method. One of these, an eight footer, was the smallest so far and went in the boat  with a wet towel covering its eyes and two deck hoses running through its gills for ventilation. 

Doing the deployment in the boat gives you a lot more control and we were able to deploy the three-year SPOT on this young male in just a few minutes. However, a 12 or 14-footer presents a different challenge, plus the system of holding them strapped to the boat but still in the water, is better for the shark.

Interestingly 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 other big 12 footers regurgitated the remains of a seabird and lots of feathers.

Meanwhile we swam with other free-swimming tigers, and saw no tags from last year. Neither did we catch a tiger that was tagged last year, nor any tagged by Brad and Mahmood in the USVI over the last 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 numbers pre-commercial exploitation of this and other species, as they sure have been hammered by shark-finning operations over the last three decades.

While we anxiously await the results of this year’s efforts, the Bermuda government is considering banning the landing of sharks, as are other countries in the Caribbean and western Atlantic.

The regular reporting by the sharks is shedding more information on their behaviour and migration. They are not the strictly coastal sharks as was previously considered, but make extensive oceanic journeys, and have an oceanic existence for much of the year. This knowledge has management implications, in that no country can consider these animals “their resource” as they have now been shown to make extensive migrations passing through the 200 EEZ of several countries in a given year.

The majestic tiger shark that grows 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.

The GHRI left several SPOTs in Bermuda with Neil and Choy in the hope that some female tiger sharks would show up later in the year. All the animals tagged so far, except for one, have been male. Another question we would love to answer…why so many males? Brad said the adults reach maturity after about ten years, at about 10 feet long. The adult females do give birth to large numbers of fully formed juveniles, up to three dozen from one large female. Growth rates can be variable depending on how well an individual shark feeds, but age in tiger sharks has been determined by counting concentric rings in their vertebra.   WH

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