Tracking tiger shark ( Galeocerdo cuvier) migrations in the western North and Central Atlantic Ocean
The tiger shark, a charismatic but little understood species, occurs world-wide in tropical and warm temperate oceans. There is increasing evidence that tiger sharks play a key ecological role as apex predators in many marine ecosystems, including shallow seagrass and coral reef habitats. Unfortunately, tiger sharks are fished heavily in some places especially for their fins to supply the global shark fin trade. Preventing their populations from collapsing due to overfishing and resulting disruptions to normal functioning of their ecosystems will require scientifically informed fisheries management and conservation efforts, including the designation of areas that are protected from fishing.
An essential requirement for such management and conservation of any shark species is a clear understanding of its migratory patterns, how it uses its environment, and identification of what is termed its "critical habitat" - i.e., areas that are key to successful reproduction and feeding.
The handful of studies that have examined tiger shark movements have revealed what appears to be inconsistent migratory behavior among individuals - with some sharks staying relatively local, others migrating long distances mostly along coastlines, and a few moving huge distances across ocean basins.
In other words, no obvious behavioral patterns have emerged and the picture of tiger shark horizontal and vertical movements remains unclear.
To understand tiger shark movement behavior in more detail and aid in conservation efforts, the Guy Harvey Research Institute/Guy Harvey Ocean Foundation and their project partners are investigating tiger shark movements in the western and central North Atlantic Ocean in long-term studies based out of Bermuda, the Bahamas, the Cayman Islands and the US Virgin Islands.
The sharks' movements are being studied by employing three types of electronic tagging technology that provide complementary information - acoustic tags whose signals are detected by strategically positioned, underwater listening devices (receivers), and two types of satellite tags that relay information on where the tiger shark is and/or its depth in the ocean.
What have we found so far?
Tiger sharks that we outfitted with satellite tags are providing remarkable and previously unknown information on their long-term movement behavior. Please see links on the right to access interactive maps of individual, animated shark tracks in different places. We have been fortunate to be able to follow many of these tiger sharks for an unprecedented length of time (10-23 months and counting).
The results from long-term tracking of sharks tagged in Bermuda are showing that in the western North Atlantic, adult tiger sharks display detectable patterns of movements and clear evidence of residency "hot-spots" that appear to be seasonal. The overall patterns detected for adult sharks are that they migrate south along a broad corridor from Bermuda to the Bahamas (mainly) and some sections of the Antilles, where they overwinter. The sharks then embark on northern migrations during spring and summer months, spending 5-6 months in the open ocean, north of Bermuda and in many cases almost in the middle of the Atlantic Ocean!
This makes one wonder what they are doing so far out in the Atlantic Ocean after spending 6-8 months so tightly associated with island habitats in the Bahamas and Caribbean.
Something must be attracting these sharks into the deep open-ocean far offshore. Are they out in nearly the middle of the north Atlantic for mating? For feeding? The causative factors driving this behavior remain unknown. Notably, these tiger sharks are displaying a remarkable ability to drastically switch their habitats comfortably, using shallow, island environments (presumably coral reefs and seagrass habitats) for part of the year and completely open-ocean, deep environments for the other part. Few other shark species show this flexibility.
The study is continuing with tracking more tiger sharks in the western Atlantic. We have also expanded this study into the Indian Ocean.