Creature of the Month

 
 

Tiger Shark
(Galeocerdo cuvier)

by Sam Andrews

 

 

 

Tiger Shark

 

 

 

At 9 foot long, not including the tail, tiger shark "Harry Lindo" is not exactly on the small side. It's not Harry's size that is exciting scientists and shark enthusiasts, nor a photograph taken in 2009 by Ian Card showing a shark - suspected to be Harry, trying to eat a 150 lb juvenile tiger shark off the coast of Bermuda.

Between 2009 and 2012 researchers tagged 24 tiger sharks with satellite transmitters in the Challenger Bank, which lies just off Bermuda in the Atlantic Ocean. Harry, it turns out, is one heck of an ocean wanderer. In just over 3 years Harry swam over 44,000 kilometres - that's more than the circumference of the Earth (just over 40,000 kilometres). Harry's track is the longest recorded for a tiger shark, and probably the longest ever published for any shark species.

Unexpected movements
Tiger sharks are often spotted in coastal waters in temperate warm and tropical seas, but they also wander out into the open ocean. The tagging study is just one of a few multi-year studies tracking individual shark migrations. The researchers' hard work was well rewarded when it revealed previously unknown shark movements in the Atlantic. Most of the sharks in this study were male, but there were a few juvenile and females too. Adult males, females, and juveniles of both sexes spent the winter months in the Caribbean and the Bahamas. All the adult males and just one adult female headed out into the open ocean of the North Atlantic, near Bermuda in the summer. The repeated use of these two vastly different types of habitats up to some 3,500 kilometres or so apart was surprising for a species previously thought to be primarily coastal. The researchers think that repeated returns to the same sites may be better for the fish than constantly looking for new habitat. They know where their food is, so why take a risk looking elsewhere only to find none?

How do you track a shark anyway?
Sharks, being a marine species, can't simply be watched by people. In this study, the sharks with tagged with something called Argos satellite platform terminal transmitters, also known as PTTs. Every time a shark goes to the surface, these little tags send location to a receiving satellite. The researchers can then grab this data from the satellites. As for getting the tags on the shark in the first place, well you have to go fishing. Once the shark is caught, the tracker device is fixed onto the shark's fin and then it is released to go about its business. There is a a decent video showing shark tagging on YouTube (start at 1:07 if you don't want to watch the whole thing).

Why is this work important?
Like many shark species, the tiger shark is an at risk species. It is currently listed as 'near threatened' on the IUCN Red List, primarily threatened because it is targeted by fisheries, and caught as bycatch. If we want to help look after these sharks, understanding their movement is really useful. For example, earlier work by the Guy Harvey Research Institute highlighted how important the waters around the Bahamas were for the sharks. The Bahamas government responded by establishing a 'shark sanctuary' in 2011, in which all commercial fishing of sharks was banned in their territorial waters. Looking after sharks isn't just important for the sharks themselves either. They are an apex predator, and considered to be a 'keystone' species, playing an essential role in ecosystem health. One impact is to alter the predator-prey ratio, with alterations being felt throughout the food web. Lose too many predators can cause herbivore populations grow. If there are too many herbivores, you could lose plant-based habitat, like sea grass beds - and the species that depend on them.

Fancy following a shark?
Head over to the Guy Harvey Research Institute Tracking Site where you can see movements of tiger sharks and other species, like blue marlin, sailfish, and mako sharks.

Images: Tiger Shark at Tiger Beach, Bahamas. Credit: 2010 Terry Goss, Terry Goss Photography USA/Marine Photobank; Juvenile tiger shark in the Bahamas by Albert Kok, CC BY-SA 3.0.

Sam Andrews is a marine conservation communicator. She is especially interested in spatial conservation strategies for mobile species/life stages, such as dynamic ocean management and marine protected area networks.

Further Reading

The travelling life of the tiger shark.

Reprinted with permission:
Copyright SCUBA Travel - http://www.scubatravel.co.uk/

 Tiger Shark