The travelling life of the tiger shark

At 9 foot long, not including the tail, tiger shark (Galeocerdo cuvier) 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.  In study lead by James Lea (The Guy Harvey Research Institute, Nova Southeastern University Oceanographic Center) and team of international collaborators, those shark movements have been compiled and analysed.  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 researcher’s 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.  When it came to the summer, all the adult males and just one adult female headed out into the open ocean of the North Atlantic 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.  The raw data itself is not 100% usable and goes through processing to ensure that the information is accurate and suitable for analysis.  You can read more about this process in the methods section of the paper (see link below).  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 Bermuda 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 seagrass beds – and the species that depend on them.

Read the research for yourself
The paper was published in Nature Scientific Reports, and has been made open access.  You can have a read of the research yourself by heading herehttp://dx.doi.org/10.1038/srep11202