Tag Archives: tiger shark

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 sea grass 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 here http://dx.doi.org/10.1038/srep11202

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.


Image: Tiger Shark at Tiger Beach, Bahamas.  Credit: 2010 Terry Goss, Terry Goss Photography USA/Marine Photobank

Shark loss hits global ecosystems

The loss of sharks could contribute to the destruction of one of the planet’s most under-appreciated sources of carbon storage — seagrasses, according to FIU researchers. Not that sharks eat the seagrass, they don’t, but they do eat the turtles which feed in the seagrass meadows. Add this to the problems of pollution, mooring and destruction of seagrass, means this vital habitat – and the sharks – need help.

“Seagrasses around the world are under considerable threat — from pollution to dredging and changes in water quality,” said Mike Heithaus, marine biology researcher who specialises in sharks. “Now, it appears that the loss of sharks, especially tiger sharks, can cause collapses of seagrass ecosystems as well.”

As global efforts are under way to conserve turtles, shark populations are suffering from overfishing, which is creating an imbalance of the two animals in the world’s oceans. The focus of the research is about sustaining the delicate food chain balance. In this case, sharks, turtles and seagrasses must all be preserved in concert. To not do so, could trigger an ecosystem collapse.

Turtle in Marsa Alam

Green Turtle (Chelonia mydas) in what was once a seagrass meadow

“Historical overfishing of sea turtles so drastically reduced the numbers of turtles in the ocean by the 16th century that impacts on seagrass meadows were rarely recognized,” said James Fourqurean, co-author of the study. “It’s only now that conservation efforts are on their way to restoring sea turtle populations that we are now experiencing the consequences of how overfishing large sharks is impacting seagrass.”

The concept is simple. Sharks feed on turtles. Turtles eat seagrass. Healthy tiger shark populations keep turtle populations in check so they do not devour entire meadows before the seagrasses are able to replenish themselves. Seagrass meadows, acre per acre, are among the world’s most valuable ecosystems, providing nurseries for species that people rely on, protecting water quality and slowing rates of coastal erosion. Seagrass absorbs carbon dioxide and outputs oxygen at twice the daily rate of tropical forests, which helps to mitigate climate change. With seagrass meadows disappearing at an annual rate of about 1.5 per cent, 299 million tonnes of carbon are being released back into the environment each year, according to research published in Nature Geoscience (DOI: 10.1038/ngeo1477).

While most seagrass losses to date have been attributed to poor coastal zone management, the food chain variable has been largely ignored.

The researchers examined the impacts of green sea turtles on seagrass communities in Bermuda, Australia, Indonesia and India, all locations with large green sea turtle populations. The findings were published this week in Frontiers in Marine Science. In each of the sites, data suggests the seagrass meadows are being disrupted by heavy grazing where turtle populations are increasing and shark populations are down. But that doesn’t mean turtles are villains. In most of the world, they are threatened with extinction and still need conservation.

“We’ve seen similar issues on land – the loss of wolves leads to population explosions of deer that then damage ecosystems,” Heithaus said. “We need efforts to protect and rebuild turtle populations in most of the world, but we also need to be working to ensure that populations of their predators are intact so the turtles don’t eat themselves out of house and home.”

Further Reading

Heithaus MR, Alcoverro T, Arthur R, Burkholder DA, Coates KA, Christianen MJA, Kelkar N, Manuel SA, Wirsing AJ, Kenworthy WJ and Fourqurean JW (2014) Seagrasses in the age of sea turtle conservation and shark overfishing. Front. Mar. Sci. 1:28. doi: 10.3389/fmars.2014.00028

Little Support for Shark Culling

Following another fatal shark attack in Australia, new research finds little support for the shark cull.

A survey of 583 aquarium visitors asked people how they thought the Government should respond to shark bites and found that despite the public’s fears, 87 per cent favoured non-lethal responses.

Only four per cent of those surveyed supported the hunting of sharks.

Another key finding was that only 2-4 percent blamed the Government and only slightly more (6-8 percent) blamed the sharks. Most responsible were thought to be either the swimmer or simply no-one.

Conducted by University of Sydney Lecturer Dr Christopher Neff and funded by the SEA LIFE Conservation Fund, the survey is the first research of its kind. Dr Neff stated, “These responses show that there is little support for government measures that kill sharks and that the public does not blame governments when these tragedies occur.”

“The Australian public is ready for some new options” said Claudette Rechtorik, Director of the SEA LIFE Conservation Fund. She added, “The findings from this data are consistent with what we hear every day. After 77 years of shark culling in New South Wales it is time to consider something else. We…believe the research is important for policymakers to consider given that it suggests that the Government should respond to shark bites with greater public education and non-lethal shark culling measures.”

The research comes as Western Australia seeks to extend its shark cull policy by three years.

Based on state figures released in March, the WA shark cull policy has killed 41 sharks of which 95% were tiger sharks. The sharks ranged in size from 1.7 m (a Mako) to 4.1 m. Ten were already dead: killed by the drum lines. The rest were destroyed. The government has not released any information on the numbers of other animals killed by the drum lines.

Photo credit: Albert Kok