tiger shark

Tracking tiger sharks

Publication specs

Title: Crossing Latitudes – Long-Distance Tracking of an Apex Predator 

Authors: Luciana C. Ferreira, Michele Thums, Jessica J. Meeuwig, Gabriel M. S. Vianna, John Stevens, Rory McAuley, Mark G. Meekan 

Journal: PLOS One

Year: 2015

Although some people may picture sharks as intimidating predators that roam long distances, that’s not the case for many of the more than 400 species. There are also small sharks and those that spend most or part of their lives in one area – a pattern known as residency. When it comes to tiger sharks, the big scary predator vision is at least partially true – they are large (sometimes more than 5m in length), can be frightening if you’re a turtle, and swim long distances (potentially up to 8000km). However, using satellite tags, this study showed some tiger sharks along the coast of Western Australia to actually be seasonal residents to the coastal waters near where they were initially tagged. The eight sharks spent about half their time in marine reserves. The sharks tended to prefer warmer coastal waters, but the results from these data demonstrated that they have the ability to migrate between tropical waters and cool temperate waters (as cold as 6°C – brr!).

Understanding tiger shark movements is important. Where sharks go influences population connectivity. In order to manage populations, we must first understand where individuals are – and in Western Australia, they are spending parts of the year along the coast near where the researchers deployed the tags. Tiger sharks in Shark Bay were shown to influence the behavior and movements of prey, so it’s no surprise that the tiger sharks likely have a sort of structuring effect on the ecosystem. Protecting sharks locally, along with both their habitat and prey, is vital.

One female tiger shark, tagged at Ningaloo Reef, had the longest duration track to date for a tiger shark (517 days) and moved 4000km in that time period. After being tagged, she moved to Rowley Shoals then impressively made it to Indonesia and back into Australian waters (a distance of about 1000km) in just two weeks. This movement pattern is known as site fidelity – where the animal makes a long-distance movement and then returns to the location where it was tagged. International cooperation between Indonesia and Australia will be needed to make informed conservation decisions since there was documented movement between both countries. For this tiger shark, local protection in Western Australia is not quite enough.

 

A predator’s role in storing carbon

Publication specs

Title: Predators help protect carbon stocks in blue carbon ecosystems

Authors: Trisha B. Atwood, Rod M. Connolly, Euan G. Ritchie, Catherine E. Lovelock, Michael R. Heithaus, Graeme C. Hays, James W. Fourqurean, Peter I. Macreadie

Journal: Nature Climate Change

Year: 2015

When you think about carbon (C) storage in an ecosystem, you may think about a lush tropical forest sucking up carbon dioxide from the atmosphere. What isn’t considered as often is coastal marine ecosystems, which take up C 40 times faster than tropical forests. An estimated 25 billion tons of C is buried in vegetated coastal habitats like seagrass meadows, mangroves, and salt marshes, making them the most C rich environments in the world. Degrading these “blue carbon ecosystems” releases C into our atmosphere, fueling climate change. But did you know that losing predators like sharks within these environments also indirectly leads to the release of C?

One example of this comes from Western Australia where sharks influence how often herbivores like dugongs and sea turtles feed in a given time period. These grazers like to eat in places where there are very few tiger sharks waiting to attack them. In habitats where sharks are present, they spend more time watching their backs. This means that less seagrass is being consumed, and in some cases is only being cropped. In the areas where sharks are present and there are fewer grazers, the seagrasses are mostly slow growing species, which promotes the storage of C since it is not breaking down quickly. In other words, where the abundance and behavior of herbivores are being controlled by predators, growth of vegetation is enhanced, which leads to increased storage of C.

Scientists don’t know the total global area affected by the loss of predators. However, if only 1% of the vegetated coastal areas were to be affected, about 460 million tons of C would be released, equivalent to the emissions from 97 million cars. In order to protect these blue carbon ecosystems, balanced conservation efforts will need to occur where the habitat, predators, and herbivores are protected together.