Australia

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.

 

Protecting migrating shark species isn’t always easy

Publication Specs

Title: Conservation challenges of sharks with continental scale migrations

Authors: Michelle R. Heupel, Colin A. Simpfendorfer, Mario Espinoza, Amy F. Smoothey, Andrew Tobin and Victor Peddemors

Journal: Frontiers in Marine Science

Year: 2015

It’s no surprise to anyone that sharks can move. What may be surprising is that some shark species can move thousands of kilometers, traveling across multiple jurisdictions. When sharks swim across state, national, and international boundaries they are exposing themselves to varying levels of threats and protections. In order for conservation measures to be effective for mobile species, knowledge of the scale of movement is required.

Bull sharks are no strangers to long-distance movements, being known to travel hundreds of kilometers. On the Australian East coast, researchers studied bull shark movements across two state jurisdictions: Queensland (QLD) and New South Wales (NSW) using acoustic telemetry. Tracking devices were surgically inserted into 114 bull sharks, which then transmitted their signals to fixed receivers. When a tagged shark swam past the receiver, data on that individual was stored and later retrieved.  

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On average, the bull sharks were found to travel 1194km with no evidence of individuals making movements based on the time of year. The sharks were detected on multiple receivers at various sites, which is evidence that these sharks are connecting temperate and tropical systems while also crossing state boundaries.

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Larger juveniles and adults were found to swim the furthest, with multiple hypotheses for this behavior:

  • The shift in diet from juvenile to adulthood
  • The need to reduce competition for food
  • To avoid eating their own kin
  • Less tolerance for nearshore freshwater
  • A combination of all of the above

Since bull sharks are frequently moving along the eastern coast of Australia, current marine protected area zoning may have limited benefits for this species. Managers face complex challenges when making conservation decisions that affect highly mobile shark species. This study highlights the need for jurisdictional cooperation between QLD and NSW that would lead to effective marine protected area zoning, which could include movement corridors for bull sharks. Nonetheless, managers face difficult decisions in terms of negotiation and coordination with different governments when trying to protect species that move across state, national, and international boundaries. Studies like this one will improve our knowledge of shark movements in order to facilitate the decision-making process that will lead to effective conservation measures.