Study finds a barrier to NT crocs returning to capture sites

6 September 2019

New research has found a natural barrier in the Northern Territory could stop saltwater crocodiles returning to sites where they were captured, potentially informing efforts to manage "problem salties" in the region.

Saltwater crocodiles have incredible homing instincts - with some able to find their way back to 'capture sites' up to 300km away.

However, the crocodiles in the study were unable or unwilling to swim around Cobourg Peninsula, preventing them from returning to their capture sites.

Lead researcher Yusuke Fukuda, a PhD scholar at The Australian National University (ANU), said the findings provide new insights into the navigation and movement of these crocodiles, and could help stop clashes between crocs and humans.

"Relocated salties often return to their original capture sites, which complicates management interventions aimed at reducing human-crocodile conflict," said Mr Fukuda from the ANU Research School of Biology.

"Cobourg Peninsula could create a navigational conflict, because salties would need to detour by swimming north in parts, away from the direction of their capture location.

"This is in contrast to what happens in Queensland, where Cape York is not an effective barrier to crocodile movement - at least, not for larger salties.

"But exactly how Cobourg Penisula interferes with the crocodiles' movements still remains a mystery."

Mr Fukuda said the study's findings supported the management of "problem salties" that no crocodile is allowed to be relocated once captured.

"Not only are they likely to return to their capture site, unless moved from one side of a movement barrier to another, but crocodiles' active movements in new areas may create the risk of crocodile attacks within other communities," said Mr Fukuda, who is also a wildlife scientist at the NT Department of Environment and Natural Resources.

The research team used satellite tracking devices on eight saltwater crocodiles of different ages and sizes to see how the peninsula interfered with their homing abilities

Five males, ranging from three to four metres long, were shifted and released 100-300 kilometres from their capture sites, while three others of similar sizes were released at their site of capture as controls.

"The salties that were moved away were highly mobile compared to the controls, and moved in the direction of their original capture site," Mr Fukuda said.

"Genetic analysis of tissue samples from nests across the NT coast also demonstrated significant genetic diversity across the coast, and confirmed that Cobourg Peninsula is a barrier separating genetic stocks across the coast.

"This suggests that more than 250 crocodiles the NT Government removes each year from the Darwin Harbour for public safety come from the western side of the Peninsula."

The study was supported by ANU, the Northern Territory Government, Australian Government, Charles Darwin University, Wildlife Management International Pty Ltd, National Geographic Society, Holsworth Wildlife Research Endowment, IUCN-SSC Crocodile Specialist Group Student Research Assistance Scheme, ACT Herpetological Association and the Northern Territory Crocodile Farmers Association.

The study findings are published in PLOS ONE.