In August 2007, Australia Zoo, The University of Queensland and Queensland Parks and Wildlife undertook a project to monitor the movements and behaviour of estuarine crocodiles in Lakefield National Park. The aim of the study was to provide better information on how adult estuarine crocodiles interact within a river system, enabling informed decisions on how to manage crocodile populations.

Crocodiles are cryptic, secretive and easily disturbed by human presence. In order to monitor their movements in the wild, a novel technology using implanted transmitters and remote listening stations was used. A total of 27 adult estuarine crocodiles between 2.1 and 4.86 metres (6’ 10” to 16’) in length were captured along the Kennedy River and in adjacent tributaries. A miniature electronic device was implanted under the skin and this transmitted a sonic pulse containing information about the animals’ identification, body temperature and diving data. Listening receivers were deployed underwater along the length of the Kennedy River and these continually listened out for the devices carried by the crocodile. The receivers had a limited range and would only collect the data when the crocodile swam within a few 100 metres.

The transmitters, which were implanted in the crocodiles, had a battery life of one year and vast amounts of data were generated from our 27 tagged crocodiles. Out of the 19 males that were tagged some maintained a small home range of only a few kilometres of river, whilst others would travel back and forth along the full stretch of the river. They could travel over 60 km in a single night and remain out at sea for a few weeks before returning to the river. Female crocodiles generally moved less distance than the males, and two of our tagged females remained in the same small water hole throughout most of the year.

The Biology of the Australian Freshwater Crocodile

The Australian freshwater crocodile is endemic to Australia and are found in freshwater creeks and rivers across northern regions. It is a small species often less than three metres in length and recognisable by its long slender snout. They have received less attention than their larger estuarine cousins and many aspects of their ecology remains unknown. In order to address this issue, we studied the movements of 28 individuals over a number of months in Lakefield National Park, north Queensland. The study revealed some fascinating insights into why crocodiles dive, what they do underwater and what factors affect their diving behaviour.

We captured the crocodiles in billabongs during the dry season, attached miniature electronic tags to them and released them back into the wild. The crocodiles would go about their usual daily routine and the tags would record information about their physiology and behaviour. This information was transmitted by radio waves to remote receiving stations and we were able to observe their secretive underwater lives.

All of our tagged crocodiles increased the duration and depth of dives in the early hours of the morning and undertook fewer, shorter and shallower dives throughout the afternoon and during the night. It was previously thought that these high periods of diving in the early morning were foraging behaviours. This study revealed that the crocodiles were in fact resting within deep holes and may have even been undergoing a crocodile version of sleeping.

Our crocodiles exhibited some of the longest dive durations ever recorded for an air-breathing vertebrate. These were even more remarkable considering the warm water temperature (> 21.5°C) and the small size of the crocodiles; a 5 kilogram crocodile dived for 5.7 hours and a 42 kilogram crocodile dived for 6.7 hours. The ability of these relatively small diving animals to remain submerged for such long periods of time reveals a unique biology, one which enables crocodiles to stay underwater longer then elephant seals and even whales.

We also observed that the crocodiles would adapt diving behaviour to modify body temperature. The deep water within the water hole was cooler than the air temperature in summer and the crocodiles would stay cool during the middle of the day by diving down into the cooler water. In winter, the deep water was warmer than the air temperature throughout the night and the crocodiles would maintain a warmer body temperature by remaining for long periods in deep water during the night.

Studying the movements of estuarine crocodiles on the Wenlock River

In 2007, the Steve Irwin Wildlife Reserve was established on the Cape York Peninsula as the government’s living memorial in recognition of Steve’s conservation work. The 335,000-acre reserve borders the upper reaches of the Wenlock River and provides an excellent platform to carry out scientific research on the river. The Wenlock River possesses the highest biodiversity of freshwater fish species of any river system in Australia, as well as a healthy population of estuarine crocodiles. Its floodplains form extensive suitable habitat for nesting females and the Wenlock functions as a crocodile breeding area as well.

Australia Zoo, in partnership with The University of Queensland (UQ), continues to undertake the largest and most comprehensive study of crocodiles in the world. Since 2008, these crocodile experts have journeyed to the Steve Irwin Wildlife Reserve, situated in far north Queensland, for the annual crocodile research trip. The research undertaken each year has yielded valuable information about estuarine crocodiles.

The research gathers data about crocodiles using capture, release and tracking methods. To capture the crocodiles, the Australia Zoo croc team set land and water-based traps along the Wenlock River, still employing the original trapping techniques developed by Steve Irwin. The crocodiles are manually restrained, as these large reptiles do not process sedatives well. Morphometric measurements are recorded, and blood and collagen samples collected for isotopic analysis.

Acoustic telemetry is used to track the crocodile movements throughout the Wenlock River system. An acoustic transmitter is implanted in the armpit of each captured crocodile. Hydrophone receivers located in the river will record the acoustic signal emitted, providing electronic data such as body temperature. This technology will allow tracking of the crocodile within the Wenlock River for up to ten years.

For more detailed movement data, some crocodiles are fitted with a GPS satellite tracking device. Satellite tracking provides the opportunity for the crocodile to tracked for a period of up to two years, wherever the crocodile may travel. This is particularly useful for tracking movements over land and especially if the crocodile has left the Wenlock River system. Data is relayed via satellite twice daily, providing details on location, dive depths and dive durations.