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 diversity of 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 rearing stockyard for other river systems.
In August 2008, we captured and tagged 15 adult crocodiles (11 males and 4 females) from the Wenlock River. A miniature electronic device was implanted under the skin of these animals and they were released back into the river. These tags have enabled us to track the movements of these particular crocodiles over the next 10 years. The signal transmitted from each receiver is recorded by underwater receiving stations. At the time, we had deployed twenty receivers covering a 64.3 kilometres stretch of the Wenlock River. In 2009 we added more receivers to the array, greatly increasing the size of our study area, and the data collection from our tagged crocodiles.Read the full story
On the 2010 expedition to the Wenlock River we tagged another 30 crocodiles, again broadening the size and depth of the sample group for the continuing study of these prehistoric creatures, as well as installing additional underwater listening receivers in popular swimming areas frequented by the locals.
During August 2011, the croc research team caught a further 22 crocs, accumulating to a total of 86 crocodiles in this ongoing research project, with 26 crocs also having been satellite tagged since the programs inception. In a first for the team, the researchers began trials of an accelerometer attached to the nuchal fold of a croc. The information gathered from the new device will help researchers graph intensity of movement of the crocs, and is hoped to indicate how regularly the implanted crocs are feeding. The research has begun to reveal the movements between the 'wet' and 'dry' season for the estuarine crocodiles, which tend to move out of the Wenlock River and into the side creeks and swamps during the 'wet'. It is also the first year that researchers have been taking blood and tissue samples from the crocs for isotope analysis, to reveal further information about the crocodiles short and long term diets.
Our 2012 trip brought 16 new crocs, as well as 9 recaptures from previous years. We continued to use the acoustic tags and satellite transmitters in order to track these amazing animals over the length of the mighty Wenlock River. The acoustic tags send out a data pulse every 60 seconds which is picked up by 54 receivers which are stretched out over 180 km’s of estuary, river and billabong.
In 2013, 12 new crocodiles had acoustic transmitters implanted and we had a whopping 15 recaptures, which is incredible! This year also saw us begin stable isotope analysis on our captured crocodiles. This ground-breaking study, which involves humanely taking blood samples from the animals, is going to give us a better understanding on what the Estuarine Crocodile actually feeds on. This is information that is previously unknown, and will teach us so much more about these mysterious creatures.
In 2014 we trapped 15 new crocodiles and had 5 recaptures from previous years. This currently brings our total croc tally to 129. 2014 also saw the use of a brand new satellite technology. The satellite tags we used this year for the first time also enable us to monitor both dive times and dive depths for the crocodiles. Historically, this kind of study has only been done with archival tags which are designed to fall off the croc and store the data on board the tag. This can be tricky as getting the tags back is not always an easy task! With this new technology though, the data is downloaded at the same time the GPS location data is downloaded, with no need to go and find the tag somewhere along the river.
All of this data from our studies is going to help us gain further knowledge on estuarine crocodile behaviour, which can help make informed decisions on the management plans for crocodiles not only here in Australia but around the world.
The project is still very much in progress and we are still learning much about the Wenlock River and its crocodile inhabitants.
Tracking estuarine crocodiles on the Kennedy River, Lakefield National Park, North Queensland
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 aims 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 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. 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 meters.
The transmitters which were implanted inside 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 ranges of only 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. These data sets are very complex and require detailed analysis, to look at interactions between individual animals and seasonal differences in movement patterns. We anticipate that many more interesting facts about the private life of estuarine crocodiles will be revealed during the continuing research.
The Biology of the Australian Freshwater Crocodile
The Australian Freshwater Crocodile is endemic to Australia and 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 devices (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 (> 21.5oC) and the small size of the crocodiles, a 5 kg 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.
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