The Catlin Seaview Survey, working with shark specialist Richard Fitzpatrick, tagged two female Tiger Sharks (Galeocerdo cuvier) in March 2013 at Raine Island, located in the Far North of Australia’s Great Barrier Reef. The sharks were named using local Aboriginal words: Moonda (meaning ‘tiger shark’) and Tarni (meaning ‘salt water wave’).
The tags enable us to monitor the Tiger Sharks’ movements using satellite tracking. By doing this we can learn about not only the movements of the sharks themselves, but also their relationship with their prey - after all a shark needs to eat!
Raine Island is home to the largest Green Turtle (Chelonia mydas) breeding population in the world. The turtles congregate around the island for their 3-4 month nesting season between November and February. An average of 5,000 turtles can attempt to nest in a single night at the peak of the season. Outside of the reproductive season, Green Turtles disperse to distant foraging grounds.
Tiger Sharks are an apex predator in these tropical reef systems and are considered to be the biggest predation threat (excluding humans) to Green Turtles. During the turtles’ nesting period, they become a predictable and abundant food source for the Tiger Sharks. Researchers would expect to see the sharks’ movement behaviour concentrated around Raine Island during the turtles’ reproductive season, starting in November.
The sharks were both tagged at the end of the turtles’ nesting season in early 2013. Once the season was over, one of our sharks, Moonda, swam north all the way to Papua New Guinea in mid-April but did not stay there long, returning to Raine Island after a few days. She also moved west towards Boydong Island on the 18th May but again, did not stay long.
We lost signal from our other shark, Tarni, between 12th June and 8th July and haven’t had any data since 13th July. There are many potential reasons for the lack of data uplinks, including exhaustion of batteries, antenna breakage, animal mortality, damage to the tag, detachment of tag, and the biofouling of saltwater, which is particularly problematic in tropical waters. Biofouling is the accumulation of microorganisms, plants, algae or animals on wetted surfaces. The fact that there was a long gap between data uplinks suggests that there could have been a build-up of algae on the tag, which was later dislodged.
On average, the sharks have shown area-restricted movement behaviour around Raine Island throughout the year so far and have not shown area-restricted movement at other locations. If the Green Turtles are known to only frequent Raine Island during their nesting season, this suggests that there is sufficient other types of prey in the region to provide food for the sharks. This also supports the view that Tiger Sharks are generalist feeders, earning their nickname ‘wastebasket of the sea’.
With Green Turtle nesting season beginning in November, stay tuned to find out whether Moonda will stay around Raine Island for the predictable feast.