| Max Length: | ~ 4.5m (15') |
| Max Weight: | ~ 2000 lbs |
| Habitat: | Coral reef, sandy shoals, seagrass beds. |
| Life Span: | max >25 years |
| Distribution: | Primarily warm coastal waters and near oceanic islands. |
| Diet: | Highly variable; includes fish, other sharks, rays, marine mammals, marine turtles, birds, and sea snakes. |
Tiger sharks are one of the largest shark species, often growing to over 4m (13’) and possibly even to 5.5m! They give birth to live young - perhaps up to 80 at a time. However, a female will give birth at most once every two years. Tiger sharks grow relatively quickly, reaching maturity at around 3m in 7-11 years.
Tiger sharks prefer warm waters and are generally found in the tropics and move into more temperate waters during summer months. They are built for eating large prey with a broad head and mouth. Their serrated teeth cut in both directions. A tiger shark can even cut through a loggerhead turtle’s shell. In Shark Bay, sea snakes, turtles, and dugongs are all favorite foods. Even though tiger sharks are impressive predators, they won’t pass up a free meal and frequently scavenge. Tiger sharks will sometimes congregate at a carcass like a whale or dugong, but they are generally solitary.
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Since 1997, SBERP has been studying the tiger sharks in Shark Bay. It is the perfect place for our study since it is one of the last places where a population of large sharks remains relatively free from human impacts. That means that we can use Shark Bay as a model to understand the role of tiger sharks in marine ecosystems and maybe learn how we can help to restore marine habitats where large shark populations have been dramatcially reduced by human fishing. Our original goals were to determine the seasonal patterns of tiger sharks abundance and to determine their patterns of habitat use. At first, we were most interested in how tiger sharks influenced habitat use patterns of bottlenose dolphins (Tursiops aduncus), but our work grew to look at the factors influencing tiger shark movements and abundance and the role of tiger sharks in the Shark Bay ecosystem. That means that we have to acutally spend more time studying the prey of tiger sharks than the sharks themselves. You can find out more about the studies of the shark's prey on our other research pages. We use many techniques to study these sharks and have benefited greatly from our collaborations with the National Geographic Crittercam team (click here to see a video of Crittercam on a tiger shark). |
| A small tiger shark is released | |
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| A satellite tag is attached |
- Determine patterns of seasonal abundance and factors that influence shark abundance.
- Determine the diet and trophic relationships of tiger sharks.
- Determine growth rates, size structure, and sex ratio of tiger shark population.
- Determine patterns of habitat and microhabitat use relative to prey distribution.
- Determine how tiger sharks influence habitat use patterns of their prey including dolphins, dugongs, sea turtles, sea snakes, rays, and cormorants.
- Determine indirect effects of tiger sharks on seagrass communities through predation sensitive foraging by dugongs and green turtles.
- Determine long-distance movements of tiger sharks when they are not present in the bay using satellite transmitters.
- Determine the seasonal patterns of abundance of large shark species other than tiger sharks.
We can estimate the abundance of tiger sharks in our study area using catch rates on drumlines. These lines consist of an anchor, a long rope, floats, and a single shark hook that dangles down from the suface about 2m (6'). We use small chunks of Australian salmon for bait.
With the high abundance of tiger sharks in the bay during some seasons, we don't have to set our lines for long. One of our biggest concerns is the safety of the animals so we check our lines often and handle the sharks with great care. Drumlines are usually set before dawn and checked about every two hours. When we catch a shark, we get it swimming next to the boat in order to tag it, measure its length, and determine its sex. Once done, the hook is removed and the shark swims away.
To determine the habitat preferences of tiger sharks we use two primary methods: acoustic tracking and Crittercam deployments. In acoustic tracking, we attach an ultrasonic pinger to the shark and use a directional hydrophone to follow the from our boat. With this method, we can plot the approximate location of the shark every five minutes. One problem is that we can't see exactly what type of area the shark is in or what it is doing. For that, we use National Geographic's Crittercam. Invented by Greg Marshall, Crittercam combines a video camera and data sensors (depth and temperature) to show us exactly what the shark is doing and precisely where it is.
To determine where the tiger shark go when they are not inside our study area, we attached Wildlife Computers SPOT3 satellite transmitters to the dorsal fins of 4 sharks. These transmitters relay the GPS position of the shark when the transmitter breaks the surface.
Knowing what the sharks are doing is only part of the research project. We also collect data on the distribution and behavior of many of their prey species to help us understand the role of tiger sharks in Shark Bay.
Tiger sharks are present in large numbers in our study area during the Australian summer, but numbers drop in the winter. In some years the sharks are virtually absent in the winter while during other years some sharks are still found in the bay.
The favorite prey of large tiger sharks in our study were sea snakes, dugongs, and sea turtles.
The abundance of tiger sharks in the bay is probably linked to the abundance of some of their main prey items in the area, and our catches of large sharks (>3.5m) are linked to the abunance of dugongs we see in our surveys.
Most of the tigers in the area are big. Our average size is almost 10' (2.9m), but we have caught tigers as big as 4.5m!
Norwest blowies eat too many baits in shallow waters so we can't use catch rates to determine habitat use of the sharks.
Tiger sharks in Shark Bay prefer shallow seagrass beds (usually <4m, 12', deep) over the surrounding deeper waters (6-10m, 20-33'). They like to swim along the edges of the banks the best, but they still prefer the middles of banks to the deeper waters.
Shallow waters have higher prey abundance than deeper waters, suggesting that tiger sharks may prefer these habitats because there are more potential prey items
Tiger sharks do not appear to attack prey that notice them. Crittercam recorded a number of encounters between sharks and possible prey, but when the prey looked at the shark, the shark turned away not wasting the effort to attack something that might just get away.
Tiger sharks bounce. The continuously swim between the bottom and surface waters. This movement pattern may help them save energy, but it may also help them detect prey floating at the surface or lying on the bottom.
The tiger sharks that we deployed satellite tags on moved large distances, but most of them stayed near Shark Bay or returned to it after being gone for a while. This suggests that the sharks have huge home ranges along the Western Australian coast, but return periodically to the bay. One shark made an amazingly long swim - all the way to the waters off of South Africa in just 99 days!
We have slowly begun to unravel the ecological role of tiger sharks in Shark Bay. Tiger sharks have a major influence on the distribution and behavior of many species in Shark Bay including dolphins, dugongs, sea turtles, and sea birds. By determining where and how these species forage, tiger sharks likely impact their population dynamics. Perhaps most importantly they may control the spatial pattern of how turtles and dugongs graze the seagrass. Studies in other areas show that turtles and dugongs can alter the seagrass beds themselves, so by changing where these graxers feed, tiger sharks probably influence the seagrass itself much like wolves influence where elk forage and in turn affect plant communities. Because tiger sharks are predators of so many species, they also are important in mediating indirect interactions among the many species of the bay. For example, dugongs influence where dolphins are willing to forage becasue the sharks go where the dugongs are and the dolphins stay away from these areas that have a lot of food because the sharks are there.
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| Turtles and other shark prey are influenced by sharks. |
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Why this research matters: The fact that tiger sharks are important in structuring the dynamics of the Shark Bay community provides some of the first direct evidence from the field that large sharks, like tiger sharks, are critical components of marine ecosystems. Our work suggests that the declines in populations of sharks that are ongoing worldwide have probably changed the marine environment greatly already and will continue to do so. Also, if we try to restore grazer populations without trying to bring back their predators (like tiger sharks) we may not restore ecosystem function in the way we hope to. The long-distance movements that we found mean that we have to be careful to protect tiger sharks over a large range. Even though tiger sharks are protected in Shark Bay and along much of the Western Australian coast, they could wander into areas where fishing is much more intense. Therefore, even a large marine park like Shark Bay could be adversely affected by fishing and events that are far away! |
| A tiger shark crusies the sand |
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- Wirsing, A. J., M. R. Heithaus, and L. M. Dill. 2007. Fear factor: Do dugongs (Dugong dugon) trade food for safety from tiger sharks (Galeocerdo cuvier)? Oecologia doi: 10.1007/s00442-007-0802-3
- Wirsing, A. J., M. R. Heithaus, and L.M. Dill. 2007. Can you dig it? Use of excavation, a risky foraging tactic, by dugongs is sensitive to predation danger. Animal Behaviour doi: 10.1016/j.anbehav.2007.02.009
All photographs © Michael Heithaus, Douglas Braun, Vanessa Cloutier, and members of the Shark Bay scientific community. Images may be used for educational purposes. For use in other forms, please contact Mike Heithaus
