| Length: | ~2m in Shark Bay |
| Weight: | ~100kg in Shark Bay |
| Habitat: | found in all waters around Shark Bay |
| Life Span: | probably to 40 years |
| Distribution: | found in all waters of Shark Bay |
| Diet: | In Shark Bay, mainly bony fish, some squid |
Tiger sharks are the main predator of dolphins in Shark Bay and many calves do not survive because of the sharks which also attack adults. Another danger to the dolphins is stingrays. At least one dolphin has been killed by a stingray spine that broke off in the dolphin.
The dolphins in Shark Bay have a complex social structure that has been investigated by the Dolphins of Shark Bay Research Group since 1984. For more information on the social lives of the dolphins, CLICK HERE.
Many people think that dolphins are able to live their lives free of constraints and worries that most animals must face. In reality, this is not the case. Dolphins are almost constantly on the lookout for their next meal, which usually is made up of relatively small fish. Dolphins also have to stay safe from predators. Tiger sharks are the major threat to dolphins in Shark Bay, especially their calves. Despite the apparent importance of food availability and predation risk, there have been no studies that have systematically addressed how dolphins deal with the demands of getting a meal and not becoming one themselves. From 1997-1999, SBERP studied this question.
To understand how dolphins balance the demands of getting a meal with staying safe from sharks, we have initiated studies of the dolphins, their fish prey, and tiger sharks. Based on studies of other animals and behavioral ecological theory, we predicted that when tiger sharks were not in the bay, dolphins would be distributed between deep and shallow habitats directly proportional to the amount of prey in those habitats. However, when sharks moved in, dolphins should spend more time in safe areas - its not worth the food to take the risk!
Methods: The first step is to understand how dolphins use their habitats and how this might change with time. We did this by drving along set lines in both deep and shallow habitats and counting the dolphins that we see. Whenever we saw dolphins we recorded where they were, which individuals were there, and what they were doing. During the study, we made almost 800 passes along our transect lines!
To find out when and where the sharks were, we used shark catches and tracked them. Check out the tiger shark page to learn more!
To learn where the dolphin's food was most abundant we set hundreds of fish traps in deep and shallow habitats throughout the year. For every fish that we caught, we measured its length and its mass. After catching over 15,000 fish we began to piece together a picture of where dolphins can find the most food - in the shallows!
We found that tiger sharks were by far the most common large shark in Shark Bay and are probably the only major predation threat to dolphins. While tiger sharks may not kill many dolphins - we rarely find dolphin remains inside tiger sharks - they attack a very large proportion of the population. More than 74% of the dolphins have at least one scar from a run-in with tiger sharks and many have multiple bites. One dolphin was attacked at least six times! We also estimated, based on the presence of fresh wounds, that at least 10% of the dolphins are attacked unsuccessfully every year. That means that every dolphin can expect to have a run-in with a shark sometime during thier life and they should be wary of tiger sharks, especially sharks over 3m (10') long. |
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We found that dolphins have to make trade-offs between getting an easy meal and staying safe from tiger sharks . . . at least during some times of the year! There is more fish for the dolphins in shallow seagrass beds than deeper waters no matter what time of year it is. When tiger sharks are not found in the study area, dolphins do exactly what we expect - most of them are found foraging in the fish-rich shallow waters. However, during the Australian summer (and some winters) tiger sharks are in the bay and they are most common in the shallow waters - exactly where dolphins can get the most food. What do the dolphins do? Most of them switch to foraging in deep waters where it is safer even if it is harder to get enough to eat. Of course there are exceptions - juvenile males seem to take more risks than other dolphins and keep foraging in the shallows. They payoff of getting a lot of energy must outweigh the risk of being attacked. Future studies by the dolphin research group may be able to answer this interesting question!
The dolphins that do stay in the shallows to forage when tiger sharks are around change the way they use the banks. Instead of feeding both in the middle and at the edges of the banks, as they do when sharks are absent, they feed almost exclusively near the edges of the banks. This tactic gives them easy access to deep waters where they can easily swim circles around tiger sharks rather than possibly being trapped far from an easy escape.
Maybe our most important finding is that the decisions that dolphins make are influenced by other species in the bay. The tiger sharks are attracted to the area and the shallow seagrass banks not by dolphins, but by their main prey: dugongs, turtles, and sea snakes. This means that decisions made by dugongs, especially, influence the dolphins because they share tiger sharks as a predator. This shows how strongly interconected the Shark Bay Ecosystem is and tells us that changes that we make to one part of a marine system may have dire, and unintended consequences for other parts of the ecosystem.
Why is this research important? Our studies on dolphins highlight the importance of the abundance and distribution of food and the risk of predation, or the fear of predators, in determining where dolphins are found. Therefore, human impacts to dolphins in Shark Bay, and around the world, that affect fish populations or that scare dolphins in ways similar to predators (like boat disturbance) may have negative consequences for dolphin populations. Also, this study shows how dolphins are influenced directly and indirectly by other species in the ecosystem suggesting that if we care about dolphins we also have to be concerned about the other species they share the oceans with.
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| Dolphin forages over seagrass |
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| Dugongs and sea snakes may influence where dolphins spend their time foraging because they attract the tiger sharks! |
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Heithaus, M. R. and L. M. Dill. 2006. Does tiger shark predation risk influence foraging habitat use by bottlenose dolphins at multiple spatial scales? Oikos 114: 257-264.
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Dill, LM, Dill, ES, and Charles, D. 2003. Feeding preferences of the Monkey Mia dolphins: results from a simultaneous choice protocol. Marine Mammal Science 19: 650-660.
- Dill, L. M., M. R. Heithaus, and C. J. Walters. 2003. Behaviorally-mediated indirect species interactions in marine communities and their conservation implications. Ecology 84: 1151-11567.
Heithaus, M. R. and L. M. Dill. 2002. Food availability and tiger shark predation risk influence bottlenose dolphin habitat use. Ecology 83: 480-491.
- Heithaus, M. R. and L. M. Dill. 2002. Feeding Tactics and Strategies. Pp 412-422 in Perrin, W. F., B. Wursig, and H. G. M. Thewissen (eds.). The Encyclopedia of Marine Mammals. Academic Press, NY.
- Heithaus, M. R. 2001. Shark attacks on bottlenose dolphins (Tursiops aduncus) in Shark Bay, Western Australia: attack rate, bite scar frequencies, and attack seasonality. Marine Mammal Science 17: 526-539.
- Heithaus, M. R. 2001. Predator-prey and competitive interactions between sharks (order Selachii) and dolphins (suborder Odontoceti): a review. Journal of Zoology (London) 253: 53-68
