Tiger sharks are known to consume a variety of prey, but an in-depth study of their stomach contents reveals the true spread of that diverse menu. A whopping 192 unique items were found in the bellies of over 600 tiger sharks, and as the title of this article suggests, some of those meals were quite extraordinary.
Before diving into digestive details, we need to touch on a potentially uncomfortable fact: all of the tiger sharks in this study, which was recently published in the journal PLOS ONE, were recovered from the KwaZulu-Natal (KZN) Bather Protection Programme between 1983 and 2014.
The programme, which deploys lethal control measures (that's shark nets and drumlines) near popular swimming beaches along South Africa's eastern coastline, is extremely controversial. We've discussed the pitfalls of such measures in detail here and here, and while it's certainly not easy to stomach the loss of so many predators, the information gleaned from these carcasses has the potential to help local scientists better protect sharks in a number of ways.
Want food, must travel
Thanks to their "can opener" teeth, tiger sharks have earned a reputation as sea-turtle specialists, but in reality, they're generalists who munch more prey types than any other shark (followed by dusky sharks, which eat about 120 species).
Learning more about what each shark population is dining on can tell us where, why and when these animals move around. And that intel is critical for those working to mitigate conflict between these predators and humans.
The study's authors found that smaller tiger sharks prefer to snack on inshore, shallow-water prey like octopuses and other bottom-dwelling molluscs, as well as a lot of small fish (67 species from 28 different families, to be exact). As the predators increase in size, so too does their prey: oceanic mantas, 20 species of sharks, 18 different rays and deep-sea fish were among the choice cuts.
"These results suggest that larger sharks are spending more time further offshore in the pelagic environment than smaller sharks," explains the team.
All this is pretty intuitive: we humans also consume larger portions with age (much to the detriment of our waistbands), and juvenile sharks are known to spend time in coastal nurseries before venturing to new foraging grounds.
But the team also observed a seasonal shift in tiger shark food choices. In summer and autumn, they tend to eat more fish; in winter and spring, their attention turns to marine mammals.
"This coincides with the northward reproductive migration of humpback whales in the winter as they move from the Antarctic to breeding grounds in Mozambique, and their return southward migration in the spring," writes the team.
This means we can expect to see more tiger sharks in South Africa's coastal waters at either end of the migration route, as the predators move inland for a biannual blubber buffet.
A similar travel-for-food approach has been seen in Hawaii, where tiger sharks are known to shake up their movement patterns to intersect loggerhead turtle aggregations and feast on the year's fledgling albatrosses. What's more, over in Western Australia, the intelligent carnivores have learned that rock lobster fishing season means lots of tasty leftovers will be discarded near boats.
Risky business
As whales migrate along 1,200 miles of eastern South African coast, many stop over in nearshore bays. The ocean giants can be seen from many popular beaches – and while that's great for whale-watching tourists, for sharks, it can mean trouble.
Some of the meat recovered from tiger shark abdomens belonged to whales far too large for these predators to hunt, like sperm whales, minke whales and humpbacks. It's possible the sharks had targeted young calves, but it's more likely they obtained their fat-rich fare by scavenging adult carcasses.
This underscores one of the most concerning – albeit unintentional – effects of using netting as a management tool to target "problem" sharks. The nets are indiscriminate, and whales are often incidentally caught. Sick or injured whales occasionally float into the nets as well. In both cases, the allure of a decomposing leviathan can bring sharks themselves dangerously close to the nets.
By matching up stomach contents with catch date and location, the research team revealed that tiger sharks are particularly vulnerable to entanglement during these events.
We saw this dangerous scenario back in 2015, when a young tiger shark was snared by a net off the coast of Durban in South Africa:
The KwaZulu-Natal Sharks Board, the organisation that maintains shark nets and drumlines in the region, has taken some steps to reduce these risks. Drumlines – which lure and capture the animals using baited hooks – have replaced the nets in some locations, but this gear also has its shortcomings. Filter-feeding whales aren't likely to bite, but non-target sharks and other marine life can still be snared. For this reason, conservationists are growing increasingly concerned about the impact of lethal control measures on the ocean ecosystem.
We still don't know why tiger sharks come to the nets to feed more often than other species, but the team suspects this has to do with their unfussy foraging habits: they're simply more likely to move in on an easy meal.
Elsewhere in South Africa, "shark spotters" have proven extremely useful when it comes to alerting bathers to nearshore predators, and a similar approach could help to reduce the number of tiger sharks that get tangled up during carcass chow-downs. More eyes on the nets during whale migration season could help prevent unnecessary deaths.
Man-eaters?
Among all the assorted food items researchers found during their work, there was one that did not show up often. Human remains – parts of tibia, fibula and pelvis bones – were detected in the stomachs of only two sharks: that's two sharks out of 628, caught over three decades. It is possible, of course, that some of the study specimens may have eaten and fully digested human tissues before they were caught. However, the study's findings do help to underscore the rarity of shark bites on humans.
Tiger sharks are often portrayed as man-eating aggressors by the media, and while the top predators can certainly be dangerous, just 31 tiger shark-related human fatalities exist on record for the past 437 years.
International Shark Attack File (ISAF) director George Burgess, who is also a fisheries biologist at the Florida Museum of Natural History, explains that while tiger sharks do rank among the "Big Three of the shark-attack world", a complex mix of factors is responsible for this.
"They are large species ... capable of inflicting serious injuries, are commonly found in areas where humans enter the water, and have teeth designed to shear rather than hold," he writes at ISAF.
Tiger sharks patrolling bathing beaches, however, are certainly not in search of human prey. As these predators age, they don't entirely abandon their "baby food": cephalopods, for example, remain an important prey group throughout their lives. During the recent study, cuttlefish (as well as 28 species of squid and five octopus species) were identified in tiger sharks of all sizes. Small reef fishes like puffers and boxfish, as well as crustaceans, were also prominent across the board.
These prey items are relatively low in calories when compared to whales or rays, but they're easy to catch – and that's a great reason for large tiger sharks to sporadically patrol the shallows to look for them.
This quantity-over-quality approach can provide an energetic advantage, too: less energy is spent diving deep, or cruising the vast open ocean in search of a meal.
Human impact
Humans may be a rare find in the belly of the beast, but our trash certainly isn't. Among the miscellaneous items recovered were condoms, crisp packets (that's "chips" to my fellow Americans), candy wrappers, chamois leather, cigarettes, tin, plastic, twine and various butcher's bags containing chicken gizzards, abattoir bones and the like.
That glaring indictment of our polluting habits aside, human activity may be affecting these animals in other ways too – and the geographical differences in tiger shark prey might help to shed some light on this.
Tiger sharks in Australia are known to hunt over 30 species of sea snakes alone, and they regularly consume both loggerhead and green sea turtles. But in South Africa, they showed little interest in reptiles (though the odd monitor lizard did turn up). A much stronger preference toward piscine cuisine could put sharks moving through this region in competition with local fisheries operations.
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Top header image: Kevin Bryant/Flickr
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