It’s generally accepted as fact that sharks are cool. People love learning about sharks, watching movies about sharks, and some of us even love swimming with sharks. We recently celebrated our shark appreciation at the Bruce Museum with a showing of Jaws, followed by a fascinating shark discussion led by Kaitlin Gallagher, a PhD candidate at the University of Connecticut. She gave us a multitude of shark information we could sink our teeth into, and left recommendations for sharky science blog posts. Today’s blog post is inspired by a particularly interesting line of discussion that came up, and a paper she sent over after.
It’s time to dive in and talk about sharks!
Tiger sharks are one of the most famous -or perhaps infamous- shark species. They are found all over the world, cruising in tropical and temperate waters. They can grow over 16 feet and account for a large number of reported shark attacks. This makes them one of the more “dangerous,” species of sharks, but attacks are still very rare. Of the thousands upon thousands of people that go swimming around the Hawaiian Islands, near which many tiger sharks make their home, there are only three to four shark bites per year. Of these, most are not fatal.
Tiger sharks actually have more to fear from humans than we do from them. They are frequently killed for their fins, flesh, liver oil, and cartilage. Demand for shark fin soup is always rising, so tiger sharks have a rough road ahead of them. They are currently considered a near threatened species due to their declines in areas where they are harvested, though their wide global distribution gives them a safety net against depopulation.
Aside from their ferocity, what most people know about tiger sharks is that they are “garbage cans of the sea” and will eat anything. While this is an exaggeration, research suggests that it is not baseless.
There have been several studies regarding feeding habits of tiger sharks, one of which surveyed populations of sharks around the Hawaiian Islands. These scientists wanted to know not only what the sharks were eating, but how their diet changed as they aged. They looked at stomach contents of 281 sharks of varying sizes noticed several intriguing trends.
Smaller sharks, under 6.5 feet, had fairly uniform diets. They ate a lot of cephalopods, like octopuses and squid. They also ate a lot of smaller fish, with puffer and porcupine fish making up a large part of their diet. Their prey tended to be slow-moving bottom-dwellers, and animals active at night. They did eat some trash, but mostly table scraps like ham, chicken, steak, spam, and citrus rinds. Some small sharks ate birds, but only two were found with land mammals in their stomachs: Two unlucky cats, probably washed away by a storm.
The larger sharks had a different feeding story. They ate a wider range of prey, and what they tended to eat was larger, like sea turtles and dolphins. While the small sharks ate no crustaceans, larger sharks ate all sorts, like lobsters, crabs, and mantis shrimp. They still ate birds, but a variety of land mammals were on the menu too. Researchers found sharks that had eaten cats, dogs, mongooses, rats, horses, sheep, and goats. In one particularly large shark they had a chilling find: The remains of a human.
Bigger sharks ate more diverse trash too. Like the small sharks, they ate kitchen scraps. However, they also ate food-related debris like tin cans, foil, plastic bags, cellophane, paper, and cardboard. Other people have found tiger sharks with tires, baseballs, and license plates in their bellies. It seems that adult tiger sharks become less discerning about what they eat all across the board.
While larger sharks ate less fish than the small ones, they had broader diversity in what fish they consumed. Smaller sharks ate more sluggish fish, but the large ones often went for faster and more challenging fish prey, like tuna and marlin.
Even though tiger sharks living in different areas have access to different food sources, the general pattern remains the same. Younger and smaller tiger sharks have a much narrower range of food selections than larger adults. In all groups, as shark body size increases, the size of their prey increases as well. When tiger sharks reach about 7.5 feet in length they start to eat human-sized prey. It’s a good bet that sharks of this size are the ones that pose the most threat to humans (though we don’t want to be shark alarmists here at the Bruce Museum and want to reiterate that shark attacks are very rare).
Now that the changing nature of tiger shark diets has been established, the question remains of why they eat what they eat when they do. Small sharks mostly eat slow bottom-dwellers, so it’s safe to assume that small tiger sharks spend a lot of time swimming and hunting close to the sea floor. Larger sharks have more prey on or near the surface, so probably forage more in those areas.
Larger sharks are more physically capable of taking on a wider variety of prey sizes, but learning may also have something to do with it. Older sharks have more time to hone their hunting expertise, so can pursue more challenging prey. This is good news for the shark, and bad news for local dolphins when tiger sharks come around!
- Kate Dzikiewicz, Paul Griswold Howes Fellow