What Do Whale Sharks Eat? Plankton and Filter Feeding

What is on a whale shark’s menu?

The whale shark is a generalist that eats whatever small food is most plentiful. Studies of stomach contents, feces and water samples taken where sharks were feeding show that its main foods include:

• Zooplankton: copepods, krill and other tiny shrimp-like crustaceans, arrow worms and the larvae of crabs and shrimp.
• Fish and coral eggs: clouds of eggs released when fish or corals spawn.
• Small fish and squid: sardines, anchovies, young mackerel and small tunas, and small squid.
• Seaweed: floating Sargassum and other algae, eaten along with the plankton.

What a shark eats depends on where and when it feeds. At Ningaloo Reef in Western Australia, krill are important; off Mexico’s Yucatán coast, sharks feed on dense patches of copepods and fish eggs; and around Christmas Island in the Indian Ocean, they are drawn to the larvae of the island’s famous red crabs.

How does a whale shark filter its food?

A whale shark’s mouth is set at the very front of its broad, flat head and can be about 1.5 m (5 ft) wide in a large adult. Inside the throat are 20 filter pads, spongy black sieves that are probably modified gill rakers. Water passes through the pads and leaves through the five pairs of gill slits, while food particles are carried to the back of the throat and swallowed.

Scientists call this cross-flow filtration. Water flows across the surface of the pads rather than straight through them, so the filters clog less easily than a simple sieve would. When they do clog, whale sharks have been seen “coughing”, reversing the flow of water to clear them.

The shark’s teeth are no help. Although its mouth holds more than 300 rows of tiny teeth, each only a few millimeters long, they play no known role in catching food.

Three ways whale sharks feed

Whale sharks use several feeding methods, depending on how dense their food is:

1. Ram filter feeding. The shark swims slowly with its mouth wide open, often at the surface, so that water and plankton are pushed into its mouth as it moves.
2. Active suction feeding. The shark opens and closes its mouth, sucking in water, sometimes while almost stationary. This works well on dense patches of food such as fish eggs.
3. Vertical feeding. The shark hangs head-up at the surface, bobbing up and down and gulping water and plankton from just below the surface.

This active pumping sets the whale shark apart from the basking shark, which feeds only by swimming forward with its mouth open. The third filter-feeding shark, the rarely seen megamouth shark, lives in deeper water.

Feasting on spawning seasons

Many of the world’s best-known whale shark gatherings are timed to short-lived bursts of food. Sharks arrive at Ningaloo Reef from about March, after the mass spawning of the reef’s corals fuels a boom in plankton. At Gladden Spit in Belize, they gather around the spring full moons, when cubera and dog snappers spawn and release clouds of eggs.

Off the northeast Yucatán Peninsula, hundreds of whale sharks have been recorded feeding on the eggs of little tunny, a small relative of the tuna. In the Arabian Gulf off Qatar, they gather around offshore oil platforms to feed on fish spawn. Being able to find these events year after year is one reason whale sharks travel such long distances.

How much does a whale shark eat?

Measuring the meals of a giant fish is difficult, so most figures are estimates. A 2010 study of whale sharks feeding off the Yucatán Peninsula calculated how much water flowed through their mouths and how much plankton it contained. Sharks there spent about 7.5 hours a day feeding at the surface.

The researchers estimated that a shark about 6 m (20 ft) long filtered roughly 600 cubic meters (160,000 US gallons) of water an hour and took in about 2.8 kg (6 lb) of plankton an hour. That adds up to roughly 21 kg (46 lb) a day, and adult sharks feeding on rich spawning events probably eat much more.

Whale sharks also feed at depth. Tracked sharks regularly dive hundreds of meters, and scientists think they may feed on deep layers of plankton and fish, which would be impossible to measure from the surface.

Do whale sharks eat plants?

Partly. A study published in 2022 found that whale sharks at Ningaloo Reef not only swallow floating Sargassum seaweed with their plankton, but also digest and absorb nutrients from it. The researchers described the whale shark as possibly the largest omnivore in the world. Whether the algae is eaten on purpose or by accident is still unclear.

Less welcome items get swallowed too. Because they filter so much water near the surface, whale sharks take in plastic, and microplastics have been found in their feces.

How whale shark feeding compares with other giants

The largest animals in the sea all feed on small prey. The whale shark and basking shark strain plankton through modified gill rakers, the giant manta ray uses filter plates between its gills, and baleen whales use baleen. The blue whale works differently again: it lunges into dense swarms of krill, taking in an enormous gulp of water at once, as described in What do blue whales eat?

Feeding low in the food chain gives access to a vast supply of energy, which is why filter feeders can grow so large. How the biggest fish and the biggest animal compare is covered in whale shark vs blue whale.