Whale Shark

The whale shark is the biggest fish in the ocean by a wide margin — a spotted giant that can outweigh a bus and stretch longer than a school bus. It is also one of the most harmless large animals in the sea.

Overview

Whale sharks roam the warm open oceans of the world, usually alone, but at certain times and places they gather in the dozens or hundreds where food is briefly abundant — a coral spawning event off Western Australia, a plankton bloom off the Yucatán, a run of fish eggs in the Philippines. These predictable aggregations are where almost everything we know about the species has been learned.

Physical characteristics

A whale shark’s back is dark grey-blue, patterned with a grid of pale spots and stripes as individual as a fingerprint — researchers photograph the area behind the gills and match the pattern with software to track individuals for decades. The belly is white.

The head is broad and flattened, ending in a mouth up to 1.5 m wide set right at the front of the snout rather than underneath as in most sharks. Inside are thousands of tiny teeth that play no role in feeding. The skin is up to 15 cm thick — the thickest of any animal — and the tail can be 2.5 m tall.

Feeding

Whale sharks are one of only three filter-feeding shark species. They use two methods:

• Ram filtering — cruising forward with the mouth open, letting water flow in and out through the gills, where 20 filter pads trap plankton, fish eggs, krill, and small fish.
• Active suction — hanging vertically in the water and gulping, pulling in dense patches of food.

A feeding whale shark filters thousands of litres of water an hour and can coordinate its arrival with events like the mass spawning of corals or snappers, timing a visit to within a few days each year.

Reproduction and life cycle

Whale shark reproduction was a mystery until one pregnant female caught in 1995 was found to contain about 300 pups at different stages of development. This showed they are ovoviviparous — eggs hatch inside the mother and the pups are born live, in enormous litters, at around 40–60 cm long.

Where whale sharks give birth, and where the pups spend their early years, is still largely unknown; almost all the animals seen at coastal aggregation sites are immature males. Growth is slow, and a whale shark may not breed until it is 25–30 years old and may live past 100.

Threats and conservation

The whale shark is listed as Endangered:

• Targeted fishing for fins, meat, liver oil, and cartilage, now banned in many countries but continuing in some.
• Bycatch in tuna purse-seine nets, which whale sharks are drawn to because the nets also concentrate their food.
• Ship strikes, a serious hazard because the sharks feed slowly at the surface, often in busy coastal shipping lanes.
• Unregulated tourism at aggregation sites, where crowding, touching, and boat propellers cause injury and stress.

Whale-shark tourism, done well, is also part of the solution — a live whale shark seen by thousands of paying visitors each year is worth far more to a coastal economy than a dead one.