Hi-tech electronic tags on whale sharks, the world's largest fish, have revealed how and where they find food.

Researchers in Belize have tracked the sharks as they dive almost a kilometre in search of food, and find shoals of spawning fish in order to eat the eggs.

The sharks grow to 20m in length, and are listed as vulnerable to extinction.

The researchers believe their findings will help to plan tourism operations around whale sharks in a way that does not harm the creatures themselves.

These new, unprecedented insights into the whale sharks' world come from the Belize Barrier Reef, the world's second largest barrier reef system and a site given UN World Heritage status.

"Our study showed that sharks dive much deeper than previously believed, reaching depths of over 1,000m in search of food," said Rachel Graham, of the US-based Wildlife Conservation Society (WCS).

"Water this deep is only a few degrees above freezing; and this explains why tropical whale sharks have an insulating fat layer just below their skins, something which has perplexed scientists for years."

Day or night

During the night, the sharks generally remain in shallow water, feeding off plankton, and reserving deep dives for the heat of the day.

Deep dives often end with a high-speed ascent, perhaps to deliver a burst of oxygen to their bodies after a period in deeper, less oxygenated water.

Around the time of the full moon, Cubera snappers come together near the shore to spawn, forming huge masses of writhing bodies in a "soup" of freshly-released eggs.

For the whale sharks, this is a feast, and they swim through the egg soup time and time again, filling their giant mouths with snapper caviar.

This habit of surfacing during spawning allowed the scientists to attach electronic tags to the whale sharks.

The tags make regular recordings of temperature, water pressure and light level. After a pre-programmed period, they automatically detach from the shark, float to the surface and beam their data back as an e-mail via satellite.

Slow and easy

The whale shark (Rhincodon typus) is found globally, both in open water and near shore.

Despite its huge size, it eats plankton rather than people, and its slow movements make it easy to catch by harpoon or net.

IUCN, the World Conservation Union, lists the whale shark as "vulnerable" in its Red List of threatened species.

Owing to a demand for fins, trade in its parts is now regulated under the Convention on International Trade in Endangered Species (Cites).

However, a different industry is now growing in some parts of the world, including Belize, using the creature as a tourist attraction.

"Knowledge of the whale shark's dive behaviour can help us tailor conservation policies in a way which minimises impact on them," Dr Graham told the BBC News website.

"We now know that the spawnings, the predictable pulses of food, are important enough to the shark that they change their regular behaviour to make use of them.

"So protection of the critical habitat that these feeding sites represent, and of the sharks when they're visiting, is key to sustaining the sharks."

The WCS and University of York scientists publish their findings in the Royal Society's journal Interface.