Secrets of the Belize Whale Sharks Revealed
By Richard Black
High-tech electronic tags on the whale shark, 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 which does not harm the
These new, unprecedented insights into the whale shark's 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
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
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
Whale sharks patrol the reef promontory waiting for snappers to spawn
[Photo credit: Rachel T Graham]
Despite its size, the whale shark eats plankton not people
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
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 creatures 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.