Secrets of largest fish revealed
By Richard Black
Environment Correspondent, BBC News website
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 creatures themselves.
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 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.
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
im****tant
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.
http://news.bbc.co.uk/2/hi/science/nature/4279278.stm
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