杏吧原创

LOVERS of sharks and shark-fin soup can both relax. Economists have worked
out a sustainable way to fish the species, and it should appease both
conservationists and soup connoisseurs.

Thanks to the burgeoning Far Eastern trade in fins for soup, sharks are one
of the most valuable food items in the world, says Quentin Fong of the
University of Alaska in Fairbanks. Demand in the Hong Kong market, which handles
some 3000 tonnes of fins a year, is so great that the best fins sell for more
than $400. They provide the much-prized noodle-like cartilage that
thickens and flavours Chinese soups.

But because sharks are slow to mature and only produce a few offspring at a
time, their populations are vulnerable to overfishing. Many fish are slaughtered
for their fins alone, long before they reach reproductive age. For instance,
most sharks caught off the shores of West Africa are younger than two years.
Conservationists say that the fin trade is threatening the survival of some
species, and that the sharks should be allowed to grow up and reproduce before
being caught.

Now help is at hand from an unlikely source: economists. Fong has combined
ecological and market data on sharks and demand for their fins to create a
single 鈥渂ioeconomic鈥� model for the shark trade.

Because Chinese chefs will pay more for the long strands of cartilage in
larger fins, Fong says that fishermen could maximise profits by allowing sharks
to reach adulthood before they are caught. In economic terms, if the sharks are
left for longer than that they become an increasingly wasted asset. But if
caught any younger, they will not fetch such a good price.

Fong鈥檚 analysis concentrates on the blacktip shark (Carcharhinus
limbatus), one of the most widely harvested species. It grows to about 2
metres in length and is, according to the World Conservation Union, highly
vulnerable to overfishing.

The economically optimum solution for the blacktip is to allow them to mature
to around 10 years old. This is only early middle age for the blacktip, which
could expect to live to around 20 years, but it is getting on for twice the age
at which it starts to reproduce.

It all sounds good in theory, but there are few effective controls on shark
fishing anywhere in the world. So the lure of an instant return鈥攑lus the
fear that if the small sharks are thrown back somebody else will catch
them鈥攎eans sensible economics usually goes out of the window.

The solution is for countries to start enforcing a tough fisheries management
policy, says Fong. That might mean setting a minimum size for sharks that can be
caught for their fins, for instance.