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Do Fish Use Cold Current To Cross Tropics?

By John Roach
for National Geographic News
February 5, 2003
 
A big, old Patagonian toothfish found thousands of miles from home is bolstering the theory that large fish can take advantage of very deep, cold ocean waters to cross the tropics from one polar region to the other, swimming under warm water in which they ordinarily could not survive.

The Patagonian toothfish (Dissostichtus eleginoides) is normally found in the icy sub-Antarctic waters off South America. So when a commercial halibut fisherman pulled one in November 2000 from the Davis Straight off the coast of Greenland, he was surprised.

"He knew he had never seen this one before and was unable to find it in any of his local fish fauna guide books," said Peter Rask Møller, a zoologist at the University of Copenhagen in Denmark.



The fisherman, Olaf Sólsker, froze the odd fish and brought it ashore. None of the local fisheries biologists were able to identify the specimen. Eventually, the fish made its way to Møller and colleagues who identified it as a Patagonian toothfish. They report the discovery in the February 6 issue of the journal Nature.

Prior to this discovery, the species, commonly known as Chilean sea bass and served as a delicacy at restaurants in the U.S., Europe, and Asia, had not been recorded north of Uruguay in the Atlantic and north of Chile in the Pacific.



"To find a Chilean sea bass in northern Arctic is certainly a novel find," said Thomas Hilbish, a professor of biological sciences at the State University of New York at Stony Brook. "It does strongly suggest there is some sort of route for trans-equatorial migration."

But how did it get there?

Transequatorial Migration

Many marine species have what scientists refer to as anti-tropical distribution patterns, meaning that the same species has separated populations on either side of the equator. Great white sharks and humpback whales are some of the most well-known examples of this.

"The present catch of a strictly southern hemisphere deep-water fish supports an old and widely accepted theory that today's anti-tropical distribution pattern is a result of earlier migration through the warm tropics," said Møller.

But since the distribution pattern is more pronounced on the family and genus level, rather than the level of specific species, other theories hold that the populations separated as tropical waters gradually warmed, pushing them north and south.

"It really is a great mystery as to how some of these cold water species can migrate across the tropics," said Hilbish. "There has been a vigorous debate about the routes and mechanisms species like this would have."

Møller and his colleagues contend that finding a Patagonian toothfish off the coast of Greenland is a strong indication that fish can and do migrate through the tropics by using the cold waters found at great depth.

Patagonian toothfish prefer water temperatures that range from 36 to 52 degrees Fahrenheit (2 to11 degrees Celsius), and occur at depths up to 9,843 feet (3,000 meters).

These features make it possible for the species to migrate across tropical waters, which at depths below 3,281 feet (1000 meters) cool off to 50 degrees Fahrenheit (10 degrees Celsius) and even cooler at greater depths.

"We know that large toothfish tend to stay in greater depths than smaller ones, so it could be expected that the straying specimen should be a large one," said Møller. "The Greenland specimen is one of the largest known."

Patagonian toothfish can grow to 6.7 feet (2 meters) long and live to 50 years old. The specimen found off of Greenland was 5.9 feet (1.8 meters), mature, and weighed an estimated 154 pounds (70 kilograms).

While the scientists do not know what could have prompted this fish to swim to the other side of the world, they say Patagonian toothfish are known to make very long migrations around the sub-Antarctic circle and that perhaps this one just ended up in the wrong current.

"In every population of animals, a small number of specimens sometimes do something unexpected and different from all the others," said Møller. "These are normally either the very strong and fit specimens, or the very weak and poor ones. This specimen seems to be one of the very strong ones."

The particular fish does have a few different features, such as shorter pectoral fins and more teeth, than the descriptions of south Atlantic populations, but the scientists believe that the fish is the same species and attribute the differences as natural results of maturation.

Møller and colleague Jorgen Nielsen have kept extensive records of fish occurring in Greenland since 1981 and seen their list grow from 116 species to more than 250 today. The toothfish has never been caught in the northern hemisphere, according to their records.

Overfished Delicacy

Møller and his colleagues are pretty certain that the fish did not swim to the northern hemisphere in search of scarce food, as much of the Patagonian toothfish stock—and thus competition for food—has been greatly reduced due to overfishing.

In fact, Patagonian toothfish has become such a popular menu item over the last few decades that the species is at risk of being fished to extinction. International conservation groups have launched campaigns to curb demand for the fish, which command premium prices at high-end restaurants around the world.

According to a report released last month by the wildlife trade monitoring network TRAFFIC, Chile exports nearly 80 percent of its toothfish catch each year to Japan and the U.S. to the tune of U.S. $90 million. Argentina exports 85 percent of its toothfish catch, earning $30 million to 36 million.

"Already, there are areas that have had their stocks of toothfish so severely depleted that commercial fishing in those areas is no longer viable," said Anna Willock, a senior fisheries advisor for TRAFFIC in Sydney, Australia. "This rings loudly and clearly the warning bell as to the conservation status of these species."
 

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