Warming tied to spread of clam-gobbling ray

Fishermen in Sanyo-Onoda, Yamaguchi Prefecture, handle long-headed eagle rays caught Friday morning.

A species of ray that is usually found in tropical and subtropical waters has been seen in waters around Japan in recent years and is believed to be responsible for damaging the local shellfish habitat.

The long-headed eagle ray, whose scientific name is Aetobatus flagellum, has been spotted in the Ariake Sea and Seto Inland Sea in western Japan. Scientists believe that a rise in sea temperature is responsible for the expansion of the creature’s habitat.

As the ray likes to feed on shellfish, the area’s asari little neck clam habitat is under threat.

In waters off Sanyo-Onoda in Yamaguchi Prefecture, which faces the Seto Inland Sea, fishermen embarked on an operation Wednesday to exterminate the eagle ray.

The ray grows up to 1-1/2 meters and weighs as much as 50 kilograms.

Three fisherman held a ray caught in a net to chop off its venomous spines from the base of its tail.

On the first day of the operation, the fisherman captured 381 rays. On Friday morning, 192 were caught over a two-hour period.

“We’ll never be able to completely get rid of them. It’s awful because the ray scares away fish from around here,” said a 77-year-old experienced fishermen with a perplexed look on his face.

It was seven years ago that damage to the asari clam habitat was first noticed.

Fishermen noticed round holes measuring 20-30 centimeters in diameter on the ocean floor in Hiroshima Bay, surrounded by large quantities of broken shell fragments.

Though they initially suspected the damage to be the work of poachers, it was later discovered that the eagle ray had dug up and eaten the clams using its sharp teeth to crush its shell.

Over the last 30 years, water temperatures in some areas of the Seto Inland Sea have increased by about 1 C.

For instance, average sea temperatures off Okayama Prefecture rose from 17 C in 1973 to 17.9 C in 2006.

According to Atsuko Yamaguchi, associate professor at Nagasaki University, the long-headed eagle ray prefers water temperatures of 17 C or higher.

“I presume the ray’s appearance in our seas is due to a rise in sea temperature,” Yamaguchi said.

While a theory has it that sea temperature rises in the Pacific Ocean are part of long-term temperature fluctuations, many scientists believe that global warming is responsible for the phenomenon.

Last year, fisherman in Yamaguchi Prefecture caught more than 3,000 rays during 10 operations to rid the area of the pest.

Though it has been either disposed of or used to feed livestock in the past, the Sanyo-Onoda city government is considering processing rays into food products and is exploring various ways to cook the creatures.

(May. 26, 2008)

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