Dolphins, Lured by Food, Moving Back to North Sea

From the wind-swept ferry bound for the Frisian island of Texel, you see more oil rigs than dolphins these days.

But the dolphins are out there, scientists say, and they are returning in growing numbers to this part of the North Sea, awash with the waste of five industrial countries, studded with oil platforms and heavily fished.

“There are now tens of thousands of white-beaked dolphins in the North Sea,” said Mardik Leopold, a marine biologist at the Dutch Institute for Marine Research.

The scientists say the dolphins are being lured back by the reappearance of vast numbers of herring--their favorite food.

For years the waters from Newcastle in Britain to Esbjerg in Denmark had been a virtual no-go area for dolphins.

But fishing curbs since the late 1970s have helped the herring population recover from the brink of extinction.

“Now you can practically walk on herring from here to England,” Leopold said.

Herring fishing was totally closed in the southern part of the North Sea in 1977 and reopened under tighter controls in 1983.

Whale numbers are also on the increase in these waters. Leopold’s colleague Kees Camphuysen, who logs sightings, estimates that there are now a couple of hundred minke whales in the area.

No sightings of porpoises, which are related to dolphins, were reported between 1972 and 1979. Porpoises were seen occasionally after that but since 1986 the institute has recorded regular sightings in the winter months.

In March, 1991, Leopold himself saw a school of 48 harbor porpoises off the coast of the Dutch island of Terschelling--the largest group spotted since the 1950s.

Both scientists say heavy fishing alone cannot be blamed for the way herring disappeared from these waters.

A current in the North Sea also weakened at the same time, so fewer newly born herring were making the journey from breeding grounds in the west to feeding sites in the east.

The scientists say the fact that dolphins are reappearing in greater numbers in this part of the North Sea does not necessarily mean that the overall dolphin population is increasing.

“There are two possibilities as we understand it,” said Camphuysen. “First, there could be more animals. But dolphins don’t reproduce that quickly.

“Second, they could be moving from somewhere else. We think that’s more likely.”

The dolphin population near the Shetland Islands in Scotland is known to be declining, the scientists say--and they could be leaving other areas.

Pollution remains a concern. “It’s nice for us to see them back. But they are entering unknown territory,” said Camphuysen. “It could be dangerous for them.”

However, the North Sea biologists have little evidence of major pollution threatening the new arrivals.

There has been no increase in the number of dolphins swimming to North Sea beaches to die--the figure for the Netherlands’ coast has been holding steady at 10 to 15 a year. Dolphins usually head for land when they feel too weak to keep swimming.

“There’s no evidence that pollution is better now, but it’s difficult to test the effect of pollutants on dolphins,” said Leopold.