The seas off the coast of Peru are some the richest and most diverse in the world. But in recent weeks both Peru's seabirds and dolphins have been dying at an alarming rate, writes Tom Clarke.

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Around 5,000 pelicans have washed up in the last month alone.

Initial fears were of an outbreak of an unknown type of birdflu. That has now been ruled out. But uncertainty about the cause has hit sales of fish and people are demanding an explanation from the authorities.

Now they think they have one but it wasn't the scientists who spotted it first. Fishermen who go out after anchoveta - or Peruvian anchovy - had seen it in their catches.

"All of this is dictated by currents, and the warmer currents are changing the direction of the anchoveta - the animals are dying because they can't get them," said Augusto Montez, who normally fishes 20 miles out from Lima, but is now chasing dwindling shoals of the fish much closer to the shore.

The force driving the fish south, and causing the Pelicans starve, is called a Kelvin wave - Tom Clarke, Science Editor

Not only are ancoveta prize by fishermen, the fish form the basis of the marine food chain along the coast of South America.

Though many pelicans have simply starved, others have washed up drowned after getting trapped in fishing nets.

"In all my years working at sea, I have never seen so many pelicans fighting it out with fishing boats in order to eat," says Yuri Hooker, a marine biologist with the marine park around the Palomino Islands near Lima.

The force driving the fish south, and causing the Pelicans starve, is called a Kelvin wave. A current of warm water, driven by changes in the atmosphere far out over the Pacific Ocean.

Kelvin waves run along the equator at speeds of up to 3 metres a second. When the wave meets the coast of South America it gets deflected. The tropical current running south down the coast displaces the cold water and the animals like anchoveta that depend on it.

So, an entirely natural phenomenon, exacerbated by pressure from man, seems to be the underlying reason for the pelicans' decline.

At least 900 dead dolphins have been washed up on Peru's shores.

Dolphin deaths

The problem for the Peruvian government is that it's not just the pelicans that are dying. At the end of last year, dolphins began to wash up on the coast a few hundred miles north of Lima. At the last count 900 had been found and many more have probably died.

It wasn't starvation that killed the dolphins, so a link to the Kelvin wave has been ruled out. Tests for viruses by government scientists have proved inconclusive too.

The leading sea mammal charity, believes the dolphins were killed by explosive seismic surveys for offshore oil - an industry that in Peru is quite literally booming.

"The organs in the fresh carcasses we found were full of gas bubbles, also the vessels in the animals ears were haemorrhaged," said Carlos Yaipen, director of Peruvian sea mammal charity Orca. "These animals were exposed to a human-made sound source that traumatized them and produced the die-offs.

The government and oil industry strongly reject the seismic shock theory - and it is hard to prove conclusively. Experts do agree the cause of the dolphin die off is probably unrelated to the Kelvin wave.

The return of El Nino?

But in the past, Kelvin waves have been a pre-cursor to El Nino, a cyclical, pacific-wide climatic phenomenon. As well as disrupting weather patterns across the pacific basin, El Ninos in the past have caused massive die-offs of marine life along the south coast of South America.

According to marine scientists, that is where the coincidental, but probably unrelated death of pelicans and dolphins share a common theme. There are twin pressures, natural and man-made on marine life in Peru.

Vast schools of anchoveta form the basis of the marine food chain here. They're also highly valued for turning into fish meal. In recent decades they have been heavily overfished. After each El Nino-related die-off natural populations rebound. But in recent decades, they've taken longer, and recovered to lower levels than the El Nino before.

According to Professor Patricia Majluf, a marine biologist who recently resigned as Peru's deputy fisheries minister, it is time to overhaul of the environmental management of this globally important marine ecosystem.

"There is the need for a major clean-up and re-thinking of the system so that it is based on sustainability, rather than maximising extraction," she told Channel 4 News.

Vast schools of anchoveta form the basis of the marine food chain here. They're also highly valued for turning into fish meal. In recent decades they have been heavily overfished.

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