OSLO - Tiny algae and a whale native to the Pacific have crossed a thawing Arctic Ocean in what may portend a marine invasion threatening Atlantic fish stocks, scientists said on Sunday.

The Pacific algae, absent from the North Atlantic for 800,000 years according to fossil records, apparently returned after climate change thawed sea ice and currents carried the microscopic plants across the Arctic Ocean, they said.

And a gray whale spotted in the Mediterranean in 2010 -- 300 years after the species was hunted to extinction in the Atlantic region -- is believed to have swum from the Pacific through newly ice-free waters in the Arctic Ocean in summertime.

"It's a Pandora's Box," said professor Chris Reid, from the Sir Alister Harvey Foundation for Ocean Science in Britain who said the algae had now drifted almost as far south as New York.

"We can expect more species to come through from the Pacific," he told Reuters. Upheavals to life in the seas have been documented by European scientists from 17 marine institutions in 10 nations in a project called CLAMER.

An influx of species could "be extremely damaging...for fisheries in the North Atlantic," Reid said. New arrivals would compete with established species such as cod or salmon.

"We have crossed a threshold," he said, with thawing ice linked to global warming stoked by use of fossil fuels from factories to cars. The algae were called Neodenticula seminae.

One-way traffic

It would be far harder for species to go from the Atlantic to the Pacific, against currents and winds, the scientists said.

The gray whale, identified from photographs taken off Spain and Israel in May 2010 as the same individual, was also believed to have swum across the Arctic from the eastern North Pacific.

"We now have two signals of organisms passing this open water in the Arctic," said Katja Philippart of the Royal Netherlands Institute for Sea Research, which leads CLAMER.

Chances that gray whales had somehow survived undetected in the Atlantic region since 1700 -- or that it had swum a far longer route such as around South America -- were far less plausible, she told Reuters.

Similarly, the scientists said it was almost impossible that the algae could have arrived another way, for instance in ballast water of ships via the Panama Canal. The algae were cold-lovers and would have died on a route through the tropics.

Pacific organisms last crossed to the Atlantic some two million years ago, according to fossil records.

"Then there was a major invasion...It completely changed the ecosystems of the North Atlantic," Reid said. The newly found algae had disappeared in the Atlantic about 800,000 years ago, coinciding with Ice Ages.

Among other shifts, CLAMER found that many species were moving north as waters warmed. "The number of fish species in the North Sea has increased from 60 to more than 80 in just 20 years," said Carlo Heip, director General of the Royal Netherlands Institute for Sea Research.

In the almost enclosed Mediterranean, species favouring cooler waters had nowhere to go. By 2060, CLAMER said a third of the sea's 75 species will be threatened and six will be extinct.