THE PACIFIC – For millions of years, fish species in the Pacific and Atlantic oceans have stuck resolutely to where they belong, kept from venturing between oceans by the cold water of the Arctic.

But new research suggests a warming Arctic could soon see fish putting aside their differences and bridging this chilly divide. And this could have implications for native species and commercial fisheries, the researchers say.

Diminishing Arctic sea ice is perhaps the most iconic consequence of climate change. And there’s a good chance we’ll lose it in summer before too long if emissions stay high.

Scientists believe the gradual warming of the Arctic Ocean over the next century will weaken a natural barrier that has separated fish from the Atlantic and Pacific Oceans for millions of years, leading to a mixing of species that could make life difficult in fishing communities from Alaska to Norway.

A new study by scientists in Denmark combined current models of climate change, and the biological water temperature and food requirements for 520 fish species native to the two oceans.

The report forecast changes in the range of these fish in five-year increments from now until 2100, when the world’s oceans are expected to heat up globally by an average four degrees Celsius.

“There will be an interchange of the fish communities between those two oceans,” beginning as soon as 2050, said Mary Wisz, lead author on the report in Nature Climate Change and a senior ecosystem scientist at Aarhaus University in Denmark.

“We know from historical examples that this kind of interchange, when biotas have been separated over long evolutionary time scales, can have huge consequences.”

In this warmer future, fishermen based in Kodiak, Alaska, could be pulling up Atlantic cod, a prized species normally caught off New England and Northern Europe.

A similar change has already started off the coast of Greenland, where fishermen in the last five years have been catching larger numbers of Atlantic mackerel, which prefers more temperate water.

Wisz and colleagues say that by 2100, up to 41 species could enter the Pacific and 44 species could enter the Atlantic, through Arctic water passages over Canada or Russia.

This interchange will have ecological and economic consequences to ecosystems that at present contribute 39 per cent to global marine fish landings.

While some fishermen may benefit from the new catches, scientists warn that it’s hard to predict exactly what kind of fish will take over, and which will be driven away by the newcomers.

It’s also possible that several kinds of fish could compete for the same food source – smaller fish, marine shrimp or larvae, for example, leading to a big reshuffling of the existing marine food chain.

“Some species when they come together they get along,” said Peter Moller, curator of fishes at the Natural History Museum of Denmark and another author on the new report.

“But of course the Atlantic cod has the potential to become extremely numerous and dominating if it has the right conditions. There is speculation if it gets to a new place, it can be a real game-changer.”

Moller said the cod is an especially voracious predator of smaller fish, and could impact commercial landings of Alaska Pollock, for example.

Around three million tons of Alaska pollock are caught each year in the North Pacific from Alaska to northern Japan. Alaska pollock is the world’s second most important fish species in terms of total catch.

Jason Link, senior scientist for ecosystem management at the National Oceanic and Atmospheric Administration, agreed that the mixing of species will cause changes in the food web in both oceans, but it’s hard to predict exactly how it will shake out.

“Another issue not noted in this paper is what happens in the ecosystem that these fish move out of, do they remain there or do other species replace them from the south?” Link said via e-mail.

Another thorny issue is how to manage fishing boats who will likely be plying the rugged Arctic Ocean once commercial harvests become feasible.

“This work raises important ramifications for fishes in response to changes in sea ice,” Link said.

Wisz and Moller say their next task is to look at realistic scenarios of predators and prey in the new warmer Arctic ecosystem.

Sea ice in the Arctic has declined rapidly in the last few decades, in large part due to greenhouse gas emissions.

In September 2012, Arctic sea ice reached its lowest extent since satellite records began in 1979.

A new study in the journal Geophysical Research Letters suggests that before 2050, Arctic summers could be almost entirely ice-free.