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* Norway, Russia see cod boom despite global overfishing
* Other revivals off developed nations; crisis still for poor
* Global warming may help in north, causes damage elsewhere
By Environment Correspondent Alister Doyle
ABOARD HAUNES, Norwegian Sea, Feb 10 (Reuters) - It was hours before dawn on a heaving Arctic sea, and snow showers were making it hard for Kurt Ludvigsen to find his fishing buoys with the trawler's powerful searchlight.
But the 49-year-old Norwegian was less bothered by the conditions than by the large numbers of cod flailing in the nets he and his younger brother Trond winched aboard.
"It's paradoxical but we have too many fish this year," the older Ludvigsen said. "Prices have fallen 30 percent ... We're having to work far harder."
Just over six years ago, an article in the U.S. journal Science projected that all fish and seafood species, on current trends, would collapse by 2048.
A cod bonanza off north Norway and Russia and recovery of some fish stocks off developed nations from the United States to Australia have led many scientists to say the future for over-fished world stocks is a bit less bleak.
Stocks off developing nations -- from the Pacific to the Caribbean -- are still in sharp decline but the recoveries give hope that the problems are not irreversible.
"The outlook is improving relative to what we saw in 2006," said Boris Worm, a professor of biology at Dalhousie University in Canada who was the lead author of the 2006 study in Science.
"It's more than isolated examples - it's a substantial number" of successes, he said.
A lot is at stake. Fisheries, both marine and farmed, provide livelihoods for up to 820 million people, according to the U.N.'s Food and Agriculture Organization (FAO), which emphasises that globally, over-fishing is still on the rise.
Cod, the 11th most caught fish species in an FAO list led by the Peruvian anchovy, skipjack tuna and Atlantic herring, has had a mixed fate.
While a 1990s moratorium off eastern Canada is still in place and European Union quotas are unchanged this year, the quota off northern Norway and Russia is a record 1.02 million tonnes, up a third from 2012 and six times as high as in 1990.
Part of the reason is that global warming has expanded the cod's habitat northwards. And strict management of quotas by Oslo and Moscow have played a role, fisheries experts say.
Among other encouraging examples, fish landings off the United States rose to a 14-year-high in 2011, "thanks in part to rebuilding fish populations," according to the U.S. National Oceanic and Atmospheric Administration.
U.S. successes include Atlantic swordfish, summer flounder, New England scallops, Pacific lingcod and mid-Atlantic bluefish, the Washington-based Pew Environment Group said.
In September, another study in the journal Science said catches of the best-studied stocks off developed nations were shifting towards sustainable levels.
"We now know that we can make fisheries recover," said Christopher Costello, lead author of that study and a professor at the University of California, Santa Barbara.
"That sounds obvious but even 10 years ago many people would have disagreed, saying 'we've already decimated them to a point of no return'."
Many experts were now dropping a belief that over-fished stocks, like cod off Canada, can never revive. Closing fishing grounds, or cracking down on illegal catches, usually gives stocks a needed respite, he said.
That is much harder for developing nations, from the Philippines to Ecuador, to enforce, with the result that better conservation in one area may simply shift problems elsewhere.
The boom in cod stocks off Norway and the drop in prices, caused partly by recession in key importers Portugal and Spain, has undermined efforts to farm cod as an alternative to preserve wild stocks. Fresh cod now costs far less than farmed fish.
Most revivals in fish stocks are in areas where only one or two nations set the rules, like off the United States or Australia, where stocks of prawns and tuna have risen.
"And the biggest gap is still fixing the high seas -- half the world's surface," said Amanda Nickson, director for global tuna conservation at the Pew Environment Group who said most successes were in national waters.
Global warming, meanwhile, is bringing all kinds of threats.
Iceland unilaterally raised its mackerel quota in 2011, saying the fish had moved north due to warmer seas, and the European Union warned it might block imports in response.
Their spat has drawn comparisons with "cod wars" between Britain and Iceland in the 1950s and 1970s, although for 2013, Iceland has cut its quota by 15 percent.
One study last year said fish would shrink in coming decades because of a lack of oxygen in warming waters.
And acidification caused by rising levels of carbon dioxide in the atmosphere could make it hard for creatures to form protective shells, disrupting marine food chains.
So far, though, only only one semi-marine fish is known to have gone extinct, Worm said, the New Zealand grayling which lived in both rivers and the ocean.
For now, Norwegian fishermen are happy to enjoy their boom. Locally known as "skrei", the cod swim south to coastal areas to spawn in vast numbers from January to April.
"Cod is king of the economy in the north," said Svein Ove Haugland, deputy managing director of the fishermen's sales organisation, Rafisklag, in the Arctic city of Tromso. Once landed, many are hung out to dry or salted, preservation methods dating back at least to Viking times.
Nature can quickly shift from boom to bust. Female cod seek an exact blend of temperature, saltiness and water pressure to spawn - a sudden natural shift in currents can destroy their eggs, said Lise Mangseth of the Rafisklag.
Marine stocks are still badly understood, which explains why fisheries went from wild optimism in the 19th century to the depths of pessimism at the beginning of the 21st.
Thomas Huxley, a leading British biologist, said in 1883 that fish stocks such as cod, herring and mackerel were so plentiful as to be inexhaustible by trawlermen of the day.
"Any attempt to regulate these fisheries seems consequently...to be useless," he said in a speech often blamed for opening the way to unfettered exploitation of the seas.
Huxley, a friend of naturalist Charles Darwin and a strong defender of his theory of evolution, said that only river fish like salmon or shellfish such as oyster beds needed oversight.
He was disastrously wrong.
A U.N. Earth Summit in Johannesburg in 2002 agreed to restore world fisheries by 2015 - a goal set to be missed. And global stocks face ever more pressure, according to the U.N.'s FAO, as the world population rises above 7 billion.
The percentage of over-exploited marine stocks has risen to about 30 percent from 10 percent in 1974. Over the same years, the percentage of fully exploited stocks, where no more fish should be caught, has risen close to 60 percent from 50.
"If you look at these trends there is no good news," said Gabriella Bianchi, coordinator of the marine and inland fisheries service at the Rome-based FAO.
"Locally there are improvements but they are not enough to influence the global trends," she said.
In the latest attempt at broader action, the European parliament on Wednesday backed reforms to end decades of over-fishing and restore the region's sea stocks to healthy levels by 2020. The 27-member bloc says about 80 percent of Mediterranean stocks and 47 percent of its Atlantic stocks are over-fished.
One worry for fishermen in developed nations, hit by weak economic growth or recession, is that prices and demand will fall because consumers are turning to import cheaper white fish such as farmed pangasius from Vietnam.
"Conservation successes may be bought at the expense of displacing fishing to less well-regulated areas," Worm said.
In developing nations, the World Bank says granting groups of fishermen exclusive rights to catches in return for respecting "no take" zones, such as near coral reefs or mangroves that are nurseries for fish, can aid recovery.
Eight Pacific island nations -- including Nauru, Palau, Kiribati and the Solomon Islands -- for instance, have tough regulations for skipjack tuna fishing.
Norway's fishermen complain that many European consumers think all cod stocks are in trouble. The Norwegian Seafood Council wants to persuade people in Britain, for instance, to eat more of the plentiful Barents Sea cod in fish and chips.
"It's like shouting in a forest. No one's listening," said Ove Johansen, a cod expert at the Norway Seafood Council.
The Ludvigsen brothers haul two tonnes of cod onto the 42-ft (13-metre) Haunes before heading for home, with Trond cutting each fish behind the gills to bleed it to death to make the flesh, prized for its taste, the pure white that buyers prefer.
They get 23,000 crowns ($4,200) for the day's catch at a fish processing plant near their home port of Sommaroya. But costs are high to maintain a boat in the Arctic and the winter fishing season is short.
"It's been an okay day, an average one. If it doesn't get worse than this we can live with it," Kurt Ludvigsen said. ($1 = 5.4313 Norwegian crowns) (Editing by Philippa Fletcher)