GLOUCESTER, Mass.-- Pete Libra is frustrated. The 40-year-old cod fisherman sees lots of fish in the ocean, and he wants to catch more. Fishing authorities see fewer, and want him to catch less.

"I'm not a scientist. But I see the fish," said Libra. His is the voice of many of the fishermen in Gloucester, the heart of a once-great fishing industry that powered fledgling America and underwrote New England's economy.

Many fishermen here feel threatened by a sweeping new set of fishing limits imposed this spring by authorities trying to rebuild fish stocks they say are depleted by overfishing and facing pressures that include climate change. Federal fishing regulators have traditionally reacted to falling stocks by putting additional curbs on fishing. But that approach may not work in the face of larger environmental changes such as global warming.

The chief fishing grounds for Massachusetts watermen are Gulf of Maine and the Georges Bank, the most westward of the famous Atlantic fishing banks off the North American coast. They are among the most famous and historically productive fishing grounds in the world; their collapse in the mid-1990s was equally historic, and the debate over how to manage the depleted stocks while nursing them back to health has been hotly contested ever since.

But the arguments are changing as scientists see more evidence of the coming impact of climate change on the Atlantic fisheries. Both the Gulf of Maine and the Georges Bank sit at the southern edge of the cod's preferred range. Fishermen have adapted to stock changes over the years, but their options may be dwindling.

"The question of the influence of fishing and the influence of the environment is tangled up," said Brian Rothschild, a professor of marine science at the University of Massachusetts, Dartmouth, who has studied aquatic stocks for more than five decades.

"There is no question that as some of the environment changes occur, some of the fish stocks are going to change," he said. "We don't know enough about it to know what's going to increase and what's going to decrease."

That gives a gloomy uncertainty to the future for New England commercial fishermen. There were an estimated 2,000 working fishermen in Gloucester in the 1940s; 50 years later the number had dropped to 400, according to a 1997 study. Fishermen glumly expect their numbers to continue to shrink.

The warming waters may hasten that. A 2007 study by the National Oceanic and Atmospheric Administration looked at codfish catch records over four decades. It concluded what fishermen who know this cold-loving fish would have predicted: As the bottom water temperature increased, the probability of catching a cod decreased.

Last year, a federal effort to coordinate research, the U.S. Global Change Research Program, found ocean warming already was forcing a migration of some species.

"The northward shifts we have seen in the area are due in part to climate change. We are starting to see some of the effects of global climate change in our area," said Janet Nye, a NOAA researcher working out of Woods Hole, Mass. She studied historical fish records and found that of 36 northwest Atlantic species, almost half had moved northward in 40 years as water temperatures warmed. She predicted the traditional stocks of cold-water fish are likely to be replaced by croaker and red hake, fish normally found farther south.

Many fishermen switched to lobster as winter flounder, a cold-water fish once abundant in fishing boat holds, declined. But lobster stocks are stressed in some areas now. Biologists on a multi-state Fisheries Commission committee have found that warmer waters, disease and fishing have depleted lobster stocks, and they recently recommended a five-year ban on lobstering from Cape Cod to Virginia.

"It's regulation after regulation after regulation," complained Michael Dearborn, 66, a commercial fisherman since 1969. "They treat the industry like a centipede, cutting off one leg at a time, snip, snip, snip, until it starves."

"Some of the fish stocks are showing improvement," acknowledged Maggie Mooney-Seus, a spokeswoman for NOAA, the agency responsible for enforcing the limits. "But there is too much fishing pressure on a lot of them. Right now, fishermen are going to have to have cuts."

Quotas on the fishermen remain the chief mechanism to react to dwindling stocks. Those quotas remain largely a function of numbers of fish caught versus a sampled estimate of the numbers of fish in any one stock; there is little room for forward calculations about how climate change will skew the future.

"I don't think the fisheries managers are equipped to quantify the effects of global warming at this point," said Peter Baker, manager of the New England Fisheries Campaign for the non-profit Pew Environment Group.  "They have no tools.

"One of the grim realities of global warming is that it is bringing change to fisheries. There are going to be regime changes in the oceans and management is going to have to adapt to that," he said.

Greg Walinski believes he has seen first-hand the workings of warmer waters on fish stock. The 53-year-old Cape Cod fisherman used to hunt for large bluefin tuna.

"In the '80s and '90s we would get 60 to 80 giant bluefin in a season," he said. "But we started to see less and less. It got to a point where it wasn't even worth going out. Most of the big fish are up in Canada," he said. "We get the little bluefin that used to be further south."

He switched to cod, but in what seems to be a repeat of the pattern, Walinski said he finds himself chasing the fish further and further out. He now travels 120 miles in a 35-foot boat - an arduous and somewhat dangerous commute - to reach Georges Bank for codfish.

"I don't think we understand the impacts of climate change very well at all," said Paul Parker, who works with the Cape Cod Commercial Hook Fishermen's Association. "We are struggling to understand it all. It's difficult to discern what causes what."

For example, the fishermen say there is a bumper crop of dogfish shark, which strip their hooks and gobble up groundfish. It is tough, they say, to sort out whether fish populations are hit harder by human fishing, the sharks' appetite, climate change or some other environmental shift.

Whatever the cause, there is a need for closer husbandry of the fish stocks, Parker said. The old system put daily limits on catches, and fishermen "were throwing overboard thousands of pounds of dead and wasted fish every day" when more fish came up in their nets than allowed, he said.

The new regulations attempt to give fishermen a share of the overall stock quotas, to limit the harvest in sync with the stock's rise or fall.

But Gloucester fishermen are unconvinced of the need for change and unhappy with the new regulations.

"This is the end. It's a slow, agonizing death," said a 55-year-old crewman on a huge drag-net fishing boat docked in Gloucester.

"The fish are steady now," added Joey Ciaramitaro, 42, who supplies bait and buys the catch from boats in Gloucester. With the new regulations, "they've gone too far."

NOAA's Mooney-Seus said of the 20 species of groundfish covered by regulations for the northeast seaboard, 13 still are listed as underpopulated and overfished. Haddock and redfish have rebounded, and there has been a spike in numbers of cod in the Gulf of Maine, according to NOAA studies.

But the regulators say they have seen little evidence of a similar rebound in cod on the George's Bank, and some other cold-water species, like winter flounder and pollock, remain low.

Libra, back in Gloucester, has been on the water since he finished high school, when he and a buddy fixed up a decrepit old cabin cruiser they got for $1. "This is what I always wanted to do," he said, pausing while working on the engine of his current boat, his fourth.

He is not sure there is a future in fishing for another generation. He faces a slew of regulations, uncertainty over fish stocks and the prospects of climate change. But he vows to keep going as long as he can.

"I don't know anything else to do."

Doug Struck covered climate change issues for The Washington Post. He is now a freelance writer and associate chair of journalism at Emerson College in Boston. is a nonprofit news service that covers climate change

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.