Atlantic salmon are returning to rivers in Maine and elsewhere along the North Atlantic this season in numbers not seen in years, suggesting to biologists and conservation groups that ocean conditions for the famed sportfish are improving after decades of decline.

On the Penobscot River, biologists have counted more than 3,100 salmon at the Veazie dam fish trap so far — more than double the number that returned last year and the highest spawning levels since 1986.

The Narraguagas River in Down East Maine, meanwhile, has recorded 186 fish so far this year compared with 76 in 2010. And biologists working the Kennebec River have counted 63 salmon, a figure still abysmally low given the river’s history but a definite improvement over last year’s tally of just seven fish.

“And it’s not just Maine rivers,” said Oliver Cox, a biologist at the Maine Department of Marine Resources. “It is up and down the North Atlantic into Canada and Nova Scotia.”

The Merrimack River, which flows through Massachusetts and New Hampshire, is experiencing the largest salmon run in at least 30 years, with 400-plus fish returning so far. Organizations in Canada — the last stronghold for Atlantic salmon in North America — are seeing larger and more plentiful fish in some rivers. And news agencies in Ireland reported this week that salmon are spawning in Dublin’s Tolka River for the first time in a century.

So what, exactly, led to this year’s unusual spawning runs? And could they signal a recovery for a fish once abundant throughout the North Atlantic?

Observers said higher at-sea survival rates are the likely answer to the first question, although determining what has changed in the ocean will take more study. As for the latter, current figures are encouraging but do not signal a clear shift for a species that would likely be extinct in the U.S. were it not for aggressive state and federal intervention.

“We need a lot of good years in a row as opposed to a few good years in a row” to begin celebrating, said Andy Goode, vice president for U.S. operations with the Atlantic Salmon Federation, a nonprofit conservation group.

Atlantic salmon are listed as an endangered species in Maine and are, therefore, protected by a myriad of laws and regulations. But biologists and other observers said the most likely reason for the sudden uptick in salmon returns can be found in the ocean environment, especially in the waters around Greenland.

After leaving their rivers of birth as juveniles, most Atlantic salmon make their way to Greenland or the Labrador Sea where they feed and grow. Adult fish that return to spawn have spent at least one and often two or more winters in the waters off of Greenland or somewhere in between.

Commercial fishing for wild Atlantic salmon has been largely halted (the Atlantic salmon enjoyed by consumers is almost exclusively farm-raised). Additionally, many government agencies and dam owners — including those in Maine — have spent large sums building fish bypasses or lifts to allow salmon to get beyond dams.

Yet salmon spawning runs in the U.S. have remained flat or nonexistent, suggesting that predators, lack of food, temperature changes or other factors in the ocean were killing juvenile and adult fish.

“Slight improvements in marine survival have resulted in more salmon arriving to feed in the Labrador Sea and Greenland and returning home to spawn,” Patricia Kurkul, Northeast regional administrator for the National Marine Fisheries Service, said in written statement.

“Hopefully we will see a large number of returns for several years,” Patrick Keliher, acting commissioner of the Maine Department of Marine Resources, said in a statement. “The next step is to review our monitoring data to see what management actions, if any, have played a roll in the increased returns. Given that many locations are experiencing high returns, an increase in marine survival is most likely the driving factor.”

Salmon watchers have been encouraged this year by not only the number of salmon returning to spawn but by the health and size of the fish.

Cox, the DMR biologist who works with Penobscot salmon, said teams tending the Veazie trap started seeing larger fish early on. Many of the fish taken to the federal fish hatchery to serve as broodstock were in the 10- to 12-pound range, he said.

Last year, just over 1,300 salmon returned to the Penobscot to spawn. That was down from nearly 2,000 the year before and more than 2,100 in 2008. But this is the first year since 1990 that more than 3,000 fish have returned.

John Burrows with the Atlantic Salmon Federation in Maine and others would like to see strong returns for four or five years — an entire generation for Atlantic salmon — before declaring a positive trend.

But Burrows optimistically pointed out that, after accounting for the fish taken to the federal hatchery for breeding, more than 2,000 fish were released upstream in the Penobscot this year.

“So there are close to 2,500 salmon swimming around in the river,” he said. “We have not had that many fish in the river in decades and decades.”