The surge of Atlantic salmon that returned to the Penobscot and other Maine rivers to spawn this year appears to be the result of improved fish survival in the ocean rather than any changes to the state’s stocking program, biologists said Thursday.

Exactly why salmon seem to be faring better during the life stage they spend at sea is still open to debate, however.

More than 2,100 Atlantic salmon were counted making their way upriver in the Penobscot earlier this year. That was the largest return since 1992 and more than double the previous year’s figure.

The number of adult sea-run salmon returning to Maine’s other rivers was still tiny by comparison. But in almost every river, biologists observed more salmon returning this year than the five-year average.

In the Saco River, for instance, 62 adult salmon returned in 2008, compared with 24 last year. Figures were also impressive on the Aroostook, where the 44 fish represented a welcome change from the five-year average of eight fish.

Rivers in Canada, Scotland and Ireland also saw larger returns this year, according to Joan Trial, a biologist with Maine’s Bureau of Sea-Run Fisheries and Habitat.

“This year was an exceptional year. I wish we knew why,” Trial told members of the state’s Atlantic Salmon Commission on Thursday.

Bureau staff had hoped that the increase in the Penobscot, at least, might be tied to a recent change in the way state and federal biologists stock salmon raised at two hatcheries operated by the U.S. Fish and Wildlife Service.

Before 2006, nearly all of the hatcheries’ juvenile salmon, known as “smolts,” were stocked above the Great Works dam in Old Town, meaning the fish had to navigate at least two dams en route to the ocean.

Since 2006, slightly less than one-third of the smolts were stocked below Great Works. But by studying the percentage of specially marked fish that return in subsequent years, the biologists could see that stocking smolts below Great Works did not appear to make much of a difference.

“That leads us to conclude that the increased run is because of increased ocean survival,” said Oliver Cox, a biologist with the bureau, part of the Department of Marine Resources.

While dams and river pollution have taken their toll on salmon over the decades, today ocean mortality is often cited as the biggest factor limiting the recovery of Atlantic salmon. Millions of young or juvenile salmon are stocked in the Penobscot and other Maine rivers every year — costing millions of dollars in state and federal money — but only a tiny fraction of those return to spawn.

It is unclear how, if at all, the single-year surge in adult salmon on the Penobscot will affect a federal proposal to designate the fish as an endangered species.

Salmon populations in eight smaller rivers or streams in the state already are protected as endangered species. The U.S. Fish and Wildlife Service and National Marine Fisheries Service have proposed expanding the designation to include the Penobscot, Kennebec and Androscoggin rivers.

Patrick Keliher, executive director of the Bureau of Sea-Run Fisheries and Habitat, said an endangered designation would prohibit the state from allowing another catch-and-release spring fishery for salmon on the Penobscot.

A lesser designation of “threatened,” as the state has proposed, offers more flexibility but would still make it “very difficult” to allow recreational fishing for salmon, Keliher said.

The federal agencies may make their decision before the 2009 fishery was slated to begin May 1. Keliher said the commission would have to decide how to proceed before spring.