SEABROOK, N.H. – When fisherman David Linney caught bluefin tuna in the Gulf of Maine in the 1990s, they were so oily they would slide around his boat deck when he tried to dress them, and it would take a long time to wash the oil off his hands.

Nowadays, most tuna have so little oil that Linney, who is 63 and from York, Maine, can clean his hands with one rinse. Tuna are a lot leaner than they were a decade and more ago, which could suggest a shift in the ecosystem.

“It’s very hard to catch a real butterball now, even late in the season,” Linney said.

It’s unclear exactly what has happened to the species, a top predator in the Gulf of Maine and the premium tuna for sushi lovers in Japan, said Molly Lutcavage, director of the Large Pelagics Research Lab at the University of New Hampshire.

But 14 years of records at a Seabrook fish auction house reveal that tuna are less abundant in the gulf, and they aren’t eating as well as they once did.

The decline, Lutcavage said, could signal changes that also may be affecting whales, seabirds and other fish that feed on the same forage fish, such has herring. Many tuna fishermen say they are seeing a lot fewer tuna in near-shore waters and also fewer whales and seabirds.

The research is based on the records kept by one man, Bob Campbell, manager of the Yankee Fisherman’s Cooperative in Seabrook.

Trained by the general manager of a Japanese auction house, Campbell has been recording the weight and date of tuna brought to the auction house, and grading each fish on freshness, oil content, color and shape.

The high oil content that makes tuna so tasty to Japanese palates – and brings the highest prices to fishermen – is the best indicator of the overall health of tuna.

Campbell’s records, which are kept in spiral-bound notebooks and date back to 1991, are valuable to scientists because they are so consistent, Lutcavage says. Between 1991 and 2004, Campbell graded 3,834 tuna.

Researchers who analyzed his records found that fish are arriving in the gulf in June in poorer condition than they did in the early 1990s, and that those caught later in the season have a lower oil content and slimmer body shapes.

The study shows a steady decline in oil content since 1991, with the steepest decline occurring between 1992 and 1995.

“What amazed all of us, the results were so consistent that the decline in every feature of condition was documented in every aspect of the quality of the fish,” Lutcavage said.

Campbell, who lives in Kennebunk, Maine, said he never knew his records could be helpful to scientists. The results, he said, confirm what he already knew.

“When I first started, at the end of September and October the fish were of really great quality, lots of fat and nice color,” he said. “That’s a thing of the past.”

The researchers have yet to publish the study, but they believe the findings are so significant that they decided to make the facts public before submitting the paper for publication, Lutcavage said.

Atlantic bluefin, which can grow to be close to 12 feet long and can weigh more than 1,500 pounds, come to the Gulf of Maine in late May to fatten up and depart for warmer waters by the end of October.

Tuna are caught in the Gulf of Maine primarily in near-shore waters by fishermen using harpoons and fishing rods. Farther offshore, fishermen also use nets.

When the Japanese economy was booming in the early 1990s, fishermen in New England were getting as much as $25 a pound for their tuna. Today, prices over $10 a pound are considered high.