CASCO – Eliot Stanley caught a 13-inch salmon in Sebago Lake on Saturday. But he didn’t even know it because it was inside a 17 1/2-pound, 41-inch northern pike that he hooked and which is thought to be the biggest pike ever caught in the lake.

The sheer size of the fish is raising concerns that northern pike – a voracious, toothy fish that eats everything from other fish and turtles to ducks and baby loons – are spreading and threatening the lake’s famed landlocked salmon fishery.

Sebago Lake is a historic destination for landlocked salmon. The northern pike was first discovered in the lake in 2003 after being introduced illegally.

Biologists say the northern pike population has grown to the point where the fish can’t be removed from the 28,770-acre lake, the state’s second-largest. Some people are fearful the fish could hurt the region’s tourism industry and the lake’s reputation as a fishing destination.

“To put pike in this lake, it’s terrible,” said Carroll Cutting, who owns Jordan’s Store in Sebago. “It’s irreversible, and it’s just going to get worse as time goes on.”

Introducing a northern pike in Sebago Lake is “like putting a piranha into a neighborhood swimming pool,” said Don Allen, president of the Sebago Lakes Anglers Association.

After Stanley hooked the pike from his dock, he fought the aggressive fish for 30 minutes before landing it with a net. The net broke, but he managed to get the fish onto his dock and into another net before carrying it to shore.

When he gutted the fish Tuesday, he found the salmon inside.

“I’ve never dealt with such a powerful fish, and I’ve fished in the Gulf of Mexico,” Stanley said.

State fishery officials two years ago declared open season on the northern pike, encouraging fishermen to catch and keep the fish to help keep the population down.

It now appears that the fish is at the top of the food chain in Sebago Lake, so landlocked salmon have to compete for smelts, said Francis Brautigam, a regional fisheries biologist.

“There is nothing you can really do in a lake the size of Sebago, especially when they start spawning. [Pike] are here for the long haul,” said Brautigam.

In the 1980s, as many as 52,000 fishermen a year came to fish for landlocked salmon in Sebago, Brautigam said. Now, only 16,000 to 20,000 come as salmon has declined and other fish such as trout have been introduced and proliferated.

Still, the salmon population had been showing signs of rebounding.

After cutting back on the number of salmon stocked in the lake and transferring smelt eggs – the salmon’s food of choice – into Sebago to improve the forage, biologists have found salmon in the lake that are fatter than they were four years ago.