Some also believe the seals' negative impact on fishing is overstated. Brian Sharp, the manager of marine mammal rescue for the Cape Cod-based International Fund for Animal Welfare, said gray seals feed mostly on fish species of little commercial value, like sand lance.
Others in the commercial fishing industry don't see seals as a threat. Lobstermen off Rockland, Maine, where gray seals are often spotted, say the seals and fishery coexist with little strife.
"Culls of gray seals have not been shown to increase fish populations. It's not that simple," Sharp said. "What we're seeing is a normal growth curve of seals repopulating an area."
The gray seals, also called horsehead seals, can grow to more than 10 feet long and inhabit both sides of the North Atlantic Ocean. They are sometimes found in the same areas as their smaller cousins, harbor seals.
Encounters with humans frequently don't end well for the seals, Sharp said. They sometimes become entangled in fishing gear, and six of them were illegally shot and killed on the southern ridge of Cape Cod in 2011, he said.
For now, the seal population is flourishing, and its ability to sustain seal watch businesses off Massachusetts and Maine is evidence that it can have an economic benefit, said Gordon Waring, fishery research biologist at the Northeast Fisheries Science Center in Woods Hole, Massachusetts.
"Seals are just another large marine predator, and they are part of the diversity of the marine environment," he said. "And they are able to thrive and recover."