SCARBOROUGH — Maine mussels are losing their muscle.

The state’s blue mussels are loved by seafood fans, but the size of the annual harvest has dipped in recent years, bottoming out at a 40-year low in 2016. Harvesters collected less than 1.8 million pounds of mussel meat in 2016, the lowest total since 1976.

The state’s mussel harvesters exceeded 6 million pounds three times in the 1980s and 1990s, and routinely topped 3 million pounds until 10 years ago.

The decline started by the early 2000s. Scientists and members of the industry blame a variety of possible factors, but a solution has proven elusive.

Some harvesters have stopped altogether because there simply aren’t as many mussels, said Carter Newell, a marine biologist and founder of Pemaquid Mussel Farms along the Maine coast.

“Some areas like Casco Bay, you just don’t see many mussels at all,” he said.

One factor in the decline is shellfish poison events that necessitate harvesting closures, said Jeff Nichols, a spokesman for the state Department of Marine Resources.

Such an event last month led the state to shut down shellfish harvesting along parts of its eastern coast because of a marine algae bloom that can carry a neurotoxin. About 58,000 pounds of mussels had to be destroyed because of the biotoxin.

Other possible factors in mussels’ decline include the increasing acidity of the ocean, the growing population of invasive green crabs, which eat shellfish, warming waters and the impact of human harvesting, said Susie Arnold, a marine scientist with the Island Institute in Rockland.

Mussels are important for the marine environment, in addition to being economically valuable, she said.

“Any shellfish that’s habitat is the seafloor, it creates habitat and structure for other important marine organisms,” Arnold said. “And mussels filter the water and improve water quality and clarity.”

Mussels can be farmed or harvested in the wild, and are produced both ways in Maine. About 50 fishermen are licensed to harvest wild mussels in the state.

In 2015, Massachusetts surpassed Maine as the biggest blue mussel-producing state in the U.S. They are also harvested in Washington state and in smaller amounts elsewhere along the East Coast.

Mussels remain abundant in stores and restaurants, in part because they are farmed extensively in countries such as Canada and New Zealand, which are big importers to the U.S. And local mussels remain a fixture in Maine seafood markets.

But as local supplies diminish, prices may creep up. Harvesters received 23 cents per pound for Maine mussels at the dock last year – the highest price in state history, according to records that go back to 1950. Such an increase will likely eventually be borne by consumers.

Consumer demand is remaining strong, said Ralph Smith, owner of Moosabec Mussels, a harvester in Jonesport.

“There’s a good demand for it and it’s a very quality product,” he said.