The phenomenon is highly localized, though. At nearby pens off Black Island, near Gotts Island, the fish have been doing just fine, he said.

"We've had what's called super-chilled water," he said. "We haven't seen water temperatures this cold for 40 years."

The amount of mortality is hard to quantify, he said, because it's been hard for divers to get into the pens to clear out the dead fish. The presence of divers would create more stress for the already struggling creatures.

Some of the fish have been pumped out and taken to a rendering plant in New Brunswick, where they're turned into fish meal.

The last time fish farmers saw super-chilled waters was in 1993.

The water temperature has been down to minus 1 degree Celsius. Sea water freezes at minus 1.8 degree Celsius, or 28.7 degrees Fahrenheit. Fish will die at about 30.6 degrees Fahrenheit.

The Swans Island pens typically have 80,000 to 100,000 salmon ready for market, at 12 to 15 pounds, at this time of year. In fact, pen operators had been in the process of harvesting them when the temperature hit new lows.

Super-chill has been a problem elsewhere along the coast, although not in Cobscook Bay, where most salmon farms are located, said Department of Marine Resources Aquaculture Coordinator Andrew Fisk.

Farms in Blue Hill Bay and Pleasant Bay have experienced substantial losses over the past few weeks, he said.

"Some farms have been completely cleaned out," he said. "We're talking tenth-of-a-degree changes that can make a difference."

Some facilities can see a one-degree difference from one end of their site to the other, depending on circulation patterns and water depths, he said.

Last winter, the water was relatively warm, never getting below 4 or 5 degrees Celsius.

This winter, "In some places, guys can walk across the harbor," he noted.

Farm operations have been tricky, he said, due to the extreme cold and the need not to disturb the already stressed fish. Some farmers have had trouble even getting out to their pens, because their boats have been iced in.

When it's this cold, Mr. Fisk said, fish farmers try to do as little as possible when it comes to disturbing the fish, which develop crystals in their cells and are extremely fragile.

"So they don't want to jostle the fish," he said.

The dead fish are not marketable for human consumption, he said, and are being sent for rendering in New Brunswick or to licensed composting sites in Maine.