Last month, lobstermen in Cape Cod Bay hauled up something disturbing. In one section of the bay, all of their traps were full of dead lobsters. Research biologists went to work trying to solve the mystery, and what they found suggests we may see more of this as the climate changes.

When the fishermen first started pulling up traps full of dead lobsters, their first call was to Beth Casoni of the Massachusetts Lobstermen's Association.

"As you can imagine, they were concerned — greatly concerned — because they didn't know how they died or why they died," Casoni said.

Over the next five days or so, she got more calls about dead animals in the traps.

"And it wasn't just lobsters,” she said. “It was skate and flounder and ling, which is an eel."

Casoni called the state Division of Marine Fisheries, which sent divers out to take a look at the seafloor in the area.

"The fishermen were fearful that there would have been a mass die-off and the bottom would be littered with carnage,” Casoni said. “And the division was happy to report that they did not see any mass die-off in the area."

But what was killing everything in the traps?

"I don't think any of us have heard reports of that before, at least not like that, where we had multiple fishermen all calling the same day, saying something's going on," said Steve Wilcox, one of the Massachusetts Division of Marine Fisheries' biologists assigned to the case.

On a breezy morning several weeks after the dead lobsters were first reported, Wilcox and biologist Alex Boeri piloted a 21-foot motorboat through choppy water on their way out to the several-mile area where the dead lobsters were found.

Steve Wilcox
State marine fisheries biologist Steve Wilcox pilots a research boat to the affected area.
Craig LeMoult WGBH News

Initially, some of the lobstermen worried recent aerial spraying of pesticides was to blame. But right away, an analysis of the water quality at the bottom of the affected area pointed to another cause.

"There were really, really low levels of dissolved oxygen in the areas where the dead lobsters were reported," Wilcox said.

Dangerously low oxygen in a water body is called hypoxia.

Dissolved oxygen is measured in milligrams per liter. "So zero to two milligrams per liter is considered lethal for lobsters," Boeri said.

In much of affected area, they found levels below two, or even below one.

"... We've never seen such low levels of oxygen in the system. This is a new one for us."
Owen Nichols, director of marine fisheries research at the Center for Coastal Studies

Casoni described this a little differently than the scientists: "I just tell people it's like that 1970s horror movie 'The Blob' was rolling around the ocean bottom floor and basically, anything that could not vacate that area died," she said.

Fish and other critters could swim or scamper out of the area. But the lobsters and other animals in the traps couldn't escape the low-oxygen blob.

As the boat rocked up and down on the waves, Boeri put a device that measures water quality overboard and lowered it to the sea floor on a long cable. The dissolved oxygen measured out at 7.89 milligrams per liter — significantly higher than it was a few weeks ago.

"It's looking a lot better," Boeri said.

But why did it happen in the first place?

Alex Boeri
State marine fisheries biologist Alex Boeri lowers a sensor into Cape Cod Bay to measure dissolved oxygen along the seafloor
Craig LeMoult WGBH News

Owen Nichols, the director of marine fisheries research at the Center for Coastal Studies, said unusually warm water temperatures this summer caused a stratification of the water.

"What stratification means is you essentially have layers of water,” Nichols said. “In Cape Cod Bay, you'll have surface waters that are warmer. The bottom waters that are colder."

That cold water gets trapped at the bottom, and its oxygen gets depleted in a few ways.

"Animals breathing, anything that's down there that's breaking down or decomposing,” he said. “All of those things will pull oxygen out of the water."

And the stratification means it can't mix with the surface water to get more oxygen.

"Now, that's to a degree to be expected, but we've never seen such low levels of oxygen in the system,” Nichols said. “This is a new one for us."

Nichols said more research is necessary to know for sure if this just was an anomaly. But with climate change already underway and water temperatures warming rapidly, he's worried this might be a sign of things to come.

"So it's certainly an area where we do have concern that we could see this phenomenon more," he said.

For now, the lobstermen's association is lobbying the state to pay for oxygen sensors at the bottom of Cape Cod Bay. So if it happens again next year, they'll know not to put their traps there.