Extended warm weather, lack of storms create 'unprecedented' conditions.

SANDWICH — Two weeks ago, lobstermen working off Scorton Creek started seeing something they had never experienced. Lobsters, in fact everything in their traps, were coming up dead.

Massachusetts Division of Marine Fisheries senior biologist Robert Glenn started fielding phone calls from puzzled fishermen Sept. 23. The fishermen were worried there might be something in the water that was killing the lobsters, fish, shellfish, even sea worms.

It turns out, it was something missing from the water: oxygen.

For the past two weeks, division researchers and scientists from the Center for Coastal Studies in Provincetown have boarded vessels and taken water samples, gathered temperature data at various depths and measured the amount of oxygen dissolved in the water. Preliminary results from testing on dozens of dead lobsters found nothing toxic in the water that could have killed them, and the focus was on a phenomenon that occurs every year — low oxygen in the layer of water along the ocean bottom, Glenn said.

“It’s typical to see that, but not below critical levels,” he said.

In his 25 years as a state scientist, Glenn said, he has never seen dissolved oxygen levels so low they couldn’t support life.

“What’s unprecedented is the moderate to large-scale reports of dead lobsters and fish in the traps,” Glenn said. The area with life-threatening low oxygen levels was largely offshore in 30 to 80 feet of water and stretched from approximately Scorton Creek to off the entrance to Barnstable Harbor.

“This is the first time I’ve heard of anything like this, and the fishermen who have been fishing in these waters for 50-plus years hadn’t seen this,” said Beth Casoni, executive director of the Massachusetts Lobstermen’s Association.

With the sunny warm weather that extended into fall, the heat was on for too long in Cape Cod Bay. Thirty to 80 feet down lies a layer of cold water. Denser than the warm water above, it remains there unless disturbed by wind and wave. As the summer progresses, the sun increases the temperature difference between the two layers, and the boundary becomes more defined. Known as a thermocline, it is the distinct temperature difference you feel when swimming and your hand penetrates beyond the warm surface layer to what seems shockingly cold water below.

Normally the onset of fall brings shorter days that lower the amount of radiational heat each day, slowly blurring the temperature and density difference between the layers. It also brings cool northern winds that chill the surface waters and, more important, mix the oxygenated upper layer with the bottom.

Without any major storms or cooling-off period, the bathwater temperatures of the bay’s surface water persisted, and the activities of the creatures living on the bottom used up the remaining oxygen. Although lobsters and fish can sense low oxygen levels and move out of the area, less mobile species essentially suffocate. Those caught in traps also suffer that fate regardless of their potential mobility.

“It’s hard to move when you’re in a lobster trap,” said Owen Nichols, director of marine fisheries research at the Center for Coastal Studies.

Follow Doug Fraser on Twitter: @dougfrasercct.