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So you want to know: Why is dissolved oxygen important?

First published in Landings, October, 2013.

Over the last few months, Maine Lobstermen’s Community Alliance (MLCA) programs coordinator Annie Tselikis has been working with lobstermen at the docks on quality handling practices. One of the most important things to keep lobsters healthy is to provide them with enough oxygen. “A lot of fishermen are installing very basic manifolds to distribute oxygen throughout their tanks, and I hear of more and more buying stations installing aeration and some buying stations in high volume areas have invested in tank systems to keep their crated lobsters in ideal conditions before shipping,” she said.

Oxygen is essential to all life. Even life in the ocean requires oxygen to survive, though it is in a slightly different form than the oxygen we breathe on land. Ocean dwelling animals breathe oxygen that is dissolved in the water, referred to as dissolved oxygen (DO). Fish, lobsters, crabs, worms and other marine animals pull oxygen from the water as it passes over their gills.

Dr. Ian Bricknell, director of the Aquaculture Research Institute at the University of Maine in Orono, said DO is very important biologically. “Reduced amounts of DO can lead to mass mortalities, like those seen in the Gulf of Mexico,” he said, referring to large numbers of fish and other animals that wash up on beaches from an area dubbed the Dead Zone. “The animals suffocate when there isn’t enough DO. The water becomes what is called anoxic.”

Bricknell said the Gulf of Maine is lucky that it has such cold waters because it is able to hold more oxygen. “Cold water holds more DO than warm water,” he explained. “If the Gulf of Maine were to warm up, it would only hold seven or eight or nine milligrams of oxygen per liter of water.” He said the Gulf typically holds around 14 or 15 milligrams of oxygen per liter of water at its current temperature.

But temperature isn’t the only thing that affects oxygen levels, warned Bricknell. He said algal blooms use up dissolve oxygen leaving little to none for animals. “Decreased DO levels leave animals stressed and vulnerable to disease.”

“Lobsters are happy at 10 milligrams per liter dissolved oxygen,” Bricknell explained. “That occurs around 10°C or 50°F. When DO decreases to 6 or 7 milligrams per liter, that’s when lobsters start to be in trouble.” Bricknell said that distressed lobsters are easy to spot in a tank. “When stressed, the lobster will move to the surface, stand upright, and move around a lot to try and find more oxygen.”

As Tselikis has traveled around the coast looking at holding systems and testing DO levels, one recent visit stood out to her. Tselikis visited the Cranberry Isles Co-op in September after visiting many other buying stations earlier in the month. “I was really impressed with their system and the general handling practices by fishermen and the crew who work with Co-op manager Mark Nighman on the wharf. They take great pride in their product and are doing everything they can to be innovative and proactive with the current market conditions.”

The Cranberry Isles Co-op has a pump system in place that keeps water and air circulating around crates of lobsters. Nighman said he prefers his Pacer Pump system to a regular aeration system for two reasons. “Most have pumps on the bottom and bring up dirt. I don’t think the dirty water is very good for the lobsters,” he said. “Our system is suspended three feet below the surface of the water.” Since the Pacer pump system intake is below the float, it moves the cold, deep water up. “When there is a lot of rain and no wind, that fresh water sits on the surface. Our system keeps the circulation rolling; it mixes up the water by bringing the salt water up,” Nighman explained.

The Co-op’s system has a three-inch water pump that picks water up below the surface. The pump is then reduced to two inches, which increases the volume and speed of the pumped water. Nighman said the water is brought back up to the surface and picks up air from small holes in the line. That air, mixed with water, is then shot back down below the lobster crates.

“We’ve had this system in place for four or five years now,” Nighman said. “I actually stole the idea from Timmy Harper at Southwest Lobster. Before that, we didn’t have any kind of aeration system.” Nighman said that it was the increased landings that pushed the co-op to install a circulation system. “We can hold 150 crates of lobsters, but the lobsters were using up all of the oxygen and were dying.” He said the increased circulation and DO that surrounds the crates now has resulted in a much higher quality product.

Bricknell noted that it isn’t difficult to provide DO to lobster. “A simple aquarium aerator even works,” he said, though noting it probably isn’t practical for a large number of lobsters. “The systems don’t have to be fancy or expensive,” he said. In fact, Nighman said the Co-ops Pacer Pump system didn’t cost more than a few thousand dollars. “And we were able to set it all up ourselves, which saved us money.”

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