Thousands of Atlantic salmon swim in Cooke Aquaculture's 24 sea sites, penned areas where the company farms salmon, off the coast of Maine. Credit: Nick Sambides Jr. / BDN

State officials say Cooke Aquaculture did not violate the conditions of its permit when more than 100,000 fish died at one of the company’s salmon farms last month.

Maine Department of Environmental Protection investigated the incident, which occurred Aug. 16, and found that the dissolved oxygen levels in water outside the pens were above the minimum limits, and that the pens where the fish died were not overcrowded.

DEP also didn’t find any evidence of excessive fouling or waste on the pens’ submerged nets, the agency said Monday.

Though it is not required to report fish death events to DEP, 11 days after the fish died Cooke Aquaculture informed the state agency that nearly 116,000 salmon had died in its pens off Black Island, which lies between Swan’s Island and Mount Desert Island. Permitted finfish aquaculture pens have been operating at the site off Black Island since 1999, according to Maine Department of Marine Resources records.

“DEP did not identify any permit violations or violations of the Clean Water Act in relation to the reported fish kill,” DEP officials said.

Cooke’s operating permit for the Black Island site requires a minimum dissolved oxygen level of 6.0 milligrams per liter in the surrounding waters within 30 meters of the pens, in what is called a “mixing” zone. The lowest dissolved oxygen level measured by Cooke in the mixing zone for the months of July and August was 6.5 milligrams per liter on Aug. 10, which later rose to above 8.0 the following weekend, DEP said.

The state does not require Cooke to track or report dissolved oxygen levels inside the pens, but the company did report that its lowest recorded levels inside one of the pens was 4.9 milligrams per liter on Aug. 15 and 16, which is when the die-off was discovered.

Cooke, which operates roughly two dozen salmon farms along Maine’s eastern coast, has been cooperative in the DEP investigation, which has since been closed, the agency said.

The company has been fined in the past, both in Maine and in Canada, for violating rules and regulations about how it has to operate.

Officials with Cooke said Monday that the fish off Black Island died as a result of “uncommonly low oxygen levels” in the fish pens.

“Throughout this event, Cooke Aquaculture USA followed all responsible steps to safely remediate the unfortunate fish loss incident as quickly as possible with protecting the environment as our priority,” the company said. “We cooperated fully with the DEP during its subsequent investigation, and we accept their reports that no corrective actions are to be taken.”

Such mortalities are unfortunate and happen naturally on occasion in aquaculture farming, a spokesman for Cooke said last week. The fish that died were only roughly 10 percent of all the fish being cultivated at Black Island.

Bill Trotter

A news reporter in coastal Maine for more than 20 years, Bill Trotter writes about how the Atlantic Ocean and the state's iconic coastline help to shape the lives of coastal Maine residents and visitors....