Deadly Casco Bay algae bloom dissipating, impacts still being assessed
Algae Bloom in Casco Bay Triggers Shellfish Harvest Closure and Ongoing Recovery Efforts
Brunswick, Maine – A bloom of the phytoplankton Karenia mikimotoi that began in Casco Bay in early September has finally begun to dissipate, but its effects on local shellfish populations and the fishing industry are still being assessed. The Maine Department of Marine Resources (DMR) and local scientists are monitoring the situation as the bloom’s decline is uneven across the region.
The bloom, which produces a brown or nearly black discoloration of the water, is not toxic to humans but it exerts severe stress on shellfish. “The phytoplankton cell counts are falling in inner Casco Bay,” said a spokesperson for Friends of Casco Bay in an October 9 update. “However, low dissolved‑oxygen levels are still suffocating shellfish in some areas.” The oxygen depletion has been a key factor in the deaths of soft‑shell clams and worms that are prized by local harvesters.
In response to the bloom, the DMR shut down shellfish harvesting in two bays last month. “We closed Maquoit Bay in Brunswick and the eastern side of Middle Bay in Harpswell because clam diggers reported dramatic die‑offs,” the department said in an email sent on Tuesday. The ban was lifted on October 8 as satellite images indicated the brown tide was receding, and the water began to look less murky.
Even with the ban lifted, the DMR warned that the bloom’s legacy remains. “Satellite imagery has shown an overall declining trend in both near‑shore and offshore bloom activity over the past week, but elevated cell counts are still present in some areas of Muscongus and Penobscot bays,” the department noted in a news release posted the following day. The agency is urging members of the fishing industry to report any signs of low oxygen, water discoloration, or foul odors, which could signal lingering problems.
Brunswick Coastal Resource Manager Dan Devereaux, who has been monitoring the bloom, said in an email on October 9 that it will take many months to fully understand both the immediate and longer‑term impacts on wild and farmed shellfish crops. “I think we are through the worst of it and are headed toward being on the mend. The mending portion will take time,” Devereaux added. He noted that oxygen levels are returning to normal in the mudflats and that his team has begun to find surviving soft‑shell clams and worms, although at densities far lower than before the bloom.
Soft‑shell clam harvesters, who have already experienced significant losses, warned that the financial blow could be substantial. “The lasting financial impact of the bloom will be substantial,” one clam digger told local media last month. The industry’s concerns are compounded by the fact that K. mikimotoi can reduce the overall yield of a harvest and increase the cost of post‑harvest processing, as dead clams can contaminate live stock.
The DMR is continuing to track the bloom’s progress and will update the public on its website. “We will share updates on the situation as new information becomes available,” the agency said. The department also noted that the algae’s appearance ranges from brown to almost black, depending on density, and urged fishermen to notify authorities if they observe any of these conditions.
As the region moves past the peak of the bloom, local scientists and industry stakeholders will need to collaborate to ensure the health of Maine’s shellfish populations. The long‑term goal is to restore both wild and farmed shellfish stocks to pre‑bloom levels, a process that Devereaux says will “take time.”
The outbreak underscores the vulnerability of Maine’s shellfish industry to environmental changes. While the bloom has dissipated in many parts of Casco Bay, the lingering effects in neighboring bays serve as a reminder that monitoring and rapid response remain critical to protecting the state’s valuable marine resources.
