DMR Updates on Karenia Mikimotoi bloom

Below are updates DMR has issued to Maine's shellfish industry and interested parties regarding the recent bloom of the phytoplankton Karenia Mikimotoi. Announcements are arranged chronologically with the most recent listed first.

It is important to note that this species of phytoplankton is not toxic to humans. Shellfish may still be harvested and sold during the bloom by aquaculturists, harvesters and dealers who are always required to meet rigorous regulatory standards that ensure the product that makes it to market is safe and wholesome for human consumption.

September 18, 2026

The Maine Department of Marine Resources announces the adoption of the following emergency rule:

Chapter 9: Harvester: Shellstock Harvesting, Handling and Sanitation [Targeted Closures]

CONCISE SUMMARY: The Commissioner adopts this emergency rulemaking to establish targeted closures in Casco Bay to include the entire intertidal growing area of Maquoit Bay in the town of Brunswick and the eastern side of Middle Bay in the town of Harpswell. The Department is concerned that continued harvesting during the phytoplankton bloom (Karenia mikimotoi) in these areas will cause additional stress on the shellfish resource, as well as increased mortality. The Commissioner finds an immediate closure is necessary to protect and conserve the shellfish resource in these harvest areas from unusual damage. For these reasons, the Commissioner hereby adopts an emergency closure of shellfish harvesting in these areas as authorized by 12 M.R.S. §6171(3)(A).

EFFECTIVE DATE: September 19, 2026

The adopted regulation is available at: https://www.maine.gov/dmr/rules-enforcement/regulations-rules.

A copy of the complete filing is available at: https://www.maine.gov/dmr/rules-enforcement/regulations-rules/emergency-regulations.

September 16, 2026

DMR Announces the Following:

REMINDER: the Karenia mikimotoi bloom is not a risk to human health either from recreational or work-related skin contact or from consumption of shellfish or other marine organisms.

The Karenia mikimotoi algal bloom centered in Casco Bay is now visible from the Maine/New Hampshire border to Penobscot Bay. Outside of the Casco Bay area, the bloom is largely offshore or in the middle of large bays without reported marine organism mortalities. Intertidal impacts are still predominantly only occurring in Casco Bay now having been observed at Maquoit Bay (Brunswick), Middle Bay (Brunswick/Harpswell) and Broad Cove (Cumberland). This week, cell counts have been generally trending downward in Casco Bay and along with other indicators such as observable decomposition on intertidal flats, suggest that the bloom may be past its peak. Algal blooms are patchy in their distribution and do not impact every waterbody or shoreline the same way. In locations where dissolved oxygen levels were taken, they show a generally stable pattern in Casco Bay with a few locations showing reduced levels. Dissolved oxygen levels may decrease creating anoxic conditions in areas where the bloom decomposes, resulting in impacts to benthic organisms. Similar to impacts from the active bloom, anoxic conditions created by decomposing blooms are often patchy in nature. Phytoplankton and dissolved oxygen testing will continue until the bloom has ended. Samples of marine organisms are actively being collected for examination by the DMR fisheries pathologist and archived for later testing.

Please visit the DMR website for notices of biotoxin and bacterial shellfish closures.

September 11, 2026

DMR Announces the Following:

Additional sampling efforts and environmental observations have been completed by DMR and other organizations during the week. The Karenia mikimotoi bloom impacting Casco Bay is persisting and elevated cell counts and water discoloration is known to be present in areas between Crescent Beach, Cape Elizabeth and the New Meadows River. Bigelow Laboratory for Ocean Sciences also completed a genetic analysis confirming the species identification. Cell counts vary from site to site with a maximum estimated cell count of 5,000,000 cells/L in Maquoit Bay. Outside organizations have reported to DMR cell counts as high as 12,000,000 cells/L. These cell count ranges are relatively similar in scale to the 2017 Karenia mikimotoi bloom. Industry members have observed marine organism impacts on the mud flats in Maquoit Bay and Middle Bay. Soft-shell clam samples are under analysis to further understand the cause of the observed stress and/or mortality. It should be noted that algal blooms such as this are typically patchy in nature and are prone to moving around with the tide and current. Impacts are often equally patchy in nature. As a reminder, this algal bloom has no human health impacts and shellfish may still be harvested during the bloom and consumed safely.

Please visit the DMR website for notices of biotoxin and bacterial shellfish closures.

September 9, 2026

DMR Announces the Following:

The following is some guidance for shellfish growers who may be impacted by the current Karenia bloom primarily in the Casco Bay area. Reminder, this bloom does not impact human health and it is safe to harvest, sell and consume shellfish.

1. First, determine if your farm is being impacted by the bloom.

This is a visible bloom. To easily determine if it is in the vicinity of your farm, take a scoop of water in a clear container and see if there is obvious discoloration of the water. This is a "you'll know it when you see it" kind of determination, if you are thinking, well maaaybe there is a very slight change, then you are not in a bloom that will adversely impact your farm at the moment. This assessment should be done every day because the bloom can grow and move around. People outside of Casco Bay likely don't have to worry but that could change and everyone west of Penobscot Bay should be making close observations on their farms.

2. Second, if you determine you are in a bloom area, consider your risk.

Where is your product in the water column, what size is your product, is it also being stressed by something else. Shellfish at smaller sizes are going to be most vulnerable, take care of them first. The most important consideration in this bloom is reduced oxygen caused at night when the cells move toward the bottom and respire and when the bloom crashes and starts decomposing. We don't have evidence that the toxins that are generated by Karenia mikimotoi have impacted Maine shellfish species in the past, but we also can't rule it out. We do know shellfish have died from reduced oxygen levels in Maine in the past.

3. Third, assess your options and take action.

• Can you get shellfish temporarily out of the water, for example by flipping the cages? During the day, the bloom will be at the surface, at night the bloom sinks toward the bottom.

• Can you take some product out of the water, for example putting it in cold storage on land (make sure it has tags)?

• Can you move product to another lease site or LPA in an area not being impacted by the bloom? Do you know someone in an area that is not impacted that would allow you to move product to their site? Keep in mind that if you move product the site you move it to must already allow that gear type and species. And make sure you follow all Chapter 24 regulations on the restriction of movement for disease considerations (e.g. MSX areas).

In 2017, the last time we had a Karenia mikimotoi bloom on this scale the entire event lasted about 3 weeks. Blooms are very dynamic and maybe gone in a localized area much more quickly.

DMR will continue to monitor the bloom, including with flights to document the extent. Shellfish will be sampled from impacted areas periodically and updates will be provided.

If you have questions please email [email protected].

Please visit the DMR website for notices of biotoxin and bacterial shellfish closures.

September 8, 2026

DMR Announces the Following:

The Department of Marine Resources is monitoring a bloom of the commonly occurring phytoplankton Karenia mikimotoi in Casco Bay. The bloom has been observed from Portland Harbor to Middle Bay. Observations have also been reported of the bloom being present in New Hampshire and Massachusetts waters. Phytoplankton counts in Portland were 2,000,000 cells/L. Observations by DMR staff and from public reports indicate brown discoloration of the water and an accompanying smell.

Unlike other toxic blooms that the Department routinely monitors (e.g. "red tide"; PSP-, ASP- or DSP-related phytoplankton), this species of phytoplankton is not a threat to human health, however, this species can have harmful effects on marine organisms. Karenia mikimotoi can produce compounds that can potentially impact fish and shellfish, and there have been fish kills and benthic mortality events reported during blooms of this species (e.g., in Hong Kong, Australia, Ireland, Japan, Korea, Alaska and in the 2017 Casco Bay event). Mortality events have been attributed to low dissolved oxygen resulting from high biomass blooms (especially as the bloom declines) and/or fish-specific toxins. The last time there was a bloom impacting Maine, Casco Bay in 2017, the highest sampled count was 44,000,000 cells/L and shellfish and lobster (unable to move out of anoxic areas due to being in traps or crates) mortalities were reported in some communities. Early reports have indicated clam mortalities in Middle Bay and Maquoit Bay that may be related to this bloom.

Industry members located in the Casco Bay region should remain alert for signs of low oxygen, water discoloration and an odor. The lowest dissolved oxygen levels may occur at night or in the early morning. Many dinoflagellate cells migrate to the surface during the morning so that they can utilize light for photosynthesis, at the end of the day, they migrate to deeper waters and respire, which consumes oxygen. Typically, oxygen is lowest around sunrise and if cells are vertically migrating, oxygen can be depleted in the bottom layers through this process.

The Department will continue taking samples to monitor and evaluate the bloom. The Department will provide regular updates on the progress of the bloom throughout Casco Bay. Questions can be directed to [email protected].

Please visit the DMR website for notices of biotoxin and bacterial shellfish closures.