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SSW: SOME Context

At the moment, there is no way to permanently rid our lakes of Starry stonewort. Research is ongoing both here and in the US into new control methods, but for the time being, SSW is here to stay.

It is vital that we learn how NOT to spread this aggressive invasive species so that we can contain its growth to the areas of the lake where it already exists. It's equally important to understand that when we dredge our waterfronts to remove weeds, we may be removing an important control on the health and growth of SSW. A healthy aquatic ecosystem, full of both native and non-native aquatic plants, may be one of the best weapons we have in our SSW-fighting arsenal. Learning to love your weeds may run against the current, but those pesky weeds could be providing an aquatic atmosphere that will, in the long run, be inhospitable to the opportunistic SSW.

New Sightings

• Submit information to Environment Council via our contact form. EC members and other lake volunteers are working to monitor and identify SSW.

• If possible send send us a photo of the area and the algae for verification.

What to do if you think you've spotted SSW

Starry stonewort at a glance.


SSW: What You Can Do To Help

Be informed, learn how to prevent its spread, and stay away from infested areas.


Be informed


Starry Stonewort (SSW) is an aggressive invasive macroalgae found throughout the Great Lakes/St. Lawrence and the following lakes: Simcoe, Scugog, Sturgeon, Buckhorn, Rice, Big Cedar and Lower Stony Lake where it has proliferated over the last few years throughout Gilchrist Bay and the Lost Channel and was most recently identified in Mackenzie Bay, near Kawartha Park.


SSW spreads easily and rapidly by the dispersal of plant fragments and bulbils (little white stars that grow on the plant like seeds) – a process accelerated when boats travel through infested areas, their propellors tearing strands of SSW and redistributing them throughout the lake.


• SSW disrupts healthy functioning ecosystems

• Displaces native aquatic plant communities and forms a monoculture

• Reduced plant diversity affects fish

• Displaces habitat for fish, frogs and turtles

• Zebra mussels thrive in it, which increases water clarity and increased plant growth

• Long-term impacts are irreversible


•  Economic repercussions result from interruptions to regular recreational activities: swimming, boating, fishing

• High economic impact to tourism – resorts, marinas, cottagers, houseboat rentals

• Increased public complaints to municipalities, TSW, conservation authorities, MNRF, MECP re: aquatic invasives, permits to remove algae/weeds, concerns about fish and wildlife, use and enjoyment of waterfront property 


• Weed harvesting will not eradicate SSW. It is often used to facilitate access or recreational use but actually increases the risk of spread by fragments

• Plant removal is regulated by MNRF and DFO as it affects critical habitat for warm water fish (e.g., perch, bass, muskellunge). A TSW permit is required to harvest and remove aquatic vegetation. A permit allows:

harvesting up to 50% of shoreline to a maximum 10 m wide and 30 m into the waterbody

• Larger areas or channels require a separate TSW permit application (e.g., Gilchrist Bay)

• Many more invasive species on the horizon (water soldier, others)

Context is key: Understanding how habitat characteristics, water composition and conditions, and sediment properties contribute to SSW's growth is critical for informing treatment strategies.


Prevent the Spread

• Avoid boat travel from infested areas into non-infested areas.

• If you absolutely need to travel in or through an infested area, stop and lift your boat motor and remove all weeds. The weeds that you remove must be placed in a bucket and taken to dry land and disposed of away from any water.

• CLEAN, DRAIN AND DRY your boat whenever launching or trailering a boat to another lake. (See detailed description, below.) This will prevent the spread of SSW out of infested lakes but also prevent other new invasive species from being spread into Ston(e)y, Clear and White Lakes.

• Ensure that any commercial harvester that you hire to remove weeds on your waterfront: 1.) Has a permit from Trent-Severn Waterway to remove weeds on your waterfront; and 2.) Is not moving in and out of SSW infested areas.

• Apply Best Management Practices – more on this later in June.


• As noted above, you can submit queries and location information to Environment Council via our contact form. In 2022 EC members and other lake volunteers are working with the Kawartha Lake Stewards Association to monitor and identify SSW.

• If possible send send us a photo of the area and the algae for verification.



• Starry Stonewort is an aggressive, plant-like microalgae that forms large, dense mats. It grows in water from 0.5>10m in depth. The mats themselves can be 3m deep.

• SSW’s branchlets and stems are very thin (like heavy fishing line), have a crisp texture, and become entwined to form mats so thick they can impede boating or paddling. At the peak of the growing season, late July to late September, they appear as hill-like mounds under the surface of the water. 

• SSW is anchored by clear filaments that produce small, white, star-shaped bulbils – the reproductive structure that gives SSW its name and makes it so difficult to eradicate.

• SSW is spread by fragmentation and the dispersal of the bulbils.


Clean, Drain and Dry your Boat

SSW can be hard to contain. If you know that you need to travel through an infested area, it’s imperative that you exercise the following precautions:

• Once clear of the infested area, stop your boat, raise your engine, and clear the prop of any SSW. DO NOT DISCARD IT IN THE LAKE! Remove the SSW and dispose of it well away from the water.

• If you are removing your vessel, make sure that it’s Cleaned, Drained and Dry. Drain all water from your boat, canoe, kayak, and other vessels and any equipment used in the water (paddles, fishing gear). Remove all plant fragments and dispose of them properly. Dry all damp areas of your boats. 



ZOOM Presentation: Dr. Brian Ginn

On July 28th, 2020, Dr. Brian Ginn of the Lake Simcoe Region Conservation Authority was our guest during a Zoom Starry stonewort presentation. He described this aggressive invasive's history in Ontario, its spread throughout the Trent-Severn Waterway, and the science behind current best management theories. Dr. Ginn's presentation was followed by an overview of the impacts of SSW and actions taking place with a variety of partners to reduce the spread and manage it in Ston(e)y Lake. Here is PART ONE of Dr. Ginn's presentation. To view more, click the link, below.

starry stonewort1.png

In the News

Drew Monkman – author, columnist and naturalist – has written about Starry stonewort's devastating growth in the Kawartha Lakes, including Ston(e)y Lake. Click the link below to read his article in The Peterborough Examiner.

Taking  Action

In 2019, as the extent of the infestation in Lower Stony became clear, EC and its association partners focussed on identifying any new areas of growth and educating the lake community about this new-to-us, aggressive invasive species via news articles, information sheets, social media posts and signs. This is one of the many signs EC posted around Ston(e)y and Clear Lakes at marinas, landings and boat launches warning boaters about the presence of SSW in our water and encouraging Clean, Drain and Dry practices (see detailed description below). In 2020, the focus is on monitoring and mapping the spread of SSW and development of a safe management strategy (now published; see link above). Taking SSW off your propellor and throwing it in the lake will cause it to spread to new areas faster.


Support EC's Efforts

• Inform your neighbours or lake association – pass along information from this website or any newsletters or other communications you receive, especially anything focussing on what SSW is and how to prevent its spread

• Discover the work we are undertaking with our partners: universities and colleges, and federal, provincial and municipal governments 

Together we can make a difference.

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