Protect Your Wisconsin Pond This Winter: Prevent Fish Kills

Winter in Wisconsin brings long periods of ice cover, heavy snow, and fluctuating temperatures—all of which can put your pond’s fish at risk. Ice and snow prevent sunlight from reaching the water, and organic material such as leaves, dead plants, and algae continues to decompose, consuming oxygen. Without proper winter management, your pond may experience a winter fish kill, threatening fish populations, water quality, and your investment.  

Why Winter Fish Kills Happen 

Wisconsin’s cold winters often seal ponds under thick ice, reducing oxygen levels and limiting natural gas exchange. Decomposing organic material further depletes oxygen, sometimes producing toxic gases that can harm or kill fish. 

Ponds at greatest risk include: 

• Shallow ponds 
• Nutrient-rich or eutrophic ponds 
• Ponds with algae blooms or murky water 
• Ponds with accumulated organic debris 
  

If your pond struggles with algae, aquatic weeds, or poor water clarity during the summer, winter oxygen stress can be even more severe. 

Aeration: Your Best Winter Defense 

A diffused aeration system is one of the most effective ways to prevent winter fish kills in Wisconsin. Aeration keeps part of the surface open, allowing oxygen to circulate and preventing toxic gas buildup under the ice. 

Winter aeration tips: 

• Run aerators year-round, adjusting diffuser placement in shallow zones to prevent supercooling and protect overwintering fish. 
• Inspect your equipment before freeze-up to ensure compressors, diffusers, and air lines are functioning properly.  

Proper aeration ensures healthier fish and a safer pond ecosystem all winter long. 

Reduce Organic Material Before Freeze-Up 

Fall preparation is key to preventing winter oxygen depletion. Removing excess debris and vegetation reduces nutrient load and supports better oxygen balance under ice. 

Recommended steps include: 

• Remove leaves, dead plants, and shoreline debris to limit organic buildup. 
• Trim back cattails, phragmites, and other emergent vegetation. 
• Apply cold-water beneficial bacteria treatments, which continue breaking down organic matter even below 52°F. 
  

A cleaner pond in the fall leads to healthier water and higher oxygen levels during Wisconsin’s long winters. 

Ice Safety for People, Pets, and Property 

Wisconsin pond ice can be uneven, especially near aeration diffusers, inflows, outflows, or shallow areas. Safety is critical for both people and pets. 

Key precautions: 

• Never walk on ponds with aeration running. 
• Mark open water and thin ice zones with posts, flags, or rope barriers. 
• Keep children and pets supervised near frozen ponds. 
• Shovel pathways and mark edges to prevent slips and falls. 
• Keep rescue tools nearby, such as a rope, pole, or life ring. 
  

Preventing accidents protects both your family and your pond ecosystem. 

Year-Round Pond Protection in Wisconsin 

A proactive pond management plan is your best defense against winter fish kills. Wisconsin Lake & Pond Resource offers customized aeration programs, water quality testing, beneficial bacteria treatments, and seasonal pond maintenance to keep your pond healthy throughout the winter. 

Start preparing now to protect your fish, water quality, and pond investment. 

About Wisconsin Lake & Pond Resource, A Jones Lake Management Partner 
Wisconsin Lake & Pond Resource specializes in comprehensive lake and pond management solutions, including algae and aquatic weed control, aeration, hydraulic dredging, erosion control, and fisheries management. As a Jones Lake Management Partner, we serve a diverse range of clients, including HOAs, golf courses, municipalities, and private lake owners. With a commitment to science-driven strategies and environmental sustainability, Wisconsin Lake & Pond Resource helps maintain and restore aquatic ecosystems for long-term health and enjoyment.