CALIFORNIA, Md. — Fish kills can be alarming for anyone spending time near Maryland’s rivers, creeks, ponds and the Chesapeake Bay. Seeing dozens or even hundreds of dead fish along the shoreline often raises concerns about pollution, public health or the condition of local waterways.
The concern is not hypothetical. In early July 2026, a fish kill was reported on the Potomac River near Seneca and upstream of Little Falls in Montgomery County after the river reached a record 94 degrees, according to The BayNet. The Maryland Department of the Environment said the extreme heat likely triggered a “summer turnover event, when layered warm and cold water suddenly mix and oxygen-poor water rises to the surface, stressing fish.”
In reality, a Maryland fish kill can happen for several reasons. While some events may be linked to pollution or other environmental problems, many occur because of natural conditions that become stressful for fish, especially during warmer months.
Understanding why fish kills happen and knowing how to respond can help residents protect local waterways while providing valuable information to environmental officials.
What Is A Fish Kill?
A fish kill is an event in which a noticeable number of fish die within a relatively small area over a short period. The size of a fish kill can vary from a handful of fish to much larger events affecting long stretches of shoreline.
Fish kills may involve a single species or several different kinds of fish. They can occur in freshwater lakes, neighborhood ponds, tidal creeks, rivers and the Chesapeake Bay.
Because Maryland has an extensive network of waterways across Southern Maryland, Anne Arundel County, Baltimore and other parts of the state, residents may occasionally encounter these events during fishing, boating or walking along the water. The Maryland Department of the Environment tracks these reports statewide and publishes an annual summary of confirmed fish kills.
Common Causes Of A Maryland Fish Kill
Not every fish kill has the same cause. Environmental experts often investigate reports to determine whether the deaths resulted from natural conditions or a human-related problem.
According to the Maryland Department of the Environment’s most recent annual data, natural water chemistry — including low dissolved oxygen, winter conditions, sudden salinity changes and temperature extremes — accounts for the largest share of reported fish kills in the state, roughly 39%. Biological changes such as toxic algae blooms, disease outbreaks and predator activity make up another significant portion, while chemical pollution from industrial or domestic discharges, pesticides, fertilizers and oil spills accounts for a smaller share.
One of the most common causes is low dissolved oxygen. Fish need oxygen in the water to survive. During periods of hot weather, oxygen levels naturally decline. If algae grow rapidly and then die, the decomposition process can consume even more oxygen, leaving fish unable to survive. The U.S. Environmental Protection Agency (EPA) explains that when algae blooms die off, they consume oxygen in the water column as they decompose, which can create hypoxic conditions where aquatic life cannot survive. Algal blooms can also harm fish directly by clogging their gills, according to the EPA.
Warm temperatures also place additional stress on fish. Shallow ponds and slow-moving waterways may heat up quickly during summer, creating conditions that reduce available oxygen. In tidal rivers like the Potomac, extreme heat can cause layered warm and cold water to mix suddenly, pulling oxygen-poor water to the surface, according to the Maryland Department of the Environment.
Heavy rainfall can also contribute. Stormwater runoff may carry sediment, nutrients and other materials into waterways. Changes in water conditions can stress fish, especially if runoff affects smaller ponds or streams.
Natural disease outbreaks and parasites can sometimes affect fish populations as well. Wildlife diseases occur in nature and may occasionally lead to localized fish deaths. In one well-documented case on Maryland’s Eastern Shore, a bloom of the toxic algae species Karlodinium veneficum on the Corsica River combined with low dissolved oxygen to kill an estimated 50,000 fish across 15 species, according to the Maryland Department of Natural Resources’ water quality monitoring program, Eyes on the Bay.
Pollution is another possible cause. Chemical spills, unauthorized discharges or other contaminants can harm aquatic life. Because pollution is only one of several possible explanations, it is important not to assume the cause without an investigation.
What Residents Should Do If They See Dead Fish
If you encounter what appears to be a fish kill, avoid jumping to conclusions about the cause. Instead, take practical steps that can help environmental agencies assess the situation.
If it can be done safely, note the location, estimate how many fish are affected and observe whether multiple species appear to be involved. Photos taken from a safe location can also help document conditions.
Residents should report suspected fish kills to the Maryland Department of the Environment’s Fish Kill Investigation Section, which asks that reports be made by phone so they can be answered more quickly. During business hours, the department can be reached at 443-482-2731 or 800-285-8195. On evenings, weekends and holidays, reports can be made through the Maryland Emergency Management Agency at 866-633-4686, which dispatches an on-call duty biologist.
Avoid handling dead fish unless necessary. If contact cannot be avoided, wear protective gloves when possible and wash your hands thoroughly afterward.
Keep children and pets away from areas where large numbers of dead fish are present until officials determine whether any hazards exist.
Can You Still Fish Or Boat Nearby?
A fish kill does not automatically mean an entire waterway is unsafe for recreation. The impact depends on the cause, the size of the event and whether any public health advisories have been issued.
Before fishing, swimming or boating in an area where a fish kill has occurred, check for any notices from state or local authorities, including advisories posted by the Maryland Department of the Environment. If officials have issued guidance or temporary restrictions, follow those recommendations.
If no advisories are in place, recreational activities may still be allowed, but residents should use common sense and avoid direct contact with dead fish or discolored water.
Why Reporting Fish Kills Matters
Public reports play an important role in protecting Maryland’s aquatic resources.
Early notification allows environmental professionals to evaluate conditions, collect samples when appropriate and determine whether additional action is necessary. In some cases, quick reporting can help identify pollution incidents or other environmental concerns before they spread. The Maryland Department of the Environment compiles reported fish kills into an annual statewide summary used to track patterns over time.
Reports also contribute to a better understanding of long-term water quality trends across Maryland’s rivers, streams and coastal waters.
Frequently Asked Questions
What causes a Maryland fish kill?
Fish kills can result from low oxygen levels, warm water, algae blooms, disease, parasites, pollution or a combination of environmental factors. The Maryland Department of the Environment attributes the largest share of reported cases each year to natural water chemistry, including low dissolved oxygen and temperature extremes.
Should I report dead fish in a Maryland waterway?
Yes. Reporting a suspected fish kill to the Maryland Department of the Environment at 443-482-2731 or 800-285-8195 during business hours, or 866-633-4686 through the Maryland Emergency Management Agency after hours, helps environmental officials evaluate the situation and determine whether an investigation is needed.
Is it safe to touch dead fish?
It is best to avoid handling dead fish. If contact is necessary, wear gloves if available and wash your hands thoroughly afterward.
Does a fish kill mean the water is unsafe?
Not necessarily. Some fish kills result from natural conditions, such as the low-oxygen “summer turnover event” the Maryland Department of the Environment linked to the July 2026 Potomac River fish kill. Check for any public health advisories or guidance from state or local officials before using the waterway.
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Potomac River Reaches 94 Degrees As Fish Kill Reported In Maryland
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