If you find a fish kill

The vast majority of fish kills turn out to be caused by a natural event. However, it’s important for biologists to rule out other causes including pollution. Natural fish kills are generally the result of low dissolved oxygen levels (anoxia), spawning stress or fish diseases. Dissolved oxygen depletion is one of the most common causes of natural fish kills. This can be the case at any time of the year but generally occurs during severe winters or late spring/early summer.

Winter fish kills

During winter, thick ice and heavy snow cover can result in low dissolved oxygen levels in ponds. With increasing ice and snow packs, there is less light penetration through the water column. This alters chemical and biological processes, often resulting in a winter fish kill. 

The vast majority of fish kills turn out to be caused by a natural event. However, it’s important for biologists to rule out other causes including pollution. Natural fish kills are generally the result of low dissolved oxygen levels (anoxia), spawning stress or fish diseases. Dissolved oxygen depletion is one of the most common causes of natural fish kills. Weedy ponds that are less than 15 feet deep are particularly vulnerable. This can be the case at any time of the year but generally occurs during severe winters or late spring/early summer.

Ice anglers may encounter signs of a low oxygen environment when they drill through the ice and notice the smell of rotten egg or observe sluggish or dying shiners. The odor is hydrogen sulfide gas which is a natural byproduct of low dissolved oxygen environments, and is not likely the result of pollution. Oxygen levels will be return to normal shortly after the ice melts in the spring.

If you observe dead fish, contact the Environmental Police's 24-hour radio room at 1 (800) 632-8075. A MassWildlife biologist will review each situation to determine whether the kill is natural or requires a site investigation.  

Spring and summer fish kills

As temperatures increase, the water cannot hold as much oxygen as when it was cold. Aquatic plants also increase their oxygen consumption. In turn, oxygen levels in shallow, weedy ponds decrease. If levels fall below that required for fish to survive, it can become critical. Late spring and early summer is also when most warmwater fish species begin to spawn. Large numbers of these species crowd in the shallow waters along the shore vying for the best spawning sites. These crowded areas are susceptible to disease outbreaks. The result is an unavoidable natural fish kill, usually consisting of one or two species of fish. This is a natural occurrence and does not pose a public health risk.

As water temperature increases over time, the water simply cannot hold as much oxygen as when it was cold. During the long hot days of summer, oxygen levels in shallow, weedy ponds can further decline as aquatic plants consume oxygen at night resulting in low oxygen levels in the early hours of the morning. This situation can become critical if the levels fall below that required for fish to survive which is approximately 4-5 parts/million. In addition to the depressed oxygen conditions, late spring and early summer are when most warmwater fish species, such as sunfish (bluegill, pumpkinseed, largemouth bass etc.), begin to spawn. At this time, large numbers of these species crowd into the shallow waters along the shore vying for the best spawning sites. These densely crowded areas are susceptible to disease outbreaks especially as water temperatures increase. The result is an unavoidable natural fish kill, usually consisting of one or two species of fish. Nothing can be done to prevent this; it is a natural occurrence and does not pose a public health risk.

MassWildlife's response to fish kills

A MassWildlife fisheries biologist will review reports of fish kills and make a determination about whether the kill is natural or requires a site investigation. Generally, pollution impacts all kinds of aquatic life, therefore the most important piece of evidence for the biologists is knowing the number of fish species associated with the fish kill. Fish kills in which only 1 or 2 species are involved are almost always a natural event.

When it is likely a fish kill is due to pollution, MassWildlife notifies the appropriate state agency which takes the lead on a formal investigation including analysis of water and fish samples to determine the source of pollution. MassWildlife provides the investigating agency with technical assistance by identifying the kinds and numbers of fish involved.