When You Find a Fish Kill

Posted by Wayne G. Barber

Report a Fish Kill - Contact the Massachusetts Environmental Police Radio Room at 1-800-632-8075.

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The sight of up to hundreds of dead and dying fish along the shores of your favorite pond or lake can be distressing and often lead to concerns about pollution. But in fact, the  vast majority of the fish kills reported turn out to be natural events.
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 anytime of the year but generally occurs during severe winters or late spring/early summer.

Conversely, in the spring and summer, 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

So how does MassWildlife know if a reported fish kill is a natural event or the result of pollution? As the lead agency in determining fish kill response, a Division fisheries biologist will review each call and through a series of questions, make a determination on 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 one or two 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. MassWildlife maintains a 40+ year database which helps track waters with a history of natural kills.