Seeing dead or dying fish can cause distress and prompt concerns about pollution, but the vast majority of summer fish kills are natural events. As warm weather arrives, lakes and ponds heat up, and natural fish kills may occur. To ensure there is not a pollution problem, please report all fish kills by calling the Environmental Police Radio Room at 1 (800) 632-8075.

 

Natural fish kills are most often caused by low oxygen levels, fish diseases, or spawning stress. Depletion of dissolved oxygen is also a common cause. Water holds less dissolved oxygen at higher temperatures, and in shallower or weedier ponds, levels can drop further as plants consume oxygen at night. Fish spawning, including sunfish and bass spawning, occurs in late spring and early summer in shallow waters near shorelines. These densely-crowded areas can grow more susceptible to disease outbreaks as water temperatures rise. The result is an unavoidable natural fish kill, often consisting of only one or two species of fish. 

 

When a fish kill report comes in, MassWildlife fisheries biologists determine if it is natural or potentially caused by pollution. Because pollution impacts all aquatic life, the most important evidence biologists look for is the number and variety of fish associated with the incident. When pollution is suspected, MassWildlife notifies the Department of Environmental Protection, who then conducts a formal investigation of the water and the affected fish to determine the source of pollution.