Mass Wildlife: Sunfish Deaths at Pond Caused by Warming Temps

Low oxygen levels in Horn Pond caused by a sudden, seasonal rise in water temperatures that occurs in the spring and summer months is being attributed as the reason behind the recent deaths of hundreds of sunfish at the Horn Pond Lagoon, according to Bridgett McAlice, a Wildlife Biologist with the Massachusetts Division of Fisheries and Wildlife.

Nothing can be done to prevent this; it is a natural occurrence and does not pose a public health risk to humans, other fish, animals or plants, according to the Division of Wildlife and Fisheries, which recommends not removing the fish, but instead allowing nature to take its course.

In the spring and summer, as water temperatures increases over time, the water simply cannot hold as much oxygen as was 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 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 to 5 parts/million.  In addition to the depressed oxygen conditions, late spring and early summer are when most warm water fish species, such as sunfish, 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.  The result is an unavoidable “natural fish kill,” usually consisting of one or two species of fish, according to the Division of Wildlife and Fisheries.

For more information, go to