Fish Kill at Little Fresh Pond is from Natural Causes

4/6/20158 years ago

caution sign The information on this page may be outdated as it was published 8 years ago.

At Fresh Pond Reservation winter killed fish will be removed from the shorelines of Little Fresh Pond and Black’s Nook starting Monday, April 6.  The dead fish were reported the weekend before.

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. Fish do act as the "canary in the coal mine", so it's natural that someone would think a fish kill was the result of pollution. But in fact, the exact opposite is true. 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 any time of the year but generally occurs during severe winters or late spring/early summer.
Winter Fish Kills

During the winter, thick ice and heavy snow cover can result in low dissolved oxygen levels in ponds. Increasing ice and snow packs limit light penetration through the water column, altering chemical and biological processes such as photosynthesis and the decomposition of organic matter (dead plants). These conditions can frequently result in a winter fish kill. Shallow, weedy ponds of 25-30 feet in depth are particularly vulnerable. MassWildlife fisheries biologists routinely find low dissolved oxygen levels in ponds statewide during these kinds of conditions. Reports of strong "rotten egg" odors are generally the first clue that a waterbody is experiencing anoxia. The odor is hydrogen sulfide gas which is a natural by-product occurring in lakes and ponds with low amounts of dissolved oxygen. This condition is natural and rarely the result of pollution such as illegal dumping, sewage or a chemical spill. Oxygen levels become fully restored when the ice melts in the spring. It is at this point that winter fish kills often become fully visible to the public.