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Hardy Pond: Concerns about fish kill

Late winter through early spring, large amounts of sunfish have died at Hardy Pond.

Hardy Pond: Concerns about fish kill

Two sunfish dead at Hardy Pond - by Ron Cohen

Many people have contacted the Waltham Land Trust and the Hardy Pond Association about the fish kill at Hardy Pond.  Here is what we know to date:

  • Thousands of sunfish are dead in the pond - they seem to be the only species affected.  We've seen lots of swimming and massing bass in large numbers.
  • There are many reasons for fish kills, most are natural events resulting in a reduction of dissolved oxygen.
  • There have been one or two larger fish kills in Hardy Pond in the past 10 years.
  • Treatment of aquatic plants is not likely to be a direct cause - the chemicals are short-lived in the water and the treatment occurred about 9 months ago.

With or without treatment, the mass of rotting vegetation in the pond contributes to oxygen depletion as do cultural inputs of nutrients. The continued dumping of materials into the storm drains, the street construction projects on Hardy Pond Road, and many other factors add to nutrients being washed into the pond.

Some experts contend that the early disappearance of pond ice and unseasonable warm weather can be problematic.  As the water warms and the water column turns over, the deoxygenated layer mixes quickly into the level where the fish are swimming. Fish become stressed by low oxygen and also become suseptible to viral and bacterial infection.  When this occurs very early in the season, there is little offsetting oxygen being delivered by living green aquatic plants.

Toxic input is certainly a cause of fish kills, and is it impossible to know when toxics get introduced through the storm drains or directly by dumping -- we are unaware of any such events recently.  The thriving pond life (other than sunfish) make this a less likely scenario.
Low oxygen and high nutrients can also encourage explosive growth of harmful algae.

All evidence so far points to a natural die-off of the fish, in a pond that suffers habitually from low dissolved oxygen, with unusual weather contributing.  This does not eliminate the possibility of toxic or damaging inputs by humans or concerns about poor turnover due to siltation downstream -- we will continue to seek more information and better answers.

Marc Rudnick for the
Hardy Pond Association

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