CONCORD — The New Hampshire Fish and Game Department and the Department of Environmental Services Monday reported that a number of dead fish were discovered as the ice receded from Hopkinton Lake (also known as Elm Brook Pool) in Hopkinton this spring. Such finds are not uncommon, according to state biologists.
Here’s how it happens: Ice, sometimes coupled with thick snow, effectively caps off water bodies in the winter. The result is that ice-covered waters are cut off from atmospheric oxygen, and the intensity of light is reduced, minimizing the ability of algae that live in the water to photosynthesize and produce oxygen.
With reduced oxygen supplies and organisms living in the water still consuming oxygen (fish, invertebrates, bacteria), low oxygen conditions can develop during the winter months. Ultimately, if oxygen levels become too low, a “winter kill,” or more specifically, a winter fish kill, is possible.
Although these conditions are more common in small, shallow (less than 10 foot depth) farm ponds throughout the state, the long winter and late ice-out this year may result in more frequent occurrences of winter fish kills on larger , shallow lakes and ponds, particularly those with relatively high levels of nutrients.
Be aware that as the ice continues to recede on small, shallow waterbodies in New Hampshire, you might find numerous dead fish washed up on or near shorelines. A complete die-off of an entire fishery is highly unlikely, and, therefore, winter fish kills are typically not a serious problem in most water bodies, as a sufficient number of fish survive to repopulate the pond or lake.
Fortunately, as the ice continues to melt, oxygen levels will be fully restored as plants begin to grow and photosynthesize, and as wind-driven air exchange resumes across the exposed surface of the waterbody. Any remaining dead fish can be picked up and buried or disposed of in the trash.