Jason Smith: (603) 271-2501
Jay Martin: (603) 271-3211
August 24, 2020

Concord, NH – Fish kills, where large numbers of fish die in a short period of time, are not an uncommon sight in the Granite State during the summer, and most are due to natural processes, not pollution. Small lakes and ponds in New Hampshire that are comprised of shallow, vegetated habitat and that support an abundance of panfish and other species can be susceptible to fish kills. Multiple factors contribute to this phenomenon. First, dissolved gases, particularly oxygen, become increasingly less soluble as water temperatures rise. As the duration of daylight decreases later in the summer, plants have fewer hours in the day to produce oxygen through photosynthesis. Oxygen levels that drop to 4 parts per million can be lethal to fish. Most oxygen available to fish comes from algae. In the overnight hours and during cloudy weather, limited sunlight causes algae to switch from photosynthesis to respiration, increasing their consumption of the oxygen also needed by fish.

In New Hampshire, the species most commonly observed during seasonal fish kills are young of the year (YOY) yellow perch. Unlike largemouth bass, considered a warmwater species, yellow perch are a coolwater species which are adversely affected by rapid increases in water temperature and decreased dissolved oxygen. These conditions may cause a large volume of mortality in a short period of time.

Most fish kills are part of natural processes, and in some instances, may be beneficial to the long-term health of a fish community. Some fish kills help maintain a balanced carrying capacity for the population in a given waterbody. However, fish kills should still be reported with as much detail as possible, including the fish species affected and any unusual site-specific information.

“Most, but not all, fish kills during this time of year are due to these natural processes rather than pollution,” said Jason Smith, Chief of the Division of Inland Fisheries, “but they should still be reported because the data can be useful to Fish and Game.” A quick description of the water body, number and species of fish found dead, along with any observation that would be considered “unusual” should be included in the report. To report a fish kill, please call the New Hampshire Fish and Game Department’s Inland Fisheries Division at (603) 271-2501.