CONCORD – Fish kills, where large numbers of fish die in a short period of time, are not an uncommon sight in the spring and 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. This can happen very quickly on a sunny, calm day in late May or early June. 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.

May and June are spawning seasons for species such as bluegill, pumpkinseed, black crappie, and largemouth bass, and these species frequently crowd warm, shallow areas in lakes and ponds. This reduces the oxygen levels because of the large numbers of fish concentrated in a small area. These conditions also put additional pressure on fish that may already be stressed from surviving the long winter, which weaken the immune system of the fish, making them more susceptible to disease-causing pathogens. Also, because they are living in close proximity to each other during this period, pathogen transmission may be increased.

“Most, but not all, fish kills during this time of year are due to these natural processes rather than pollution,” said Scott Decker, acting 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 provide this information to the New Hampshire Fish and Game Department’s Inland Fisheries Division at 603-271-2501.

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