CONCORD, N.H. – Fish kills are not an uncommon sight in spring and summer, and most are due to natural processes, not pollution. Small lakes and ponds in New Hampshire that have a lot of shallow, vegetated habitat and a high abundance of panfish can be susceptible to fish kills. Multiple factors contribute to this phenomenon. First, dissolved gases (particularly oxygen) become increasingly less soluble as the water temperature rises. This can happen very quickly on a sunny, calm day in late May or early June. When oxygen levels decrease to 4 parts per million, it can be lethal to fish, or at a minimum, cause the fish some stress. Most oxygen available to fish comes from algae. During nighttime and cloudy weather, limited sunlight causes algae to switch from photosynthesis to respiration, consuming oxygen needed by fish.
Spring time is also spawning season for species such as bluegill, pumpkinseed, black crappie, and largemouth bass. These species will often seek out and crowd these warm, shallow areas in lakes and ponds in pursuit of adequate spawning habitat. Not only does this natural crowding deplete oxygen levels because of large numbers of fish concentrated in a small area, it also puts additional pressure on fish that might already be stressed from surviving the long winter. These additional stressors can weaken the immune system of the fish, making them more susceptible to disease-causing pathogens. Also, because they are living in close proximity during this period, pathogen transmission is increased.
Most, but not all, fish kills during this time of year are due to these natural processes rather than pollution but should still be reported. A quick description of the water body, number and species of fish found dead, along with any observation that would be considered “unusual” or