Spring and Summer Fish Kills in New Hampshire

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.

https://www.kevindahlkeoutdoors.com/single-post/2018/06/11/Spring-and-Summer-Fish-Kills-in-New-Hampshire

https://nhfishgame.com/2018/06/08/fish-kills/