Fish kills occur most often in late July and accelerate in early August. In virtually every case, it has been possible to link these fish kills to hot, dry weather conditions and/or algal blooms.
In some cases, water temperatures in ponds have warmed above tolerance levels for trout causing mortality. More commonly, the fish kills that have been reported have been large-scale kills of many species of fish. These fish kills have usually occurred as a result of algal blooms and subsequent death of the algae, resulting in reductions in dissolved oxygen in the water.
Symptoms of these incidents include large numbers of affected fish across all species, fish gulping at the water surface, and snails and crayfish leaving the water and congregating around the pond edge. Unfortunately, there is little that can be done quickly to prevent these fish kills. They can be prevented by treating or reducing algal growth in the pond earlier in the summer using herbicides, dyes, or other techniques. Aeration devices can also be used to increase dissolved oxygen in the pond during blooms.
When snow and ice have melted from ponds, pond owners may notice dead fish, frogs, or other pond life. In most cases this is probably due to winterkill. Winterkill is rare in Pennsylvania ponds but does occur frequently in ponds in colder climates. It occurs when a pond is frozen and snow covered for an extended time. It occurs most frequently in shallow ponds with abundant aquatic plants. Plants and other organisms in the pond use oxygen during the winter. If the pond is not frozen, the oxygen is quickly replenished from the surface. If the pond has a lot of plants, snow on top of ice shades the plants, causing them to use oxygen rather than produce oxygen.
What can be done to prevent winterkill?
Since winterkill is a relatively rare event in Pennsylvania ponds, you may wish to take no action on your pond and simply live with occasional winterkills during especially harsh winters. If you want to prevent winterkill, you can try one or more of the following:
- Control aquatic plant and algae growth during the summer to prevent them from using oxygen during harsh winters.
- If your pond ices over, plow the snow from the ice to allow sunlight into the pond to keep plants growing and producing oxygen.
- Install aeration in the pond to maintain some dissolved oxygen in the water.
- Dredge the pond to create deeper water that is less susceptible to winterkill.