Fish Kills in Pennsylvania

By Tess Flynn-Belles, Natural Resource Specialist
     Imagine this; it’s an early spring morning, the sun is shining and most of the snow has melted. You decide to take a walk down to your pond and see if the ice has melted off. You’re enjoying your walk, the warm sun softening the crisp of the morning air. As you approach your pond, you take in the light sparkling off the water. YAY!! The ice is off, time to start casting a line out and see what you catch. You’re cruising the edge of your pond when you notice a dead sunfish…. A little more walking… a dead largemouth bass. You look up and notice that the shoreline of your pond is littered with dead fish!! It’s then that it hits you in the gut…… FISH KILL!!!

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     Although winter fish kills are not common in Pennsylvania, they do occur when presented with a harsh winter, most frequently in shallow ponds with an abundance of aquatic plants. The fish most likely die in the late winter, but aren’t noticed until the ice has melted off. In winter months, when there are prolonged periods of ice and snow cover, fish have to compete with plants for oxygen. During low levels of sunlight and at night, plants actually use more oxygen than they produce, therefore competing with the fish. This is not suggesting that if you remove all the plants in your pond, you will never have a fish kill. Decomposing plant material and mucky bottoms also use up dissolved oxygen. Aquatic plants are beneficial to waterbodies because they do produce oxygen through photosynthesis. If your pond does have an abundance of aquatic vegetation, it may be beneficial to control them in the summer months to prevent them from using too much oxygen during a harsh winter. Removing any snow covering the ice will also allow sunlight to shine through and aquatic plants to photosynthesize. Drilling holes in the ice can allow air to reach the water surface and oxygen to dissolve into the waterbody.
     Summer fish kills are a more common occurrence in Pennsylvania, most often happening in late July and early August. When presented with hot, dry weather conditions, ponds with a low flushing rate (the amount of time it takes for water to enter and leave a waterbody) tend to develop algal blooms. Large-scale blooms and the subsequent death of algae will result in the loss of dissolved oxygen in the waterbody. If there is a large enough fish population, this lack in dissolved oxygen could result in a fish kill. A few dead fish is not cause for alarm, but a large number of fish of all species and sizes is when there is a problem. Fish gulping at the water surface or snails and crayfish leaving the water and congregating around the water’s edge are signs that a waterbody is being affected by low dissolved oxygen levels. Treating and reducing algae growth in the early summer is one way to prevent large blooms from occurring. Aeration devices, although pricey, can supply a constant source of dissolved oxygen to a water body, cutting down on algae blooms and therefore keeping your fish alive. 
     Taking the time to observe and preform some preventative maintenance on your pond could save you the headache of losing your fishery in the future.
     The Bradford County Conservation District is committed to helping people manage resources wisely.  You can visit the Bradford County Conservation District at 200 Lake Rd in Wysox across from the Wysox Fire Hall. Contact us at (570) 485-3144 or visit our web page at www.bccdpa.com.