COLUMN: Naturalist notes

     Why do some ponds and lakes experience winter fish kills? Many landowners may be wondering this as our cold temperatures and snow hang on into March and the talk of possible winter kills on Iowa’s lakes and ponds circulates.

     Winter fish kills in Iowa ponds occur most commonly when heavy snow completely covers the ice of a lake or pond. When the absence of sunlight in a pond stretches out for extended periods of time, the plants and algae under the ice cannot produce oxygen through photosynthesis. This situation leads to a decline in the dissolved oxygen content of the water and the death of fish, frogs, and turtles living within that pond.

     Winter kills occur more frequently in shallow, nutrient-rich ponds with large quantities of aquatic plants and algae.  Decaying plant material within the pond exasperates the decline in oxygen levels when oxygen is used in the decaying process.

     Long, hard winters with heavy snow cover produce the greatest number of winter kills. Owners of small shallow ponds can use aerators or diffusers to increase dissolved oxygen content in the ponds water – similar in function to the aerators of a fish tank. These often lead to open areas or areas of very thin ice on a pond when they circulate the water. Anyone using the pond for winter recreation needs to use extreme caution on ponds and lakes with aerators present.

     Ice fishermen may observe live fish coming up into their ice fishing holes in search of oxygen in shallow ponds being affected by declining oxygen levels. Winter kills are most often discovered following ice-out, or shortly thereafter, when dead fish are observed in the affected pond.

     Winter fish kills can be caused by other factors including extreme fluctuations in water temperature, pollutants, and fish diseases. If any of these causes is suspected please call the Iowa DNR Field Office at 563-927-2640. Winter fish kills due to cold and snowy winters are a natural process and can sometimes be beneficial if a farm pond fish population is unbalanced. Fish populations can recover over time on their own or landowners can restock the population, if desired.

     For information on managing Iowa ponds visit the Iowa DNR website at https://www.iowadnr.gov/Fishing/About-Fishing-in-Iowa/Iowa-Ponds. If you have a natural resource related question email naturalist@co.jones.ia.us or mail questions to Jones County Conservation, Attn:  Naturalist, 12515 Central Park Rd, Center Junction, IA 52212.