Spring is the time of the year in the Iowa when Iowa’s landscape awakens from winter’s cold and the remaining snow and ice melts. In most years, ponds survive winters with minimal issues. However, there are those rare occasions that pond owners see a number of dead floating fish in the pond. Although the fish may look like they just died, they likely actually died prior to the spring thaw. This fish loss is often referred to as a Winterkill.
Winterkills result from low winter dissolved oxygen levels. Shallow nutrient-rich ponds often have high amounts of aquatic vegetation going into the winter (decaying vegetation takes up oxygen) and are more susceptible to winterkills. Excessive aquatic vegetation comes from both the shallow depths and nutrients. Shallow depths can be managed by removing sediments around the pond parameter by using backhoes but the costs are often high. The best solution to both is to control nutrients and sediments is through sound watershed management practices that include buffer strips. The mere presence of aquatic plants does not necessarily mean that control measures be adopted. Instead it is the amount of aquatic vegetation and their decay during the winter that can cause winterkills.
Although first reaction that a pond owner might be that all of the fish are dead and that the pond needs to be restock with all new fish there might still some fish that survive. If the dead fish census only consists of largemouth bass and bluegills and not channel catfish, there might be some remaining fish as those species are more susceptible to low dissolved oxygen than channel catfish.
To check for possible largemouth bass and bluegill survival, the pond owner should wait until late May or early June and sample the ponds for these fish with a common seine, sometimes referred to as bait seine and is often available are larger farm supply stores. To seine a pond, select a shallow area of the pond, 2-4 feet deep, and pull the seine along the pond edges with a companion. If no fish are collected and you are not able to catch any fish later in the year, you will likely have to restock the pond with new fish. There are a number of private aquaculture farms in Iowa that you can contact for stockers.
Another solution to winterkills uses aeration in summer, fall, or winter. Fall or summer aeration reduces the amount of decomposing organic wastes that results in decreased oxygen demand in the winter. Winter aeration may be dangerous due to the resulting hazardous opening in the ice or management practices during inclement weather. It is important to match a suitable aeration device to the appropriate pond size to maximize aeration.
Finally, sound pond management is similar to the care used by homeowners for their yards in that the best ponds are those that have maintenance done a regular basis.