Winter Fish Kill Survey

Summit Laboratory is thrilled to be back for our fourth season of the CLAM Program!
Our first responsibility for the 2007 season was to conduct a post ice winterkill survey. During the “last ice” periods of 2005 and 2006, we conducted thorough dissolved oxygen testing on all the lakes to determine if lack of oxygen was stressing the aquatic ecosystem. This data demonstrated that though oxygen levels were low enough to cause stress in some of the sample sites, no significant winter fish kills were observed.
Once the ice cover begins on northern lakes, they are cut off from atmospheric mixing and the dissolved oxygen this action supplies. Over the winter, as organic matter (plants, algae, leaves, dead organisms) decays, it slowly consumes the available dissolved oxygen in the water. During winters with long periods of ice cover, and heavy snowfalls (no photosynthesis for plants to add some dissolved oxygen), northern eutrophic lakes (like those of CL) oxygen levels can become depleted due to the biological oxygen demand. Being that the winter of 2007 was mild with ice cover not forming until late in the season and receding early in the spring, we did not expect to see winterkills. It isn’t safe to just assume no winterkills occurred though. In smaller aquatic systems, winterkills can seriously degrade a fishery resulting in the need for specific restocking. I went to all the C.L.P.O.C. waterbodies and with only one exception, witnessed no winterkill issues! The aquatic ecosystems have come through another tough Michigan winter intact.
The one exception was Lily Pond. I witnessed numerous small bass had died, most very recently. Was it a DO related winterkill? Unlikely, as both smaller and larger living bass were observed. The mortality event was most likely related to starvation. Adult fish were stocked into a system that doesn’t have enough food to support them. It takes time to develop and aquatic ecosystem diverse enough to support large predators.