Ponds and Winter

I had a report the other day of dead bluegills on the shore of the pond at Ryder's Woods in Woodstock. Yuck. I didn't have to clean them up (phew), but I did want understand how it happened. Winterkill seems to be the most likely culprit, although no one is sure why it took the fish so long to make it to the surface.

Here's a bit more detail: According to Randy Schietzelt, Biology Professor at Harper College, ponds need to be at least 9 feet deep in Northern Illinois to prevent winterkill of the fish. But, even if the pond is deep enough, it is still very possible for the fish and other critters to use up all of the oxygen in the pond if the freeze last a long time. As Randy pointed out, this year's freeze lasted well into March.

David Wolf, attorney, and owner of two ponds reports that he has had winterkill in both is ponds, even though one is 14 feet deep at its deepest point. As a result, he has added an aerator to the ponds which keeps the oxygen level up all winter long. David also provided some additional wisdom that he gained from his friends at Keystone Hatcheries, who have helped David out with his ponds: essentially, when a pond is frozen over solid, the water cannot gain any oxygen through a process called diffusion - this is the process whereby water absorbs a certain amount of oxygen from the atmosphere based on air temperature and barometric pressure.

Additionally, the heavy (and long-lasting) snow cover blocks off any sunlight from reaching the pond, and that means that no photosynthesis can take place. When plants photosynthesize to create food, they release oxygen as a byproduct. If they stop photosynthesizing, they also stop producing oxygen.

Finally, Duane Ambroz, former Deparment of Natural Resources employee, and current restoration specialist with Red Buffalo Nursery, explained winterkill this way: "Sounds like a combination of thick ice from the early freeze and lengthy snow cover. That equals lack of oxygen in the water. Large bodies of water, or those with inflow of water, can usually get through the winter without running out of oxygen. Smaller ponds almost always run out of oxygen without an aerator."

So, the dead fish were most likely the result of winterkill where oxygen levels dropped so low that the fish could not breathe. They may have been lodged under something and only rose to the surface when the pond started to mix as it warmed during the few warm days earlier in the week. Either that, or it's the work of the Ryder's Woods ghost...?

P.S. you probably noticed that the picture has nothing to do with the story -- I just like the colors, and thought it was much nicer than a picture of dead fish!