Fish Kill

by Carl Strang

On Tuesday of last week I returned to Mayslake Forest Preserve after the Labor Day holiday to find hundreds of small, dead bluegills floating in the east end of May’s Lake.

A sample of the fish kill.

They had been dead for a day or two, and they or possibly other afflicted fish had attracted a flock of gulls, a few cormorants and several herons of three species to the lake. I left a message with Forest Preserve District fisheries biologist Don LaBrose, and he got back to me later with the news that the immediate cause of death was columnaris disease, a bacterial illness in which Flexibacter columnaris produces lesions in the gills, and other damage.

Sometimes the lesions are obvious externally, sometimes you have to examine the gills.

Don’s reconstruction of events is that the shallow lake had a greatly reduced dissolved oxygen level, in part a consequence of this year’s hot summer. Lake water had reached a temperature of 80°F. This had stressed the fishes for a long period of time, and had them hanging close to the surface where the oxygen level was marginally higher. Over the weekend a strong cold front came through, accompanied by rain which suddenly dropped the lake surface water temperature by several degrees. This would have been enough to shock the already stressed fish and make them susceptible to the disease, which is known both to spread rapidly and to kill rapidly. Young fish are particularly sensitive to columnaris. By Friday the lake temperature had dropped enough to knock out all but a residual remnant of the bacteria.

Part of the record high count of herring gulls on May’s Lake, drawn by the bounty.

Don pointed out that there had been two recent years of high bluegill reproduction in May’s Lake, resulting in so many young fish that there was the danger of an overcrowded, stunted population. Consequently, this fish kill may have been a blessing in disguise. Certainly to the many predators with full bellies, it was a great boon.



  1. kattalina said,

    September 13, 2011 at 9:23 pm

    Thanks for writing about this! I saw this last Tuesday and wondered what was going on. Will the FP clean out all these fish? Also, there were these crab like creature – they looked like some kind of scorpion type thing, hanging out at the edge of the lake. i have NEVER seen these before, and on that day i saw quite a few. ANy idea what these were? thanks. I like your blog and LOVE the fact that you write about Mayslake. It used to be a favorite space for me. And as it was, on tuesday, i believe a great blue heron came and ” hung out” with me on my side of the lake.

    • natureinquiries said,

      September 14, 2011 at 5:44 am

      Hi, Kattalina,
      Yesterday I only saw a couple fish at the edge of the lake, though I wasn’t systematically following the entire shore. I think our local scavengers have been removing them for us. The crustaceans you saw were crayfish. They mainly stay in the deeper water, but the low oxygen level brought them into the shallows. They will retreat as the cooler weather and wind regenerates the lower levels. Thanks for your interest!

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