I am the applicator that treated Crystal Lake. Here are the facts about the treatment...
We treated 28 acres for the invasive plant Eurasian watermilfoil, using an amine formulation 2,4-D product. (See the WDNR fact sheet here: WDNR 2,4-D Fact Sheet) The amine formulation is NON-toxic to fish at max label rates of 4 mg/L. This toxicity testing was conducted on larval fish, which would be much more susceptible than adult fish. We treated the 28 acres at half of that, 2 mg/L. On the day of treatment we noticed many 6" sized bluegills dead around the lake, prior to treatment, and I personally notified our DNR coordinator of these fish (and this internet speculation is why I did so).
We are mandated by the DNR to treat Eurasian watermilfoil and Curlyleaf pondweed early in the season so that we minimize the impacts to native plant species. This is so that the invasives are knocked out, and there is room for native plant communities to establish themselves. We aren't just going out and, willy-nilly, spraying "poison". We make many trips to the lakes to survey plant populations for our acreages and so that we are sure that conditions are good for an effective treatment. Then, we use herbicides in concentrations with very narrow ranges of susceptibility for the targeted species. The treatment on Crystal Lake was very effective at this goal, as EWM has been knocked down where, on the day of treatment, it was topped out or nearly topped out in almost 100% of the treatment zone acreage.
As far as fish dying, I have a fisheries biology bachelors degree and a masters degree in fish health/nutrition, so I have some expertise on this subject. You have to realize that fish are coming out of a very harsh winter, and then experienced a very compressed spring warm-up. This dumped them straight into "spawning mode" (more stress) before they could recover a bit from the winter. A fish's immune system is strongly tied to their stress levels. When fish are stressed, their immune system is compromised, and this opens them up to infection from many angles including opportunistic bacteria and fungus that are not pathogens under normal conditions. Seeing a hundred fish die in a population of 10's of thousands, after a winter we had, you should feel good about it.....
Look at Little Elkhart. Nobody treats EWM out there. The EWM biomass is so large, that when it starts to decompose in late winter it strips the entire lake of oxygen...complete fish kill. Then EWM chokes the lake out in summer so you can't fish (irrelevant now), swim, or boat. These conditions are what we are preventing on Crystal Lake; Three weeks after treatment, and the EWM is gone and native plants are flourishing. You won't see surface matting of EWM this year on Crystal. The native plants will allow native insects to flourish, which in turn feeds the fish.
Trust me when I say this, if 2,4-D would harm a single fish, the DNR would not allow us to treat. Period. They routinely deny us treatment areas if there is a small chance that a native plant will be harmed (not even killed, simply set back a bit this year). They are very strict on what they allow us to do.
All of the information about this treatment and every other lake treatment in the state is freely available to anyone that asks us or the DNR. We put up posting signs that have all of the information about the treatment and all of the contact numbers. We do this for a reason, so that if you have questions/concerns/comments, you are able to get a hold of us.
My cell phone number is 920-579-1878...call me anytime you have questions.
As a side note, a friend and I fished Crystal Lake on Sunday, June 22nd, for two hours in the late afternoon and boated 17 bluegills that we kept.