If you go back and look at the history of botulism, and this kind of kill-off, it's only in the past couple of decades that we have been seeing regular occurrences of this. It's a bit of a perfect storm. It's not just the invasive species; it's a bit of the warming of the water. It's the clarity, which is also invasive-species-related, in that the zebra mussels and quagga mussels are now clearing out the water columns, so that the sun is penetrating lower and growing algae at the bottom of some of these lakes that have never had algae growth there before. It's a bit of a perfect storm that this is happening there.
On April 30th, 2012. See this statement in context.