According to this report at The Mining Journal (and many other sources), a recent government study concluded that sport fishing in Lake Michigan has the serious potential to be negatively impacted by invasive species. A team of scientists from the USGS Great Lakes Science Center surveys the lake annually, and in fall 2006 they found that while prey fish were at their lowest levels since the 1970s, populations of invasive species like the round goby and the zebra mussel were at their highest ever recorded. Unfortunately, prey fish are crucial food sources for larger game fish, like salmon and alewife, that fishermen seek. The researchers are concerned that zebra and quagga mussels are consuming nutrients that would typically be taken up by zooplankton and other tiny organisms that prey fish typically eat, causing prey fish populations to dwindle and leading to crashes in game fish populations as well. Add to this the confounding fact that Lake Michigan is stocked with millions of game fish annually just to keep populations "stable" and you've got...a complicated mess.