@dill1971: That’s a really good question. I did a report on it in my Biology class and it’s really hard to understand. The way that the UMESC explains it is that the chemicals are poured into the lakes that are infested with the sea lampreys. It kills the sea lampreys when they are young. When the lampreys eat the chemicals or get covered in the chemicals, it will die before it matures. That means that the lamprey will not be able to harm any of the native fish or produce any more lampreys.
From what I’ve read, this method greatly reduces the lamprey population and somehow does not affect the other fish. Of course, some of the other fish die but the chemicals seem to attract the lampreys the most.