The US Geological Survey (USGS) studies the invaders using marine lampreys caught in traps. They separate the females and kill them to restrict their reproduction. Before being released back into the Lakes, the males are injected with a sterilant to prevent subsequent progeny. It's unknown why they don't kill the males as well, but it's possible that such attempts would be too swift and disrupt the ecosystem's equilibrium (even if it is currently weighted in favour of the destructive sea lampreys).