Sea lampreys travel through many of the rivers in Europe and North America as they migrate between the places where they lay eggs and the ocean. Now they face a new threat: the enormous European catfish people have brought to the rivers where sea lampreys have always traveled. So what happens when sea lampreys meet these catfish? To answer this question, we attached sensors to sea lampreys: one that would let us track their movements in the Garonne and Dordogne Rivers in France and one that would tell us if they were eaten by another fish. We found that many sea lampreys are eaten by European catfish, and these new fish pose a serious threat to the survival of sea lampreys in their native habitat.