L. salmonis is a salmon-infesting specialist that plagues aquaculture operations, which explains why it’s drawn the most attention. But it is not the only louse that hurts salmon—Caligus clemensi is a generalist that attacks salmon as well as other fish. “One of the things that is still unclear in the world of salmon lice is how these two species co-infect different Pacific salmon species,” says Cole Brookson, a biologist at the University of Alberta.

To answer that question, Brookson and his colleagues analyzed a huge data set, compiled from observations made between 2015 and 2019, that showed how many lice were infecting juvenile wild salmon of different species off the coast of British Columbia. Using this data, they determined how each kind of louse affects the different species of salmon. They focused on juvenile salmon because the period when a fish is leaving its natal stream and is heading out to sea is the most crucial part of its life cycle. This is when it faces the most danger from predators, and when lice infestations can do the most damage.