From NPR.

Traces of pharmaceutical drugs are finding their way into waterways around the world. A new study in the journal Science suggests that this kind of pollution can mess with salmon migration in somewhat surprising ways.
Jack Brand, a biologist at the Swedish University of Agricultural Sciences, and his colleagues implanted slow-release pharmaceuticals into 279 hatchery-raised Atlantic salmon.

Special trackers helped the team monitor both exposed and nonexposed fish as they swam from release sites across multiple dams to eventually reach the Baltic Sea. Surprisingly, they found that more clobazam-exposed fish eventually migrated through the river and reached the Baltic Sea than unexposed fish.

The team found that fish exposed to drugs traversed often harrowing dams two to three times faster than fish that weren’t drugged.

“We suspect that these fish that are exposed to clobazam are more risk-prone, more solitary, and therefore just sort of beelining it through the dams rather than waiting around for their salmon friends,” said Brand.

However this change in behavior would likely have negative consequences in other contexts. For example, subsequent lab experiments found that clobazam made salmon less likely to school with other fish. That could make them more vulnerable to getting picked off by a predator.