The study looked at lakes in Wyoming’s Wind River Range, a stretch of the Rocky Mountains that is home to glaciers, high peaks and alpine lakes that historically had no fish. Beginning in the early 20th century, wildlife managers began stocking the lakes with trout for fishing enthusiasts, leading the lakes’ native zooplankton to evolve to become smaller as larger plankton turned into fish food.

During the summers from 2018 to 2021, researchers collected hundreds of fish from 18 high-elevation lakes along with information about the lakes’ size, depth, elevation, vegetation and fish stock history. Then they compared the fish they found with fish born in Wyoming hatcheries.

The lake fish, it turned out, had higher numbers of structures called gill rakers — bone or cartilage in the esophagus that allow fish to strain more plankton into their stomachs. Researchers found more, longer gill rakers in the gullets of fish that lived in lakes stocked decades ago. They think the structures evolved as a result of the need of the fish to feed in the zooplankton-rich lakes, and that natural selection favored fish with bodily structures that allowed them to gather as many zooplankton as possible. The researchers write that the evolution probably happened because of these external pressures and not simply random genetic variations.

The pace of change varied by lake, with some fish taking 47 years to develop higher numbers of gill rakers and others not showing more gill rakers even 88 years after their lake habitats were first stocked.

Overall, though, the researchers say the evolutionary changes took place rapidly and may show that some animals can respond quickly to other human-caused habitat changes. They call for more research on whether fish in other lakes evolved similarly or as rapidly.