https://phys.org/news/2018-05-ocean-migrating-trout-freshwater-environment-years.html

The steelhead introduced into Lake Michigan continue to spawn in small freshwater tributaries and streams, but now treat the entirely freshwater habitat of the Great Lakes as a surrogate ocean. After their introduction into Lake Michigan, steelhead began to naturally reproduce and established self-sustaining populations throughout the Great Lakes. To examine how these fish adapted to this novel environment, a team led by Mark Christie, an assistant professor of biological sciences at Purdue University, sequenced the complete genomes of 264 fish. The team then compared steelhead from Lake Michigan to those from their ancestral range, searching for outlier regions associated with genetic adaptation.

The research, which was published in in the journal Molecular Ecology, found that regions of three chromosomes in steelhead evolved after they were introduced in Lake Michigan, offering insight into how this ocean-migrating fish adapted to an entirely freshwater environment.