Found mainly in freshwater, Blue catfish are native to the Mississippi drainage west of the Appalachian Mountains. These fish were intentionally introduced to the Chesapeake Bay and other East coast rivers under the assumption that they were exclusively freshwater species (NOAA 2020).  Because Blue catfish can tolerate brackish water for moderate periods, they quickly spread along the US East Coast and are now considered an invasive threat to the ecosystem. While the population structure is unknown efforts are underway to stop or slow the spread and eradicate the species in many locations (Chesapeake Bay Program 2020).