The brook trout (Salvelinus fontinalis) is a non-native fish species in the UK, introduced from North America for sport fishing in the late 19th and early 20th centuries. It is primarily found in cold, clear streams and upland lakes, often cohabiting with or competing against native species like brown trout. While its ecological impact varies, brook trout can displace native fish in some areas due to their adaptability and high reproductive rate. Conservationists monitor their presence to assess potential threats to local biodiversity.