The ruffe (Gymnocephalus cernua) is a small, spiny freshwater fish found in lakes, rivers, and canals across the UK. While native to parts of Europe and introduced to the UK, it has become an invasive species in some water bodies, particularly in Scotland, where it has disrupted local ecosystems. Ruffe are opportunistic feeders, preying on invertebrates and fish eggs, which can impact native species, including important populations of whitefish.