Grey mullet belong to a large family, which comprises some 80 species of marine fish, known as Muglidae, and is a common inhabitant of marine coastal waters in Europe. The thick-lipped grey mullet is the most common of three species which occur in northern European waters. They are slow-growing, long-lived and late-maturing fish, which makes them susceptible to overfishing. They live to around 25 years old and mature around 9 years old (42 cm) for males and 11 years (47 cm) for females. Thicklipped mullet are thought to spawn on alternate years. Grey Mullet spawn in the English Channel and Irish Sea and potentially in estuaries in Eastern England. They spawn in open water and after around two to six weeks, the then juvenile fish move into inshore waters, especially estuaries. Their occurrence is impacted by temperature and they are often observed near the surface (0-10 m depth). Because of this, their large size, and east to capture, they can be popular with anglers.

Grey Mullet are catadromous (which means that they migrate from freshwater to seawater to spawn) and can be found in brackish lagoons and at the heads of estuaries because they can tolerate very low salinity levels. Grey Mullet are grazers, the scrape the surface off mudflats to eat diatoms and other algae off surfaces.