The Thicklip Mullet is widespread and generally considered common. However, fisheries data suggests that the British Isles and western Mediterranean subpopulations have declined significantly since the early 2000s. There is no reason to believe that this trend is not reflected elsewhere, since the ongoing threats are comparable throughout its range. It is therefore assessed as Near Threatened under Criterion A (A2cd+4cd), pending confirmation of the global population trend.

This species is present throughout the Mediterranean and Black Sea basins, plus the eastern Atlantic Ocean where its range extends southwards from Iceland, the Faroe Islands, and southern Norway to the Cape Verde archipelago. The Azores archipelago appears to represent the westernmost limit of its range.

In commercial catch records this species is typically combined with related taxa, especially other members of the genus Chelon and flathead mullet (Mugil cephalus), under the generic term 'mullets nei' (nei = not elsewhere included), thus complicating analysis of its population trends. Nevertheless, evidence from fisheries landing data, recreational catches and studies on juveniles demonstrate the overall population of these fishes is declining in the Mediterranean and northern Atlantic portions of their ranges.

For example, data provided by members of the National Mullet Club suggest the England and Wales population declined by more than 30% from 2006-2016. A decrease in average specimen size is an indicator of over-exploitation in a fish population, and the average weight of individuals captured by anglers in southern England declined by more than 40% from 2008-2017. Anecdotal evidence from Association of Inshore Fisheries and Conservation Authorities officers based in northwest England suggests a concurrent decline in the Irish Sea (A. Burt pers. comm.).

A major decline in the western Mediterranean 'mullets nei' stock saw average annual catches fall from c. 40,000 tonnes per year during the period 1996-2001 to 13,000-16,000 tonnes per year from 2013-2017 (D. Crosetti pers comm.). A recent study demonstrated that grey mullets have become increasingly rare in the Neretva River estuary, Croatia, having once been the most abundant fish species present.

A primarily pelagic species frequenting coastal waters, brackish lagoons, and estuaries, but often enters the lower reaches of rivers. Studies suggest that in the northern part of its range growth is slow and the fish can be more than 10 years old before they become sexually mature, whereas in the Mediterranean basin they mature much earlier.

Some adults are thought to reproduce on a biannual basis. Adults migrate offshore to spawn before returning to coastal habitats. Spawning and is followed by young-of-the-year recruitment to monospecific schools in nursery zones along coastlines or in estuaries. In the winter months adults tend to aggregate in large numbers for overwintering and pre-spawning in established locations. In the Mediterranean, and probably elsewhere, the growth of juveniles and immature individuals is highly dependent on availability of suitable brackish habitat, where they feed primarily on zooplankton. The range of sizes within each age class differs between Atlantic and Mediterranean subpopulations.

The diet is diverse, ranging from detritus and diatoms to amphipods, snails and larvae of chironomid midges. Stable isotope analyses have revealed that the majority of assimilated nutrients derive from the meiofauna, seston and suspended organic matter, with the precise contribution of each resource variable depending on location.

In the Atlantic Ocean this species' slow growth, late maturity, complex reproductive cycle and high site fidelity render it highly vulnerable to over-exploitation, and this is likely to be reflected throughout its range.

The ongoing decline around the British Isles is thought to be driven by commercial overfishing (in particular for juveniles, since where size limits are set they are below the size at which adults are known to become sexually mature), habitat degradation due to increased construction of coastal infrastructure, and water pollution caused by chemicals and compounds used in agriculture and water treatment. These combined pressures are driving an ongoing reduction in available nursery zones for juveniles and subadults.

Winter aggregations of adults are also vulnerable to pair-trawling, tangle netting and shore seine nets. Proposed hydroelectric schemes and tidal barriers are likely to prevent some subpopulations reaching their summer holding areas and block access to nursery zones. In the Mediterranean basin, fry and juveniles are harvested in large numbers to grow on in aquaculture schemes.

The thicklip mullet is harvested commercially (often alongside other grey mullets as 'mullets nei') with gillnets, trammel nets, beach seines, cast nets, and sometimes purse seines or handlines. It is highly valued for the excellent quality of its flesh and marketed fresh as a food fish.

Globally, combined capture fishery production increased by an average of 2.7-4.1% annually in the four decades from 1954-1993, 8.6% annually from 1994-2003 and 2.3% annually from 2003-2014. According to FAO FishStat Plus, the total fishery production for 'mullets nei' in the Eastern Central Atlantic amounted to 30,257 tonnes in 2010. In the UK, commercial landings were historically small, e.g., between 1923 and 1967 the average was 31 tonnes per annum with a maximum of 87 tonnes. This has increased considerably since the turn of the century, with an average of 151 tonnes from 2006-2017 peaking at 209 tonnes in 2010, after which a steep decline to 81 tonnes in 2016 was observed. The increase is thought to have been driven by restrictions placed on landings of European seabass (Dicentrarchus labrax), but the subsequent decrease has seen market values rise and demand outstrip supply.

Fisheries statistics reveal that a total of 13,186 tonnes of grey mullets were produced by capture fisheries in European marine and inland waters during 2013, of which almost 10,000 tonnes was listed as 'mullets nei', and 975 tonnes identified specifically as thicklip grey mullet.

Grey mullets have been exploited for aquaculture for centuries, and have been reported in FAO FishStat statistics since records began in the 1950s. Production in aquaculture intensified during the 1990s and drove a threefold increase in global aquaculture production from 1999 to 2007, peaking at 271,816 tonnes in 2007, but this has since dropped to annual totals of 133,000-150,000 tonnes. Production is mainly concentrated in the Mediterranean and Black Sea basins plus Southeast Asia, with 84% taking place in Egypt. In Europe the biggest producers are Greece, Spain, Italy and Ukraine, and the thicklip grey mullet is among the species utilised, although it is not reported separately in official statistics.

Despite extensive research and experimental trials, artificial propagation has never been successful at a commercial scale, and grey mullet culture is still mainly based on collection of fry and juveniles from the wild. In Italy, 30-58 million 'mullets nei' fry were harvested annually from 1988-2000. In the Mediterranean basin, fry collection for aquaculture continues in Albania, Greece, Israel, Egypt, Algeria, Tunisia, Spain and Italy, but is banned in Croatia, Cyprus, France, Morocco and Türkiye, although many unlicensed fishers also take fry.

There are no specific conservation efforts and few catch limits in place. In the UK, Association of Inshore Fisheries and Conservation Authorities in the southern English counties of Devon and Cornwall have introduced bylaws to control netting within estuaries. The UK Marine Conservation Society considers the species as a 'fish to avoid' from eating.

Evidence that the species is declining in the northern (Atlantic) and western Mediterranean portions of its range suggest that in-depth population studies are required to assess whether this is true elsewhere.