Subpopulations inhabiting the upper Danube River system may represent a distinct taxon (Weiss et al. 2021), but are included in this assessment pending a definitive review.

Global and European regional assessment: Least Concern (LC)
EU 27 regional assessment: Least Concern (LC)

Although this species' population trend may be decreasing due to habitat degradation, there is no evidence that the rate of decline approaches the minimum threshold for Vulnerable under Criterion A (≥ 30% over the longer of 10 years or three generations). It does not approach the range thresholds for Vulnerable under Criterion B (extent of occurrence (EOO) < 20,000 km², area of occupancy (AOO) < 2,000 km²) or D2. The population size significantly exceeds 10,000 mature individuals, hence it does not approach the thresholds for Criteria C or D. There exists no quantitative analysis which would permit application of Criterion E.

Therefore, the European Grayling does not currently meet the thresholds for any Red List criteria, and it is assessed as Least Concern both globally and for the EU 27 member states.

This species is native to the European region, where its range extends westward to Great Britain and France (including the Rhône River system in the Mediterranean Sea basin), northward across Fennoscandia to the Kara River and upper Volga river systems in northeastern Russia and southward to the Danube River system.

In Great Britain, native subpopulations are understood to be restricted to the Ouse, Trent, Hampshire Avon and possibly the Severn, Wye, Ribble and Welsh Dee river systems.

In the Danube system, it is absent from the majority of the main stem and larger lowland tributaries.

It has been extirpated from numerous locations (see 'Population' and 'Threats'), but these events are not reflected on the range map accompanying this assessment due to a lack of comprehensive data.

It has been introduced to rivers outside of its native range in the United Kingdom, France, northern Italy, Slovenia, and most of southern and central Finland.

This species' population size is unknown, but it significantly exceeds the minimum threshold for Red List criteria (< 10,000 mature individuals).

The current population trend has not been quantified, and the number of subpopulations is unclear. An accurate global estimate is complicated by a lack of consistent reporting and extensive supportive stocking in many areas.

Overall abundance has contracted significantly since the 19^th century, particularly in the United Kingdom, Central and Western Europe.

The majority of these declines have not been quantified, but in southern Germany it was estimated to occupy > 50% of its original range by the 2000s.

In some cases such patterns may be ongoing at the subpopulation scale, while in others there are signs of recovery (see 'Conservation').

This benthic, rheophilic species predominantly inhabits fast-flowing, well-oxygenated and unpolluted middle reaches of rivers and streams which are often referred to as the "grayling zone".

It is sometimes found in slower-moving downstream stretches, but appears to prefer medium to large-sized channels measuring at least five metres in width and does not occur in headwaters. Lacustrine and semi-anadromous subpopulations exist in Scandinavia and the northern Baltic region, respectively, and enter mildly brackish water in the latter.

Some subpopulations undertake seasonal movements related to foraging or avoidance of winter ice cover, particularly in the northern part of its range. These tend to involve a shift from comparatively shallow summer habitats with swift currents and high prey densities to deeper areas with more stable flow and temperature conditions as drift decreases in winter.

The adult diet largely comprises benthic and drifting invertebrates, alongside smaller quantities of terrestrial insects and smaller fishes.

Adult individuals are territorial and select stream positions in dominance hierarchies based on maximising their energy intake, usually favouring depressions near the centre of the current where they employ a "sit-and-wait" foraging strategy.

Maximum lifespan is 10-13 years, and sexual maturity is reached at age 2-6+, depending on latitude, with females maturing later than males and most individuals spawning annually thereafter. Adult males possess a significantly larger and more colourful dorsal fin than females.

The annual reproductive period extends from late March or April to June at water temperatures of 3-11°C (sometimes to 14°C), although the precise length and timing can vary interannually depending on environmental factors. Subpopulations inhabiting larger river channels and lakes typically migrate to spawning sites in smaller affluents. Both lacustrine and main stem fluvial spawning has been reported in Sweden and Switzerland.

Spawning normally takes place in flowing shallows with well-washed substrata of fine gravel, sometimes mixed with small cobbles or sand. It tends to occur during the later hours of the day when water temperatures are at their peak.

Sexually active male individuals arrive at spawning sites prior to females and aggressively defend territories which may measure several square metres in size. Visual barriers such as boulders or woody structures often play a role in reducing territory size and aggressive encounters between rival males. Females remain in deeper pools and only enter male territories for short periods to spawn. Both males and females may spawn on multiple occasions with different partners during a single season, and some non-territorial males exhibit sneaking behaviour.

Fecundity ranges from c. 10,000 to 31,000+ eggs per kilogramme of body weight, and the eggs remain in the substrate for 2-4 weeks before hatching. The emergent larvae spend a further 4-10 days within the substrate until their yolk sacs are absorbed, then spend a number of weeks foraging in marginal zones with low flow and abundant cover. They later occupy the upper third of the water column close to stream banks, where they prey on drifting and terrestrial invertebrates until reaching a size of 25-28 millimetres, when they move into benthic habitats.

This species' decline has primarily been driven by river regulation and other forms of habitat degradation, which have resulted in widespread loss of the heterogeneous, interconnected fluvial habitats required to complete its life-cycle.

In particular, the construction of dams, weirs and other barriers has altered natural flow and sedimentation regimes, blocked access to spawning and seasonal refuge sites, fragmented subpopulations, interfered with the distinctive habitat shifts required during early ontogenetic development, and generally reduced the extent of suitable habitat for all life stages. The quality of available habitat has been further diminished by bank stabilisation, channelisation and other efforts to enhance flood protection or river transportation links.

Hydroelectric dams have created regular fluctuations in discharge and water temperature (hydropeaking and thermopeaking) which cause dewatering of spawning sites and loss of stable nursery habitat for juveniles, plus downstream displacement and stranding of individual fish.

Furthermore, the combined effect of hydropeaking, dam flushing operations, changes in land use, and the removal of riparian vegetation has increased accumulation of fine sediments at spawning sites, impairing the hatching and survival rates of eggs and larvae.

The industrial extraction of riverine gravel and other sediments for urban development has further reduced the extent of available spawning sites.

This species is also likely to have declined due to widespread agricultural, domestic and industrial pollution during the 20^th century, some of which persists today.

Predation pressure from piscivorous birds, particularly Great Cormorant (Phalacrocorax carbo), is believed to represent a significant threat at some sites.

Interspecific competition with introduced fish species that occupy comparable habitats, e.g., Rainbow Trout (Oncorhynchus mykiss), is sometimes problematic.

Molecular studies have revealed that the European Grayling demonstrates a naturally complex phylogeographic structure which is assumed to be a consequence of this species' limited dispersal ability, relatively narrow ecological niche and the historical existence of distinct glacial refugia. The long-term and often unchecked supplemental stocking with individuals of non-native origin (see 'Use and Trade' and 'Conservation') has thus led to the genetic structure of many subpopulations being significantly compromised through introgressive hybridisation.

Stocking may also drive reduced fitness to local environmental conditions, impair natural recruitment, and impose artificial sexual selection on native subpopulations, while there is no evidence that it leads to increased abundance when ecological conditions are suitable for natural reproduction to take place.

Relatively few unstocked subpopulations now remain, and the practice has also contributed to a dramatic decline of the congeneric Adriatic Grayling (Thymallus aeliani) in the Po River system, Italy.

While pressure through recreational angling is unlikely to have driven significant subpopulation declines, it is understood to compound the effects of other threats at some intensively-exploited sites.

Research suggests that the extent of suitable habitat is likely to be significantly reduced by increased water temperatures, diminishing river discharge and shifts in food availability driven by climate change.

This species is a popular game fish and is particularly targeted by fly-anglers.

It is also taken as food throughout most of its range, but is not harvested on a commercial basis.

In Finland, the recreational catch and financial value exceed those of Atlantic salmon (Salmo salar).

Many rivers throughout its range have been stocked to create or maintain recreational fisheries. Some of these efforts have been carried out on an unauthorised basis, and may threaten native subpopulations (see 'Threats').

This species is included in Appendix III of the Bern Convention and Annex V of the European Union Habitats Directive.

At the national scale, it has most recently been assessed as Endangered in Switzerland, Vulnerable in France, Croatia, Romania and Ukraine and Near Threatened in the Czech Republic and Slovakia. The subpopulation from the northern Baltic Sea was assessed as Critically Endangered in 2013.

It occurs in numerous protected areas of differing designations throughout its range, including some national parks and many sites included in the European Union's Natura 2000 network.

Efforts to supplement declining subpopulations based on stocking and supportive rearing have been ongoing since the 19^th century, but have gradually increased since the 1960s. They have in the past relied on the establishment of brood stocks and failed to take into account the natural genetic divergence between wild subpopulations, leading to widespread translocation and stocking of domesticated and/or non-native individuals.

However, it is increasingly understood that a conservation-oriented management framework must consider the genetic structure of the native subpopulations, the source of the stocking material, and breeding or rearing techniques that aim to maintain genetic diversity.

As a result, there is a gradual trend towards releasing individuals reared from brood stock collected from their native river(s). The use of brood stocks may also be avoided entirely, with gametes instead collected from ripe wild individuals and the larvae and juveniles reared in hatcheries prior to being released as fingerlings. In cases where hybridisation has already occurred, individuals exhibiting the least degree of introgression with non-native lineages are sometimes reared selectively.

Furthermore, management efforts have increasingly focused on habitat improvements such as the removal of barriers, installation of fishways, restoration of heterogeneous natural features, increased residual flows in regulated river stretches and improved control of reservoir flushing operations. The European Grayling is also likely to have benefitted from measures to improve fluvial ecosystems within the framework of the European Union's Water Framework Directive 2000/60/EC.

Species-specific angling restrictions comprising annual closed seasons, daily bag limits and minimum catch size limits are established in a number of countries and in some cases, e.g., Czech Republic, have recently been modified for conservation purposes.

The body of research investigating suitable mitigation measures for the effects of hydropeaking and thermopeaking has grown extensively in recent years, leading to a number of proposals. However, the interactions between hydrological and morphological factors in relation to daily and seasonal aspects of this species' ecology remain poorly understood and require further research.