This species was revalidated in 2014, after a series of studies highlighted genetic and morphological disparities between grayling subpopulations from the northern Adriatic region and elsewhere in Europe (Sušnik et al. 2001, 2004, Geiger et al. 2014, Bianco 2014).

Earlier publications referring to graylings from the northern Adriatic as T. thymallus thus correspond to T. aeliani and/or hybrid individuals (see 'Population').

The Adriatic Grayling has a restricted range (area of occupancy (AOO) c. 400 km²), which meets the threshold for the Endangered category under Criterion B2 (AOO < 500 km²). It is present at four locations, and the quality of habitat is estimated to be declining. Therefore, this species is assessed as Endangered under Criterion B (B2ab(iii)).

This species is endemic to the northern Adriatic Sea basin, where its native range extends eastward from left-bank tributaries of the Po River system in Italy to the Soča (it. Isonzo) River system in Slovenia, and thus includes the Adige, Brenta, Piave, Livenza, Meduna and Tagliamento rivers plus some smaller systems such as the Sile and Lemene rivers. It is absent from the northeastern Adige River catchment, comprising the Isarco River and its tributaries (see 'Population').

It has effectively been extirpated from the majority of known locations, but putatively native subpopulations inhabit the middle Sesia, lower Pellice and lower Adda rivers in the Po catchment, plus parts of the Adige and probably the Livenza systems (see 'Population' and 'Threats').

Some rivers within its range have not been exhaustively sampled, therefore the possibility that additional native subpopulations exist cannot be discounted.

This species' current population size and trend have not been quantified. It is understood to have declined significantly since the early 20^th century, but quantitative analyses of patterns in its abundance and distribution have been hampered by a lack of both baseline and recent comparative data.

Signals of genetic admixture with non-native grayling lineages have been detected throughout most of its confirmed range (see 'Threats'). A number of analyses have shown that the extent of introgression varies depending on location, ranging from relatively minor to the total collapse of native gene pools, e.g., in the Soča River. Although the majority of analysed subpopulations have been impacted, some have not yet been examined and others may still be discovered (see 'Geographic Range').

Molecular research has demonstrated that non-introgressed native subpopulations occupy short stretches of the Sesia and Pellice rivers, while individuals inhabiting parts of the Adda and Adige rivers exhibit low to very low levels of introgression. For example, recent genetic screening of individuals collected from the Adige by authorities in South Tyrol (Autonomous Province of Bolzano) has revealed that most retain > 95% of their native genetic ancestry.

In addition, unpublished genetic data indicate that at least one native subpopulation still exists in the Livenza River system (Gandolfi, pers. comm., Bravničar pers. comm.).

Significant divergence has also been detected between native genetic clusters corresponding to individual river catchments, including different tributaries within the Po system. These findings have important implications for the conservation management of each subpopulation (see 'Conservation').

Adriatic Grayling genetic signatures are completely missing from the northeastern Adige River, where the resident grayling subpopulation is closely-related to the European Grayling (Thymallus thymallus) genetic lineage inhabiting the Drava River (Danube River system). No recent stocking with non-native individuals has taken place in the Adige, and the most likely scenario appears to be that these fish were introduced several centuries ago in response to the natural absence or local extirpation of Adriatic Grayling.

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".

The adult diet largely comprises benthic and drifting invertebrates, supplemented by terrestrial insects and smaller fishes.

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

Its life history has not been well-studied, but is presumably comparable to that of the European Grayling (Thymallus thymallus). The annual reproductive period is thus likely to extend from spring to early summer, at which point sexually mature adults migrate short distances to their favoured spawning sites.

Grayling typically spawn in flowing shallows with well-washed substrata comprising fine gravel, sometimes mixed with small cobbles or sand. Spawning usually takes place during the latter part 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 tend to exhibit sneaking behaviour.

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 was initially driven by river regulation and other forms of habitat degradation, which 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 foraging 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 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 individuals.

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), has been suggested to represent a significant threat in the Po River system.

In response to declining abundance driven by these factors, restocking with hatchery-reared domesticated and/or non-native individuals originating from the Danube River system and elsewhere in Europe has taken place throughout the Adriatic Grayling's range since the 1960s.

As a result, hybridisation and introgression with these lineages has occurred on an extensive but variable basis, ranging from considerable persistence of native genetic profiles to comprehensive admixture (see 'Population').

The stocking of hatchery-reared individuals also increases the risk of introducing novel pathogens or parasites to native fish communities.

The extent of suitable habitat is likely to be impacted by climate change, due to increased water temperatures and diminishing river discharge.

This species is a popular game fish throughout its range, and this has driven extensive restocking of recreational fisheries with non-native graylings (see 'Threats').

These activities continue at a number of locations today, e.g., the Adda and Brenta rivers, despite being prohibited under national legislation (see 'Conservation').
This species is also widely targeted by recreational fisheries.

This species is included (as Thymallus thymallus) in Appendix III of the Bern Convention and Annex V of the European Union Habitats Directive.

Its native range overlaps the boundaries of numerous protected areas, some of which are included in the European Union’s Natura 2000 network.

In Italy the stocking of non-native grayling lineages is prohibited according to national legislation, but this rule is only enforced in a handful of cases, e.g., since 2012 in the upper Adige River.

In Slovenia, the translocation of grayling individuals between river systems has been prohibited since 2006 and a dedicated conservation programme has been established since 2004. The latter involves selection of individuals with at least c. 70% of native genetic structure for ex situ supportive breeding and subsequent restocking.

Efforts to reinforce abundance at locations where natural reproduction is low or absent have been underway in South Tyrol, Italy, since 2019. Each year, a genetic screening process is applied to several hundred wild age 0+ fry collected from the Adige River. Individuals retaining at least 95% of native ancestry are raised under hatchery conditions until age 2+, before being implanted with a PIT tag (an electronic microchip used for animal tracking) and released at selected sites.

This species is one of two taxa targeted by the European Union co-funded LIFE project "GrayMarble" (LIFE20 NAT/IT/001341), which aspires to improve its conservation status in the Dora Baltea River, a tributary of the Po River in northwestern Italy. Stated aims of the project, which is scheduled to run from 2021-2026, include ex situ propagation of genetically-pure Adriatic Grayling, restoration of connectivity to a section of the river, eradication of non-native fish species and hybrid lineages, and reintroduction of captive-bred, non-hybrid individuals.

Extensive management has been carried out in the Sesia River, including restocking with hatchery-reared individuals, habitat restoration based on improving connectivity and ecological flows and management of Great Cormorant numbers.

Conservation actions led by Turin City Council's nature protection department alongside local anglers and NGOs have been underway in the Pellice River since 2020. A fishway has been constructed on the lower part of the river, close to its confluence with the Po, and genetic monitoring activities have taken place.

Annual efforts to restock recreational fisheries are carried out by local angling associations at numerous locations within the Adriatic Grayling's range. Although these are typically approved by the relevant fisheries authorities, the origin of stocked individuals tends to be disregarded. In some cases, hybrid individuals have even been marketed and stocked as Adriatic Grayling within the framework of official conservation projects.

Native Adriatic Grayling subpopulations exhibit microgeographic genetic divergence (see 'Population'). It is therefore strongly recommended that each should be treated as a separate management unit in order to preserve their unique gene pools by preventing the transfer of individuals between them.

Furthermore, a comprehensive understanding of this species' current distribution represents the major research priority. Many potential locations in Italy have not yet been sampled, and it is therefore plausible that additional native subpopulations have been overlooked or remain undiscovered.