This species is not recognised by the relevant authorities in either of the two territories to which it is native. Alongside the Sonaghen (Salmo nigripinnis) it represents one of two endemic trout taxa inhabiting Lough Melvin, which is also occupied by the congeneric Ferox Trout (Salmo ferox).

These three are morphologically and genetically distinct, have contrasting feeding preferences (benthivorous, planktivorous, or piscivorous) and are reproductively isolated by their natal homing to discrete spawning sites. Mitochondrial DNA analyses suggest that they did not evolve via adaptive radiation, but diverged allopatrically and colonised Lough Melvin independently during the postglacial period (Ferguson et al. 2019; Ferguson and Prodöhl 2022).

European regional assessment: Vulnerable (VU)
EU 27 regional assessment: Vulnerable (VU)

The Melvin Gillaroo is putatively endemic to Lough Melvin in the Drowes River system, northwestern Ireland. It has a restricted range (extent of occurrence (EOO) c. 32 km²), which meets the threshold for the Critically Endangered category under Criterion B1 (EOO < 100 km²). It is restricted to one location, but there is no indication of continuing decline or extreme fluctuations, hence it does not qualify for a threatened category under Criterion B.

There is no evidence of any population size reduction that would approach the threshold for Vulnerable under Criterion A (≥ 30% over the past ten years or three generations). The population size does not approach the thresholds for Vulnerable under Criterion C (< 10,000 mature individuals) or D1, and there exists no quantitative analysis of extinction probability which would permit application of Criterion E.

Therefore, this species is assessed as Vulnerable under Criterion D2, based on its presence at one location which is threatened by pollution and the introduction of non-native species that could drive it to Critically Endangered or Extinct in a very short time period.

This species is putatively endemic to Lough Melvin (ga. Loch Meilbhe) in the Drowes River system, northwestern Ireland. This lake spans the boundary between County Leitrim in the Republic of Ireland and County Fermanagh in Northern Ireland (United Kingdom).

This species' current population size and trend have not been quantified, but the results of recent field surveys indicate that it remains stable in the lake.

Naturally oligo-mesotrophic and polymictic Lough Melvin was formed after the last glacial period and has a maximum depth of c. 45 metres, although c. 46% of the lake is less than five metres deep. It is located close to the Atlantic Ocean so is exposed to westerly winds that promote thorough mixing, and only a handful of short, sporadic stratification events have been recorded. The lake's water is notably stained by peat, and the resulting dark colouration is understood to strongly limit primary production. The surrounding land is sparsely-populated but largely turned over to forestry and low-intensity agriculture, particularly livestock grazing.

The lake is inhabited by a unique sympatric salmonid community comprising three trout (genus Salmo) taxa, Atlantic Salmon (Salmo salar) and Alpine Charr (Salvelinus alpinus).

This species primarily occupies the benthic zone where it feeds on molluscs and other zoobenthos, but typically approaches the surface to feed on terrestrial insects during the late summer.

The annual reproductive period extends from November to December, when nuptial individuals migrate to spawning sites in the outflowing Drowes River.

Rainfall and runoff in the Lough Melvin catchment are relatively high and the lake is therefore threatened by nutrient enrichment.

A rapid increase in phosphorous levels observed between 1995 and 2001 was linked to the clearcutting of conifers on surrounding peaty soils, and the lake has since remained within the upper limit of mesotrophic classification. However, the abundance of algae has not increased significantly due to the peat-stained water limiting the depth at which photosynthesis can occur (see 'Habitat and Ecology').

Diffuse agricultural pollution is estimated to account for > 60% of nutrients entering the lake, with additional sources including forestry activities plus point source discharges from small wastewater treatment plants and septic tanks.

Should the lake ever reach a eutrophic status, the extent of suitable Melvin Gillaroo habitat could be reduced through oxygen depletion in deeper areas.

The most abundant fish species in Lough Melvin is currently the non-native Eurasian Perch (Perca fluviatilis), of which older individuals may prey on small trout while most life stages compete for benthic resources.

Other non-native fish species recorded in the lake include Common Roach (Rutilus rutilus) and Eurasian Rudd (Scardinius erythrophthalmus), in addition to hybrids between these two species plus Common Roach x Common Bream (Abramis brama). Although survey data suggest that their abundance is currently low they are nonetheless expected to compete with native fishes for food resources, and their presence has been linked to declines in other Irish salmonid species.

Alongside eutrophication and the expansion of non-native fish species, rising water temperatures due to climate change represents a plausible ongoing and future threat.

Lough Melvin is among the most important salmon and trout recreational fisheries in northwestern Ireland. This species is among those targeted, and is also taken for human consumption.

The portion of Lough Melvin located in the Republic of Ireland is a Special Area of Conservation and included in the European Union's Natura 2000 network of protected areas (site IE0000428).

In the late 2000s, an integrated Catchment Management Plan was developed within the framework of the European Union's Interreg IIIA programme in order to advance the fulfilment of good ecological status in the lake and address the threat of nutrient enrichment. Recommendations covered agriculture, forestry and urban wastewater, but the project did not remain active beyond 2008.

Official fish stock surveys have been carried out in Lough Melvin every 3-4 years since 2008, but there are no specific management actions for the Melvin Gillaroo.