Setpember 2025
Encouraged by cooling autumnal water temperatures and stronger river flows, a remarkable species of fish will shortly begin its annual migration upstream to spawn in rivers across Wales. To give their offspring the best chance of survival, the fish will try to reach as far upriver as possible, often into streams only a couple of feet wide where the water is at its purest and where there are fewer predators. This fish is not the Atlantic salmon but a close relative; our native brown trout (Salmo trutta) or “brithyll du” in Welsh.
Brown trout first arrived in the UK around 13,000 years ago at the end of the ice age. Making use of an ability to live in saltwater as well as fresh, the fish migrated from the continent and took up residence in the newly flowing Welsh rivers that had burst into life after the glacial retreat.
Brown trout in rivers across Wales will soon begin their journey upstream to spawn in headwaters.
Evidence suggests that the Usk’s trout are getting bigger.
A March brown (Rhithrogena germanica) on the River Usk. (Photo: Tim Hughes)
Downloadable River Usk fact sheet.
It is an exceptionally adaptable and resilient species, one of the most genetically diverse vertebrates. Trout populations can vary widely between rivers and even within the same stream, with different genetic strains adapting to different conditions.
According to the Wild Trout Trust, British trout have far more genetic variation than exists between any human populations anywhere in the world. Exported to the far corners of the world during colonial times, this northern European fish species now resides in rivers as far afield as East and South Africa, Tasmania, New Zealand, Sri Lanka and Argentina.
The river abounding in fish
One Welsh river in which Brown trout has flourished is the River Usk. In fact, so much so that the translation of its Welsh name (Afon Wysg) is “river abounding with fish.” Of course, this name could also refer to other species once highly prolific in the river, such as salmon, lamprey and shad. Early literary references to the Usk and its fish do not tend to differentiate between species, rather remarking on the total volume of all, but the river has long held a reputation for the number of brown trout it supports.
Rising on the slopes of Mynydd Du (Black Mountain) in the western area of the Brecon Beacons, the Usk quickly enters into the highest of several reservoirs in its ctachment – the Usk Reservoir. From there, it flows in a general easterly direction through Sennybridge and Brecon, before taking a more south easterly route through Abergavenny and finally turning south through Usk Town and, some 126km from its source, entering the Severn Estuary at Newport. At just 4km longer than the Tywi, the Usk is the longest Welsh river to flow entirely in Wales.
It is one of Wales’s most protected rivers, designated a Special Area or Conservation (SAC) for its salmon, lamprey, shad, bullhead and otters, amongst other features. It is also a Site of Special Scientific Interest (SSSI) throughout its length.
Pressures on the Usk’s trout
Although largely spared the high levels of industrial pollution of Welsh rivers to the south, the Usk and its trout populations have experienced their fair share of issues, particularly human over-exploitation in addition to water quality and quantity problems.
According to George Agar Hansard in his 1834 book “Trout and Salmon Fishing in Wales,” the river was not worth bothering with from an angling perspective downstream of Crickhowell because of the polluted Clydach tributary, poisoned by a local iron works. An 1878 report in the Monmouthshire Merlin describes a sewage pollution and severe fish kill on the Olway Brook “killing everything in it from the works to its junction with the Usk.” To this day this lower Usk tributary remains blighted by poor water quality, mainly as a result of agricultural pollution.
The “finest trout stream in the world”
At the start of the 20th century, the Usk’s trout numbers appear to have held strong, so strong in fact that the local clergy were concerned how much of a threat the lure of the river presented to Sunday Service. In 1916 a Brecon vicar, the Reverend D. Saunders Jones, requested the Usk Board of Conservators put a stop to all Usk fishing activity on the Sabbath, a request that appears to have been completely ignored. A few years before that, the Board’s Chairman had boasted that the Usk was “the finest trout stream in the world.”
And a year later, a Captain Adams wrote to the editor of the County Times stating that “The Usk is an exceedingly prolific trout river,” arguing that the fish should be harvested to ease food shortages that were affecting the country in the latter stages of the Great War.
With occasional fluctuations, the Usk’s trout numbers seem to have remained healthy over the proceeding century and a bit, at least in the middle and upper reaches of the river. However, in his article for the 2025 edition of the Wild Trout Trust’s magazine (‘Salmo Trutta’), Dr Guy Mawle noted a relatively recent change in the dynamics of Usk trout populations, in that the fish appeared to be getting bigger. He pondered over a number of potential reasons, including climate change leading to quicker growth rates and the higher catch-and-release rates by modern anglers.
March browns
Another possible factor could be changes in the trout’s available food sources. Usk brown trout, as is normal for the species, have a varied diet: a wide range of invertebrates and other creatures, including smaller trout. The river used to be just as famous for early spring hatches of a particular aquatic insect, the March brown (Rhithrogena germanica). George Hansard noted in 1834 that the greatest hatch he had ever seen of the fly was on the Usk at Brecon. For whatever reason, however, March browns had all but disappeared from the river (and those nearby) by the early 2000s.
In 2019, the Wye and Usk Foundation noted that March browns were returning, perhaps not in the same numbers that Hansard witnessed, but nevertheless present. Good hatches of the insect have been reported every spring since, providing an important food source to hungry trout eager to retain condition following their winter spawning. While the return of the March brown probably has little bearing by itself on the overall dynamics of the river’s trout population, it does show that their food sources can fluctuate quite dramatically.
Free passage
In addition to good water quality and quantity, an important factor in the health of a trout population is the ease with which they can migrate both up and downstream. Most fish species rely on the ability to migrate to another section of river, or in the case of brown trout, even to sea. Populations that are restricted in this ability are far less resilient to threats such as disease, predation and pollution.
Since starting their Usk Project (UP!) in 2000, the Wye and Usk Foundation have been working hard to restore access to tributary streams throughout the catchment. The work has involved the removal of small weirs and obstructions to construction of large, complex fish passes. Over the next twenty-five years, the trust completed around 50 fish easements, improving or restoring access to over 100kms of tributary stream for migrating salmon and trout. Despite this monumental effort, there is still more to be done to improve fish migration in the Usk catchment by the Foundation.
Fish migration easements can involve complex fish pass construction or simple removal of blockages. The upper two photos show the clearance by Wye and Usk Foundation staff of debris blocking an existing fish pass on the Honddu, an upper Usk tributary. The lower photos show fish pass constructed on the Cynrig (left) and the Riangholl (right), two middle Usk tributary streams.
Meanwhile, Natural Resources Wales are also looking into improving migration in the main stem of the Usk, particularly over the bridge footings at Crickhowell and Llanfoist in Abergavenny. This is mainly for the benefit of the river’s Twaite shad who unlike salmon and trout, do not leap and struggle to find their way over relatively small obstacles.
It is also resolving the centuries-old issue of fish migration over Newton Weir in Brecon, built in the late 18th century to supply water to the Brecon and Monmouthshire Canal. This weir has been shown to be a major obstacle to salmon smolts migrating to sea, delaying their journey and making them especially vulnerable to predation.
Natural Resources Wales are working to improve fish migration over the bridge footings at Crickhowell. The 18th Century bridge is a Grade I listed building and a Scheduled monument. The work will benefit the Usk’s Twaite shad in particular.
In 2025, ospreys bred for the first time in the Usk valley for hundreds of years.
A new threat to the Usk’s trout
Brown trout in the Usk and across Wales continue to face challenges from agricultural and water company pollution, over-abstraction, barriers to migration amongst others.
However, one new threat is more welcome. In 2023, a pair of ospreys built a nest near the river a few miles downstream of Brecon, the first time the birds had been recorded in this part of Wales for hundreds of years.
In 2025, the pair successfully raised a chick; good news for this very rare bird and for ornithologists, but perhaps not such good news for the Usk’s trout and other fish. The website set up to celebrate the event headlines with an image of an osprey clutching a twaite shad – a highly protected bird predating a high protected fish. In the Usk Valley, work to protect and preserve the latter has become even more important.
Wild populations of Brown trout, or ‘Brithyll du’ in Welsh, can be found in rivers, reservoirs and llyns throughout Wales with variations in size and appearance showing how versatile the species is in adapting to different conditions.
Some waters have been stocked artificially. 40,000 trout from Loch Leven were introduced into the Vyrnwy reservoir, for example, shortly after it was constructed. In the Usk, stocking of brown trout continued up until the early 2000s to enhance angling prospects.
Sea trout, known as ‘Sewin’ in Welsh are brown trout that for reasons still unclear (possibly food abundance), migrate to saltwater for various lengths of time before returning to the rivers to spawn. The Usk is not among the famous Welsh sewin rivers, which include the Teifi, Towy and Dyfi, probably because there is enough food abundance to support most of its resident trout.
Our thanks to the Wye & Usk Foundation for information and photos.
References & More Information:
“A tale of two fisheries. The Usk past and present.” Dr Guy Mawle, 2025 Salmo Trutta, the magazine of the Wild Trout Trust.
“Trout and salmon fishing in Wales.” George Agar Hansard 1834
Key species on the menu for fish in the Usk and Wye catchments.