River conservationists are working to find out if two salmon found dead in the Wye south of Hereford are a sign of a bigger problem in the river.

“We are trying to work out if it’s localised or more general,” Wye and Usk Foundation chief executive Simon Evans said.

“Fish in the river are being assailed by multiple pressures – high temperatures and low oxygen caused by algal blooms as well as disturbance from canoes.”

The two dead fish were reported and photographed yesterday (June 28) lunchtime by the Wye Salmon Association.

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The Wye would be expected to be around 16-18°C, but has been at 23°C for a week – high, but not as high as the 27°C considered lethal to river salmon, Mr Evans said. “So we think it’s something more insidious.”

Algae photosynthesise (that is, produce oxygen) by day but respire by night, meaning oxygen levels in the river are at their lowest in early morning, he explained.

Algae thrive in water enriched with nutrient runoff from farms and other sources, which has been linked with the Wye's official "declining" environmental status.

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The Environment Agency’s monitoring stations, including at Hereford’s Victoria Bridge, have shown oxygen levels to have been dangerously low in recent mornings.

“So it is likely to be a combination of the two,” Mr Evans said.

Migrating salmon are an iconic species of the river, which is both a site of special scientific interest (SSSI) and special area of conservation (SAC) for most of its 152-mile length.

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“The plight of the Atlantic salmon is a recognised barometer of overall water quality and the health of the environment,” the Wye Salmon Association says.

But it reckons numbers are now only around a twentieth of the river’s historic peak with between 2,000 to 3,000 salmon “runs” a year.