Salmon spotted in a Yorkshire river after nearly 150 years could signal the success of engineering work undertaken to restore the polluted river, according to the UK's Environmental Agency.
The juvenile 14cm (5.5 inch) salmon was found in the River Dearne, a tributary of the River Don, is evidence of fish spawning in the waters since mid-19th century when industrialisation first took its toll.
Weirs built to power industry or passage to boats had become barriers to the fish reaching their spawning grounds, causing their fast disappearance.
However, the agency completed a fish pass last year at Sprotbrough Weir, opening up 55km (34 miles) of the River Don, almost half the length of the river to salmon and other fish.
The size of the fish found indicates it was born in early 2014, indicating its parents probably used the fish pass shortly after it opened, according to the agency.
The pass reconnects the rivers Don and Dearne back to the Humber Estuary and is part of a bigger plan to create "fish highways" between the sea and the upper reaches of rivers, the Environment Agency said.
Salmon have to live in both freshwaters and the sea to complete their lifecycle, having to cope with threats in both environments in order to survive.
Pollution by chemicals and raking up of sediments by cattle and gravel mining affect spawning of salmon which need gravel beds with clean, cool, oxygen-rich water to hatch eggs.
Jerome Masters, Environment Agency fisheries technical officer, said: "Our rivers are the healthiest for more than 20 years and otters, salmon and other wildlife are returning for the first time since the industrial revolution."
The fish pass was constructed by the EA working closely with the Canal & River Trust, Don Gorge Community Group, a local landowner and Doncaster Council.
Over-fishing has sent numbers plummeting while climate change is also believed to affect salmon populations with the rising sea temperatures.