They were so small, you couldn’t even see them with the naked eye. But their deadly legacy suffocated 150,000 fish.
Scientists now believe tiny bacteria reached “unnaturally high” levels in the Old Bedford River around Welney last August.
When the microscopic organisms died, their decomposition robbed oxygen from the water – killing tens of thousands of roach, bream, perch, pike and zander, in what the Environment Agency described as one of East Anglia’s worst-ever fish kills.
Low flows and increased nutrients leaching into the shallow, man-made waterway created ideal conditions for the microbes, a 60-page report into the incident concludes.
EA fishery chiefs are now drawing up an action plan, in the hope future outbreaks can be prevented.
“The deaths appear to have been caused by the sudden explosion of small organisms,” said agency spokesman Paul Waldron.
“The conditions were just right for it to happen and we want to avoid the conditions being right again.
“It will take a partnership approach because there is no simple solution.”
Fish stocks in the Bedford will be surveyed in the spring, before officials decide whether to restock the affected stretch, or allow fish to recolonise it naturally.
A rare stone loach, found in just a handful of rivers, was feared to be among the casualties.
But other stretches of the drain, including Purl’s Bridge and Welches Dam, escaped the catastrophe.
Smaller fish kills are common on the Bedford and the neighbouring Delph, which runs alongside it.
Both can suffer as stale water is pumped off Welney Washes – a 20-mile-long area of land used as a flood control reservoir. But nothing on the scale of last summer’s kill, when dead fish were removed by the lorry load, has been seen before.
