Fish died in an ‘ecological disaster’ at Warnham Mill Pond Reservoir after a failure with the sluice gate caused it to empty on Monday morning.
During a planned Environment Agency (EA) operation to lower water levels the main sluice gate failed to close properly leading to fish being washed downstream into Boldings Brook before its officers were able to manually close the gate.
The agency confirmed that around 250 adult fish had died and many smaller fish were also killed.
Dog walker Geoff Nichols described seeing hundreds of dead and dying fish on the banks of Boldings Brook as he walked his border collie cross Freddy along the Riverside Walk on Monday morning.
Works are part of £1.3m project to improve the reservoir’s safety during extreme flood conditions, which is being part funded by Horsham District Council (HDC).
Christian Mitchell (Con, Holbrook West), a Horsham district councillor and a local ward member, said: “This is nothing short of an ecological disaster and it is difficult to see how the situation could be much worse than it in fact already is.
“All the hard work of rangers and ecologists over many years in building up this nature reserve has been lost, which must be devastating to all those involved.
“The Environment Agency have some urgent questions to answer on how and why this happened and I look forward to reading their report on it. I want and expect them to produce an interim report within a week. They will need to set out what risk assessments were done, if any, by the EA and how foreseeeable this was.”
He voted against the ‘wasteful’ and ‘absurd’ £1.3m scheme, which will allow the reservoir to safely withstand a one in 10,000 flood event, and called on the EA to set out how it would restore the ecology of the mill pond.
Christine Costin (LDem, Trafalgar) added: “It is difficult to express how depressed this situation makes me feel, I am frustrated to know that key issues had not been resolved after past events. Everyone who has any connection with the Warnham Nature Reserve is bound to be upset by what has happened.
“Volunteers and staff have worked tirelessly to manage and improve this area and by creating interest and fostering enthusiasm in people of all ages, especially the young.
“This is a calamity that should not have happened.”
Following the incident HDC undertook an ‘ecological survey’ to discover the extent of damage to the ‘aquatic ecology’ on Tuesday.
Jonathan Chowen (Con, Cowfold, Shermanbury and West Grinstead), HDC’s cabinet member for arts, heritage and leisure, expressed ‘serious concern’ for the reservoir’s ecology, and praised council officers for ‘working tirelessly’ to support the EA.
He added: “The rescue operation that was mounted saved many fish that had got caught downstream. They did an excellent job in very difficult circumstances.”
The EA has inspected the river downstream and is investigating why the sluice gate failed to close.
David Willis, area duty manager for the EA, said: “Our initial investigations have shown that this was because an alarm sensor prevented the gate from closing in the normal manner and manual over-rides did not function properly.”
Apart from the fish who had died he said that there had been no impact on nature conservation downstream of the sluice.
A spokesperson for HDC said: “A new steel sheet pile protective wall was being installed in front of the existing brick wall of the embankment as part of a £1.1 million improvement scheme.
“It was planned to reduce the water level in the mill pond by 0.5 metres to enable driven steel piles to be set in to place. It was during this process that the gates failed.”
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