An emergency has been declared after slurry split from a container into a mjor river, killing dozens of fish. 

The spill came from Plumpton College and has left Bevern Stream -which  into the River Ouse - black and covered in a foam.

At least 70 fish have been found dead but it is feared more could have perished.

The slurry, which is a mixture of urinary and and faecal excerement from a dairy head, spilled out of the container following a mechanical fault. 

The Environment Agency has declared it a level one incident - the highest possible - with the slurry spread across 4.3 miles of the stream. 

Sam St Pierre, vice-chairman of the Ouse and Adur Rivers Trust said: “These streams are most important trout breeding streams in the whole catchment and its just been absolutely ruined.

“At this time of the year the sea trout are coming up to spawn and we have found at least 70 dead sea trout in peak breeding condition.

“It’s heartbreaking and an absolute tragedy.”

Plumpton Mill Stream is a classic chalk stream and a rare habitat supporting an abundance of wildlife rarely found elsewhere.

The Environment Agency said the pollution has been contained with officers now battling against the slurry in the Bevern stream. Efforts to oxygenate the water are expected to continue over the weekend. An investigation has also been launched. 

Michael Turner,  Environment Agency area environment manager, said: “We are taking this incredibly seriously in our response and in our desire to make sure it doesn’t happen again.

“Once we have completed the investigation we will weigh up the evidence and the impact of the pollution

“We have worked for many years with Ouse and Adur Rivers Trust to improve this water course for brown and sea trout so we are disappointed and frustrated this has happened.”

Mr St Pierre added:"We were told last year that they had refurbished their slurry management infrastructure and reviewed operational procedures so that these episodic pollutions would now cease."

A spokeswoman said: "Unfortunately, we believe that the mechanical failure of a pump led to us having to distribute some dirty water late on Tuesday night and as a result, some of that entered on of out streams. 

"Thankfully, this was noticed quickly and we are pleased to know that the watercourse is now running clear."