MYSTERY still surrounds how 60,000 fish were killed in one of Dorset's worst pollution cases.

After two-years the Environment Agency say the file on the matter remains open although they are no longer actively investigating.

Following the spill of highly poisonous farm waste wiping out fish stocks in the River Cale - a tributary of the River Stour - in North Dorset in July 2000, Environment Agency inspectors visited 20 farms in the area but failed to find the direct cause of the pollution.

However, in the course of their inquiries they did discover a discharge containing massive amounts of ammonia flowing into the river from Higher Nyland Farm at Gillingham.

At Blandford magistrates court on Monday, farmer Brian Bennett was found guilty after a trial of allowing poisonous, noxious or polluting matter to enter the river on August 2, 2000.

Mr Bennett was not directly linked with the pollution incident which had occurred days earlier.

Julian Wardlaw, from the Agency, told magistrates that during his routine inquiries he found a discharge pipe dispensing a green-brown liquid at the rate of two litres per second from the farm.

Prosecutor Simon Rundle said a lab report revealed the discharge contained a Biochemical Oxygen Demand reading of 701 milligrammes per litre and an ammonia level of 185mg/litre. The BOD for raw sewage is between 300 and 400 mg per litre.

A sample of river upstream showed healthy water quality but downstream of the pipe there was evidence of increased waste in the water.

In court, Mr Wardlaw said: "The readings are consistent with some pollutant having entered the watercourse. The BOD reading would certainly have began to stress aquatic life had there been any in there but there wasn't because of the previous pollution incident."

Bennett was given a six-month conditional discharge and ordered to pay £500 costs.

After the case Mr Wardlaw said: "The polluting discharge was revealed during our investigation into one of the worst fish kills on a Dorset river for years.

"It is vital that everyone is aware of the potential for pollution and the devastation that could occur."

Anyone with information on the fish kill should contact the Agency's pollution hotline on 0800 80 70 60.