United Utilities has been fined £14,000 over a water pollution incident in 2009.
According to the Environment Agency, over 6,000 fish died at the Three Pools Waterway in Southport due to a pump failure.
A spokesperson for the organisation said pollution had a devastating impact on marine life in the area.
"This incident caused significant damage to the environment, which could have been avoided or considerably reduced if the company had correctly reset a pump when it responded to an alarm," the Environment Agency representative added.
The incident caused oxygen levels to fall, resulting in the death of roach, bream, tench, pike and perch of all sizes.
Officials expect it to take between seven and ten years for the river to recover from the effects.
In related news, the Environment Agency recently took action after a south-west hotel was found to have discharged poor quality sewage into Cornwall's famous Lamorna Cove.
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