Trout were among the fish killed

The Environment Agency started an investigation last week after untreated sewage leaked into Howl Beck at Guisborough

It happened when a Northumbrian Water sewer tank overflowed into the watercourse.

The Environment Agency said all life along a 4.5 mile stretch was wiped out with about 6,000 fish dead.

Victims of the spill included thousands of trout, along with several hundred eels, stone loach, bullhead, minnow and stickleback.

Restock fish

But when the discharge first happened, the number of fish killed was put at only a quarter of that figure.

Agency officers had been on the scene within an hour but for the majority of the fish it was too late.

An investigation is ongoing although the Environment Agency has described the impact on the beck as a "disaster."

Alastair Baker, from Northumbrian Water, said the company reacted quickly to the incident.

"It was the monitoring equipment which alerted us to the fact there was a problem.

"If we hadn't taken the action that we did the environmental impact would have been more immediate and greater because the effects would have happened from the pumping station rather than the treatment works."