The Environment Agency restocked the River Witham with 200,000 tench this summer after pollution previously killed tens of thousands of fish.
As previously reported, the EA found back in March ammonia pollution was responsible for the death of the fish in the River Witham near Woodhall Spa.
They were also deployed to the Southrey area after reports of a mass of fish carcasses floating in the water between Bardney Bridge Lock and Anton Gowt.
Environment Agency staff took regular samples of the water quality in the Witham as the river recovered and fish larvae from the EA’s national fish farm in Calverton were released into the river.
Over a million fish larvae have been put back into the Witham.
The investigation is still ongoing as officers continue to gather evidence.
A spokesperson for the Environment Agency told Lincolnshire Reporter: “Our officers are still investigating and gathering evidence. We’ve also undertaken some re-stocking of the river – with over a million fish larvae put back into the Witham.
“Our last batch of re-stocking was 200,000 tench at the beginning of July.”