Staff at the Environment Agency will be working with Yorkshire Water to improve water quality close to a waste water treatment works after pollution escaped into the River Calder.
The Environment Agency investigated the incident which was reported by members of the public on April 25.
Officers found dead trout and grayling between Copley and Cromwell Bottom, near Brighouse and traced the pollution to Halifax Waste Water Treatment Works at Copley.
This stretch will now be surveyed to assess whether the pollution has caused any long-term impact, and the results will be used to determine whether fish restocking is needed.
Environment Agency team leader for Calderdale Tracy Langdale said: “Our investigation found that the problem was caused when the power supply was cut to the treatment works. This meant that the sewage being discharged into the river was only partially treated.”
Halifax treatment works is allowed to discharge under these conditions as part of its consent permit which is issued by the Environment Agency.
Staff are now reviewing this permit to reduce the amount of ammonia that is allowed to discharge into the river. Ammonia is present in sewage and is toxic to fish.
Yorkshire Water is also carrying out a programme of improvements at the treatment works which will further reduce the risk of pollution.
Tracy said: “Our rivers are a valuable part of the environment and good water quality is vital for the health of wildlife and their habitats. We are working with Yorkshire Water to make sure that everything is being done to protect the River Calder.”
The power supply was cut to Halifax treatment works on the afternoon of April 24, and Yorkshire Water brought in an emergency generator later that day.
Environment Agency officers attended the pollution incident and counted more than 200 dead fish but a survey of the area showed the impact on the river was limited to the stretch between Halifax Copley Outfall and Cromwell Bottom.