Water monitoring techniques are being stepped up following a suspected pollution incident that led to the death of up to 400 fish in Huddersfield.
The Environment Agency has taken water samples from both the River Colne and Huddersfield Broad Canal in a bid to identify the cause of the deaths.
Almost three kilometres of the river are believed to have been affected and environment officers are carrying out water quality tests as they seek to assess the impact on other river life.
Environment management team leader Mark West said: "There is no obvious evidence of pollution but we are trying to trace the source."
He added that it is "very unlikely" that the fish had been killed by natural causes, such as a shortage of oxygen in the water.
The incident comes after Anglian Water Services received a fine of £75,000 for allowing sewage to leak into the Pix Brook, affecting at least four kilometres of the river and killing more than 2,600 fish.
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