The Marlborough District Council is investigating why more than 200 eels have died in a creek near Blenheim.
Council scientist Pete Hamill said a vineyard worker found the eels this month along several hundred metres of the creek and notified authorities.
He said the investigation was looking into whether the deaths were the result of a chemical or biological cause.
"Our thought looking at the waterways is that it's more likely to be chemical, but until the tests come back we can't really ascertain one way or the other."
Mr Hamill said tests included isolating the range of chemical sprays applied in surrounding land uses.
He said it was not clear if other fish and aquatic species in the stream had also died.