Tests have failed to reveal what killed more than 200 eels in a creek near Blenheim last month.
The Marlborough District says only one of a possible 68 different pesticide chemicals was identified - the broad spectrum herbicide Glyphosate.
Council compliance manager Gina Ferguson said it was found at barely detectable levels.
A vineyard worker found the dead eels along several hundred metres of the creek and reported it to the council on 10 February.
The council said its compliance team will talk to the owners of all neighbouring properties to remind them about the risks and procedures when dealing with agri-chemicals.