The reason for a large number of dead fish floating about the head of the Waikareao Estuary this week remains unknown.
The regional council's senior pollution prevention officer John Morris assessed storm water outlets upstream and failed to find any sign of discharge from the industrial area that might have killed what looked like horse mackerel.
Dead fish in the estuary.
According to BOP regional council communication advisor Linda Thompson, the dead fish were found throughout the estuary, not just at the outlet of the Kopurereroa Stream.
They were all located in the shallows, which indicate they had been washed up.
There was a northerly wind and an incoming tide when the fish were found and no other aquatic life was affected.
The regional council science team is confident the deaths are the result of a natural event, because of the lack of more sensitive species or a wider range of fish affected.
John is satisfied that the fish kill was not a result of an unauthorised discharge or from contamination.
Fish samples have been gathered and these will be held in the event that information comes to light of any potential source discharge.