A faulty submersible pump that electrified the water in Mill Creek near Arrowtown is the most likely cause of death for about 20 brown trout.
Otago Regional Council environmental monitoring and operations director Scott MacLean praised the ''detective work'' by the council and Otago Fish & Game staff who worked with local fishing guide Grant Adolf to investigate the cause of the fish deaths earlier the month.
Fish & Game notified ORC of dead fish in the creek at the beginning of June - several days later, during a site visit, more dead fish were found.
Water samples, sent for analysis, showed no high levels of chemicals or heavy metals except aluminium in the vicinity of the pump.
Two of the dead fish were sent to the Cawthron Institute in Nelson, where autopsies showed no sign of poisoning or disease.
However, while ORC environmental officer Richard Heyward was collecting water samples with colleagues he noticed a submersible pump in a privately-owned wetland adjoining the stream.
Mr Heyward recalled Fish & Game staff's comment it was as if ''something'' was causing a magnetic field attracting the fish, and realised most of the dead fish had been reported in the vicinity of the pump’s intake.
He spoke to the adjacent landowner and an electrician checked the pump and found it had a faulty float switch, sending current directly into the water and electrocuting the fish.
The pump was not operating all the time, so fish were being affected sporadically.
It was removed as soon as the fault was identified.
Mr MacLean said it was a ''relief'' to find water pollution was not a factor and there was a ''simple'' explanation and an easy solution.
