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Several factors may have contributed to the deaths of hundreds of fish at a Dunedin stream last month.
Dead fish, including smelt, flounder, giant bullies, trout and inanga, were seen in the Kaikorai Stream and estuary at the Brighton Rd bridge on February 20.
The Otago Regional Council has investigated, but said it found no conclusive evidence of an upstream pollution event.
However, scientific advice identified that degraded water quality, high temperatures and the opening of the coastal mouth were all possible contributing factors to the deaths.
Council regulatory general manager Richard Saunders said initial findings showed work undertaken by the council’s operations team complied with its coast plan.
The investigation’s findings would be independently reviewed.
"Having a qualified third party evaluate these findings will provide an independent assessment of the legality of the activity."
Mr Saunders said the Kaikorai was known to stratify in deeper locations of the estuary, meaning there is a warmer top layer of water with higher dissolved oxygen levels, and a colder layer with lower dissolved oxygen levels.
"Opening the mouth may have allowed the oxygen-rich layer to drain out of the estuary, exacerbating natural factors such as temperature and algal growth."
Under the new National Policy Statement for Freshwater Management 2020, the council will be required to develop an action plan to halt water quality degradation and improve the health of water bodies that do not meet the policy statement’s bottom lines.
The council’s engineering team has engaged independent expertise who were assessing both the incident at the Kaikorai Estuary and an earlier incident of mass fish deaths at Silverstream in January.