Salmon mysteriously dying at Marlborough farm

Salmon are dying in unprecedented numbers. Photo / Getty Images

Salmon are dying in a Marlborough Sounds farm in unprecedented numbers, and scientists are working to establish whether bacteria never before found in New Zealand is to blame.

The Ministry for Primary Industries reported that salmon at a farm in Waihinau Bay experienced higher than normal death rates in the early months of 2012, and the trend has continued in subsequent summers.

A range of factors including warm waters, the flow of water, fish diet and sea anemone stings were at play, Chris Rodwell, response manager for MPI, said.

"In addition, new genetic tests developed by our Animal Health Laboratory have revealed the presence of two bacteria not previously detected in New Zealand."

The bacteria can cause disease in fish but do not affect people, Dr Rodwell said.

"There are no concerns about food safety associated with this situation. Our focus now, is to determine what, if any, link these bacteria have with the higher summer mortality rates."

MPI would collect information to determine whether the bacteria exists outside of Marlborough.

"We know the bacteria have been present in New Zealand for a number of years. Our new genetic tests enabled us to retest samples from the first mortality event in 2012 and these revealed the bacteria were present then. It is possible it may even naturally occur here," Dr Rodwell said.

However, chief financial officer for NZ King Salmon (NZKS), Andrew Clark, said there was no proven link between the bacteria and the death of the salmon.

"Fish welfare and the health of the marine environment is paramount to us and we have invested significant resources to better understand this episode," he said.

NZKS was focusing its research on the high water temperatures, diet and sea anemones.

There was not enough data to establish a correlation between the bacteria and the deaths, he said.

MPI said there have been no reported increased mortality events in wild fish or in farmed salmon outside of the Marlborough Sounds.

Biosecurity controls have been put in place at the Marlborough farms to prevent any spread, and Mr Clark said NZKS was following the controls.