Opponents of New Zealand King Salmon plans to expand in the Marlborough Sounds say nine new fish farms would discharge equivalent nitrogen waste to half a million people.
The figure is not disputed by New Zealand King Salmon but the company says it is wrong to compare the effects of fish to human waste.
The comparison with half a million people comes from ecologist and ornithologist Rob Schuckard, who lives at French Pass in the Marlborough Sounds. Mr Schuckard is an adviser to Sustain Our Sounds, a lobby group opposing King Salmon's application for plan changes and resource consents that would allow it to develop its new farms.
Mr Schuckard said he based his comparison on information in an Assessment of Environmental Effects supporting the New Zealand King Salmon application and four or five international scientific reports for which he could provide references.
The comparison was based on the amount of nitrogen in fish and human faeces, Mr Schuckard said. However, he concedes there is a difference in impact, as salmon waste did not support human diseases like salmonella, campylobacter and E coli.
King Salmon proposes to increase production from about 8900 tonnes of fish a year to 15,000 tonnes in 2015 and 30,000 tonnes in 2020.
Mr Schuckard calculates growing 15,000 tonnes of salmon would release about 1500 tonnes of nitrogen waste into the sea each year; the equivalent to nitrogen in untreated sewage from 250,000 people.
At 21,000 tonnes of salmon, fish faeces would contain the same amount of nitrogen as sewage from a city of 350,000 people, he estimates. Once salmon production reached King Salmon's 30,000-tonne target, the equivalent nitrogen would be released to sewage from a city of half a million people.
For every tonne of salmon produced in their 17-month growth period, 32kg of nitrogen from feed would be retained in fish flesh and 98kg would be excreted as faeces, Mr Schuckard claims. Waste from one human over 17 months contained about 5.7kg of nitrogen.
King Salmon chief executive Grant Rosewarne agrees with Mr Schuckard that 15,000 tonnes of salmon discharge does contain 1500 tonnes of nitrogen. However, comparing fish faeces to human sewage was misleading, he said.
In 25 years of farming, there had never been an environmental effect remote from the farms. The Sounds contained tens of millions fewer fish than in the past, which was why a few million salmon were not causing problems.
"I have seen some of our most vitriolic opponents fishing right next to our farms so they clearly don't believe their own propaganda."
Faeces from farmed salmon was broken down by native fish and micro-organisms beneath the farms and mussels harvested from the Sounds absorbed equivalent nitrogen to output from the company's salmon.
Increased concentrations of nitrogen could enhance the growth of phytoplankton including harmful algal bloom species, but output from proposed new farms was unlikely to cause problems.
- The Marlborough Express