Opponents say an estimated almost $900 million benefit to the Marlborough region from further salmon farm developments could cost New Zealand its international reputation.
The proposal for nine new farms by New Zealand King Salmon has attracted 1400, of which 800 are against.
At a hearing in Blenheim on Tuesday, the Environmental Protection Authority was told that building the farms would cost $40 million, with the Marlborough and Nelson economies benefiting by almost $30 million.
Over the next 25 years, the benefits are estimated to be $880 million.
But Danny Boulton, chair of environmental group Sustain our Sounds, says the figures are unproven and the environmental effects of large-scale salmon farming will destroy New Zealand's clean, green image.
"There is a huge, overwhelming body globally who buy our products who do not see salmon farming as not being an environmentally friendly practice. And you compare the carbon footprint of producing their fish - it is probably one of the highest footprints that you'll find in an industry."
Some Maori say the development will destroy their customary rights.
New Zealand King Salmon says the alternative could be operating in Chile or the United States.