A ban on the sale of mussels from the River Fal due to polluted water will see jobs lost.
Water quality was downgraded by the Food Standards Agency (FSA) in Ruan Creek and King Harry Reach, blamed on raw sewage pouring out of storm overflows during bad weather.
The Grade C rating, after levels of E coli found were above permitted levels, means a potential risk to human health, even after the shellfish are cleaned.
The downgrading means that mussel from the area are off the menu at restuarants locally and across the UK.
There are also fears about a downgrading for mussel beds in the Helford River, which would hit the industry hard.
The ban will be in place for a year.
Mark Mercer, from the Duchy Oyster Farm, told the BBC: "We'd been building up stock for two years and we're now at a point where we were about to harvest 40 tonnes a year. That is now impossible with the downgrading in place.
"We're applying to have an official relay site on the Helford [River], but we have got a potential downgrade for mussels on the Helford as well.
"If that goes ahead, the stock will go waste. It's a great local resource being lost and the lead on could result in lost jobs."
A spokesman for Truro Port Health Authority, said: "It's disappointing the last two areas in the River Fal where mussels were commercially gathered are failing to meet requirements.
"We are monitoring the situation very closely. We want to facilitate business but at the same time we have to ensure that adequate protection for public health is in place."