FISHERMEN and environmentalists claim silt from the dredging of Falmouth harbour could spread across the area, killing off a vast array of sea life.
They raised their concerns at a public meeting organised by Friends of the Earth at The Poly in Falmouth.
The event was aimed at explaining the threats to the environment posed by the proposals (see below).
A trial dredge is being carried out to help determine the potential effects.
Mylor fisherman Dave Thomas said when 47,000 tonnes of dredged material were dumped in 2001 it destroyed the mackerel and whiting fishery for several years.
"The fishing didn't really recover until 2009," he said. "So you can just imagine what 1.1 million tonnes of the stuff is going to do."
He said the silt could contain pollutants, such as TBT from anti-fouling paint, and just 1.7mm settling on oyster beds could prove fatal. "The trial (test dredge) will do nothing to allay the concerns of fishermen. We do not want to see the demise of the docks, but we do not believe the rhetoric, we do not believe they have made their case.
"We know there is a disease in oysters and it flares up when the animal is under stress and we don't know how close it is to being terminal stress – a sediment plume could cause mortality."
A spokesman for Falmouth Harbour Commissioners, who are leading the dredging plan, said: "The issue of sediment movement has been extensively modelled and is something the Marine Management Organisation (MMO) will examine closely as part of its independent and detailed deliberations into whether or not to grant consent to dredge.
"The Port of Falmouth Development Initiative partners are considering potential uses for spoil to reduce the amount that would be deposited offshore from any dredge, and we have offered to meet representatives of the fishing industry to explain this."
Mr Thomas also claimed fishermen had not been consulted at any point regarding the plans or invited to stakeholder meetings.
But Cornwall Development Company, one of those leading the masterplan, said it co-ordinated a stakeholder event in May 2010 and a representative from Cornwall Inshore Fisheries Conservation Authority was invited but did not attend.
"During the full consultation it was made clear that the dredging would undergo a separate approval process to the masterplan, via the MMO, and the fishermen have had every opportunity to be involved in that process over the past two years," said a spokesman.
Marine biologist and part-time fisherman Miles Hoskin addressed the business case, which he claimed "did not hold up to scrutiny", and Jean-Luc Solandt, of the Marine Conservation Society, explained why the Falmouth and Helford Special Area of Conservation was unique and why the society did not support the dredge.