Fishermen and marine experts are concerned fish could be endangered by dredging in the Thames Estuary.
The dredging is due to start next year as part of the construction of a £1.5bn London Gateway container port.
Fishermen say fish stocks will be affected and marine biologists fear "untold" species will be displaced as a channel is deepened for the port.
Developer DP World said the dredging would be carefully monitored and 10,000 jobs would be created by the project.
'Absolutely disgusting'
The London Gateway container port and business park will be built on the former Shell oil refinery site at Shell Haven in the Thames Estuary, Thurrock.
Dredging will deepen the channel to enable large ships to enter the port, which received the go-ahead after a public inquiry last year.
But Paul Gilson, whose ancestors have fished in the estuary since 1780, said it could have severe consequences for fishermen.
He said: "If someone comes along with a dredger and vacuums these little fish up there's nothing left for us to work on. It's just absolutely disgusting."
Fisherman Paul Gilson fears the project will deplete fish stocks
Marine biologists at Sealife Adventure centre in Southend fear the dredger could unearth toxins in the mud and displace fish such as bass and Dover sole, and other marine life.
Curator David Knapp said: "Untold fish species will be displaced - the river's a big breeding ground. There's a lot of fish which call this river their breeding base."
DP World said it would spend millions of pounds monitoring the dredging operation.
Environmental manager Marcus Pearson said: "We've had a lot of surveys done on the actual fish in the estuary and we have actually shown that there is going to be minimal impact to the fishing stocks as we do the dredging."
He added: "The fish will actually move out of the way and then re-colonise later on."
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