Coorong fishers are struggling to stay afloat financially as the New Zealand fur seal invades waters near Meningie.
The Standard reported last June that the seals had become a nuisance since they were first sighted in March 2007.
Meningie fisherman Gary Hera-Singh said the seals had increased rapidly in the past 12 months with damaging effects on commercial fisheries and the Coorong's ecology.
"As far as commercial fishing goes, it's having a diabolical financial impact; seals are a bit like foxes, they kill all the chickens and they can only eat one," he said.
"They're increasing in numbers, quite a significant increase and more widespread.
"I believe there's quite significant numbers in the Murray Mouth; they've been seen in Lake Alexandrina ... where I fish in Pelican Point I would see anywhere between 15 to 50 in a single day and I talk to other fishers and they also tell me they see 20 in their areas."
According to Mr Hera-Singh, Coorong fishermen can no longer leave mullet nets unattended and are required to move gear at all hours of the night.
"I support protection and conservation of Australian seals and Australian fur seals because those numbers are critically low but these New Zealand fur seals are pushing these animals out of their natural areas," Mr Hera-Singh said.
"There needs to be some sort of ecological risk assessment done or look at some sort of management assessment."
A spokesperson from the Department of Environment, Water and Natural Resources said New Zealand fur seals (NZFS) had been hunted to the point of extinction in the early 1900s but numbers were returning to pre-hunting levels along the South Australian coastline.
"A survey undertaken by SA Museum in 2010-11 indicated that (NZFS) across South Australia numbered 110,000 and this number was predicted to increase about 11 per cent per year," the spokesperson said.
"The Coorong, where there are no natural predators and abundant fish, is an attractive alternative for NZFS populations.
"The issue requires detailed understanding and SA Murray-Darling Basin NRM Board is working with the local community to assess the ecological impact of increased NZFS numbers."
The New Zealand fur seal has also been known to maim local bird and fish species but, as marine mammals, they are protected under the Natural Parks and Wildlife Act; anyone caught harming the seal may receive a fine.