The public are being warned against purchasing illegally caught salmon or sea trout.
Inland Fisheries Ireland says it could have a knock-on effect on future salmon stocks.
Deputy CEO and Head of Operations at Inland Fisheries Ireland, Dr Greg Forde told RTÉ's News at One that from early July, they have noticed a significant increase in the number of illegal nets off the Cork coast targeting wild salmon returning to rivers.
He said there were also incidents off the Mayo and Donegal coasts.
Last Monday, fisheries protection officers boarded a fishing boat off the Cork coast and seized 98kg of salmon and 256m of illegal netting.
Mr Forde said "the real trouble is that a net, in the right place as far as a poacher is concerned, or the wrong place as far as wild fisheries are concerned, can take virtually the entire run going into a river, at a particular time".
He said salmon are not running year-round, but over the next fortnight, they are coming back in peak numbers and every effort must be made to protect these fish.
Mr Forde said in two incidents in the past week, they seized over 1,200 yards of illegal drift nets.
He appealed to people not to buy a salmon unless it is from a wild source and has a tag on it.
He said people are selling these fish on the black market to unsuspecting individuals.
Mr Forde said only salmon or sea trout that have been caught by licenced commercial salmon fishermen may be sold and must have a colour-coded green or white gill tag, or for wild imported salmon, a yellow tail tag.
