"There is a new reality on the market, especially in southern Greece and the Dodecanese islands where there is a strong presence of non-native species," WWF marine programme associate Panagiota Stappa told a news conference.

"Little by little, many more non-native fish species will appear on the market," she said.

Greek fishermen are keen to market species that they currently throw overboard because consumers do not know they are perfectly edible, said WWF field fishery manager Michalis Margaritis.

He cited the case of one Greek island, where 400 kilos of spinefoot (Siganus luridus) had been discarded.

"People do not choose non-native species because they are afraid of them," noted chef Giorgos Tsoulis.

"It will take, I believe, many more years for us to achieve the goal," he said.

Though particularly destructive to the seabed because of its voracious appetite, the spinefoot is nevertheless "incredibly rich" in fish oil and "very tasty", Margaritis said.

WWF's responsible seafood guide, whose revised version (www.fishguide.wwf.gr) was unveiled Wednesday, features over a hundred species available on the Greek market.

Among them are 13 invasive species which were not on the previous guide in 2015.

"Some may be already eating non-native species and not know it," said Margaritis.

"There are more than 13 species, we just listed the ones easiest to find," he said.

He noted that on his native island of Rhodes, there were three mullet species crowding out the native red mullet.

The invaders include the Atlantic shrimp (Penaeus aztecus) and blue crab (Callinectes sapidus) in the northern Aegean Sea, and the lionfish (Pterois miles) in waters further south.

According to "realistic estimates", it is believed that 65 percent of seafood consumed in Greece is imported, Margaritis said.

According to fisheries sector data, Greeks consume 19.6 kilos of seafood per year, compared to an EU average of 23.5 kilos, the organisation said.