(CNN)The sawfish is one of the ocean's stranger sights -- with its unique long, narrow nose lined with teeth, it resembles a hedge-trimmer or chain saw.
But the shark-like rays are on the brink of extinction, having vanished from half of the world's coastal waters, according to a new study.
Sawfish were once found on the coastlines of 90 countries. But now they are some of the world's most threatened marine fish, and are already presumed extinct in some 46 countries, researchers from Canada's Simon Fraser University said.
At least one species of sawfish is missing in 18 countries, and two species have disappeared in 28 countries, the researchers said.
Three of the five species of sawfish are critically endangered, and the other two are endangered, they warned.
Despite their alarming appearance, the fish are not considered dangerous to humans, but their saws -- which can reach a third of their total length -- are used for feeding, digging out animals, and killing or maiming other fish.
But the teeth on their "saws" -- known as rostra -- mean the creatures are easily tangled in fishing nets, while their fins are valuable in the shark fin market. Their rostra are also sold as novelties, for medicine, and are used as spurs for cockfighting, the researchers said.
"Through the plight of sawfish, we are documenting the first cases of a wide-ranging marine fish being driven to local extinction by overfishing," Nick Dulvy, a professor in marine biodiversity and conservation at SFU, said in a statement.
"We've known for a while that the dramatic expansion of fishing is the primary threat to ocean biodiversity, but robust population assessment is difficult for low priority fishes whose catches have been poorly monitored over time," he said.
In a study published in the journal Science Advances, researchers warn that complete extinction is possible if overfishing is not curbed, and if threatened habitats where sawfish live are not protected.
They say they have identified "priority" countries where there is still a chance of saving the species.
Conservation efforts should be made in Cuba, Tanzania, Colombia, Madagascar, Panama, Brazil, Mexico and Sri Lanka, where protections could save the imperiled species. Countries like the US and Australia, which already have adequate protections and some sawfish, should be considered "lifeboat" nations.
"While the situation is dire, we hope to offset the bad news by highlighting our informed identification of these priority nations with hope for saving sawfish in their waters," Helen Yan, a researcher in marine biodiversity and conservation at SFU, said. "We also underscore our finding that it's actually still possible to restore sawfish to more than 70 per cent of their historical range, if we act now."