The arapaima, a giant species of fish that lurks in the Amazon river, may be threatened by overfishing.
Studies reveal that errors in the classification of the species could mean that it is being pushed closer to the edge of extinction than thought.
The arapaima is the largest freshwater fish with scales in the world.
But there may actually be four species rather than one, say scientists, and a lack of research and management may allow some to be fished to extinction.
The threat to the future of these fish has been revealed in research conducted by Dr Leandro Castello of the Woods Hole Research Center in Falmouth, Massachusetts, US, and Professor Donald Stewart of the State University of New York in Syracuse, US.
They have reviewed what is known about populations of the arapaima, and conducted detailed investigations into the status of the fish in the wild.
Previously, it was thought there was one species of arapaima (Arapaima gigas), which also goes by the common names pirarucu or paiche.
They have the curse of being tasty and of having to breathe air
Dr Leandro Castello
Woods Hole Research Center, US
This perspective is based on a taxonomic review done over 160 years ago.
Adults grow to almost 3m in length and can weigh more than 200kg, making the fish the largest with scales living in freshwater anywhere in the world.
They are also air-breathers, coming to the surface every 5 to 15 minutes to gulp air, a behaviour which allows them to colonise muddy oxygen-poor rivers and lakes within the Amazonian basin and prey on other fish that find it difficult to move in such conditions.
However, in an ongoing study, Prof Stewart has analysed nearly all preserved specimens of supposed arapaima available in museums in the world.
So far he has only found one specimen of Arapaima gigas.
The others are suspected to be closely related species, including some as yet unreported.
"Our new analyses indicate that there are at least four species of arapaima," says Dr Castello.
"So, until further field surveys of appropriate areas are completed, we will not know if Arapaima gigas is extinct or still swimming about."
Concern about the fish's numbers comes from other work done by Dr Castello and Prof Stewart.
For a split second, an arapaima surfaces to gulp air
That suggests that arapaima sexually mature relatively late, and need very specific habitats to both live and reproduce.
Their research also shows that populations of the fish are being put under severe pressure by fishermen.
Because of the fish's huge size and habit of coming to the surface, it has long been a favoured fish to catch, with fisherman using harpoons and gill nets to land their prey.
"They have the curse of being tasty and of having to breathe air," says Dr Castello.
Fishermen have been catching large numbers of arapaima in this way since the 1800s.
But now, while a few populations are increasing, others are being overfished, say the researchers, who have published a paper warning of the fish's fate in the Journal of Applied Ichthyology.
And while Brazil implemented regulations to manage arapaima fisheries some 20 years ago, most fishermen do not follow the regulations, say the authors.
Fishermen capture a young arapaima for ecological studies
"Arapaima can be viewed as badly overexploited and under some level of threat of extinction," says Dr Castello.
One solution, they say, is to encourage community-based schemes for fisheries, and there is much need for additional action on the part of the government.
For example, their research shows that fishermen who specialise in hunting arapaima with harpoons can accurately count the fish, due to the fish's habit of breaching the surface for air.
The fishermen can then select a sustainable proportion of the population to hunt.
"Populations of arapaima managed with this system increased about 50% annually, while yielding increasing catches and hence economic profits to the fishermen," says Dr Castello.
Around 100 such community schemes are in place, and some previously overexploited populations have recovered.
"Such results are extremely rare in wildlife conservation, especially in tropical countries where wildlife conservation challenges are greater than elsewhere," says Dr Castello.
But much more needs to be done to research these fish in more detail and prevent overfishing, the scientists warn.
In particular, "the present situation may be one in which one species of arapaima is recovering in certain areas, while unrecognised species are going extinct," they say.