London, Nov 15: Biologists have discovered 57 new freshwater species in the rivers and lakes of Europe, which is more than previously thought.

The findings lengthen Europe's list of freshwater fish to 522 species.

Study authors say many more undescribed fish have been found or are suspected to exist, potentially taking the total number of confirmed species to 600 or higher.

The new species were discovered during a seven-year assessment of the conservation status of freshwater fish in Europe that was conducted in collaboration with the World Conservation Union (IUCN).

"The new species come from all over Europe, " said co-author Jörg Freyhof of the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin, Germany.

According to Freyhof, the new freshwater species might have remained undiscovered for so long because until now, scientists had not compared fish closely enough across countries' boundaries.

"Fish are a massive part of our fauna, but people don't tend to notice them because they are out of sight, " said Kevin Smith of the IUCN Freshwater Biodiversity Unit.

"Unlike free-ranging animals such as birds and mammals, fish are often isolated from each other because river basins and lakes act as barriers that keep unique populations apart, " he said.

The new species include various types of fishes.

One of them is the world's smallest known fish cisco, which is a type of whitefish. It was found in Germany's Lake Stechlin, north of Berlin.

This fish, silvery pink in colour, was found to be distinct from a much larger cisco species from the same lake.

Another remarkable discovery was of two new species of trout like Charr in alpine lakes in Germany and Switzerland.

The study team also named eight new Sculpin, a type of small, squat river fish often found under stones.

One of these freshly named species, 'Cottus perifretum', had been labeled as another European sculpin, ' Cottus gobio'.

But according to Freyhof, the two species are relatively easy to tell apart.

"The skin of Cottus gobio is very smooth, but perifretum's is like sandpaper, " he said. "There are many molecular markers which also distinguish the two species, " he added.

Another species which were even harder to tell apart were members of a group of lake fish from Central and Eastern Europe called Shemayas.

But the study team was able to identify four new Shemaya species.

"At first glance, they appear like herrings. But you really have to look at them in detail, " said Freyhof. "Shemayas have a wide distribution, but they are highly localized and are usually difficult to collect, " he added.

"There are very few of these species in the museum collections. Only after we checked almost all the populations in detail did we realize there are so many, " said Freyhof.

"Once you realize a fish looks different, then you can go into more detail, analyzing its bones, scales, DNA, and so on, " Freyhof added.

A recent research also determined that more than a third of Europe's 522 freshwater fish species are at risk of extinction and that 12 species are already extinct.

Regions where freshwater fish face the highest risk of extinction include the lower reaches of the Danube, Dnister, Ural, and Volga rivers in Eastern Europe. (ANI)