One of these freshly named species, Cottus perifretum, had been labeled as another European sculpin, Cottus gobio.
Yet the two species are relatively easy to tell apart, according to Freyhof.
"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."
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—you really have to look at them in detail," Freyhof said.
"Shemayas have a wide distribution, but they are highly localized and are usually difficult to collect," he added.
"In the museum collections there are very few. Only after we checked almost all the populations in detail did we realize there are so many."
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.
But the Grecian fish has black fins, while the British variety has orange fins.
"This color fades when the fish are preserved, so maybe a Greek scientist had seen a [British museum specimen] and so couldn't see it didn't have black fins," Freyhof said.
The new Greek fish, called Squalius orpheus, is one of five previously unrecognized chub species identified by researchers during their search.
"Once you realize a fish looks different, then you can go into more detail, analyzing its bones, scales, DNA, and so on," Freyhof added. "Then you realize, Oh, it really is different!"
Kevin Smith, of the IUCN Freshwater Biodiversity Unit, said that deciding which distinct fish merit species status can lead to lengthy scientific debate.
But the new species included in the handbook have been individually described in scientific journals, he noted.
They have also been added to the Catalog of Fishes, the leading global checklist of fish species, which is maintained by the California Academy of Sciences.
"Fish are a massive part of our fauna, but people don't tend to notice them because they are out of sight," Smith said.
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.
Moving fish between rivers or lakes through practices such as stocking for angling and commercial fishing therefore presents a serious conservation threat, he added.
"Drought is also a major threat, especially in the Mediterranean region," Smith said. "With the onset of climate change, rivers are starting to run dry in the summertime.
"Whether [the fish are] unique populations or you call them separate species, either way, once their river runs dry, they're not coming back."
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