A sizeable population of tropical fish native to Venezuela and Guyana has been thriving in the polluted sewer system of Kyiv, in Ukraine, since the 1980s.

The ability of guppies (Poecilia reticulata) to adapt to various environments is well documented, but few people would expect to find a full-blown colony of the popular aquarium fish living in the sewer system of Ukraine’s capital city. And yet, a scientific study found that the little tropical fish native to South America has not only been surviving, but actually thriving in the dirty sewers of Kyiv. Guppies were first observed in the Bortnicheskaya treatment plant on the outskirts of the Ukrainian capital in the 1980s, and have been able to breed there despite the high level of pollution and unpleasant odor.

Photo: Diogo Cardoso/Unsplash

Popular in aquariums worldwide, guppies have an unusually high tolerance for different water qualities and temperatures and can adapt to human-modified environments. They usually live in the warm and clean waters of South American streams, but apparently have no problem adjusting to the very different conditions of Kyiv’s sewer system.

Guppies have been observed in irrigation channels, lagoons, lakes, ponds, and reservoirs all over the world, but the sewers of Kyiv are on a different level in terms of less-than-ideal conditions for life. Yulia Kutsokon, an ichthyologist of the Institute of Zoology at the National Academy of Sciences of Ukraine, recently confirmed the guppy’s incredible power to adapt to extremely hard conditions, confirming the thriving of the colony that calls the Bortnicheskaya treatment plant home.

Photo: Denny Muller

“Although the water is polluted and smelly, they live there all year round and reproduce successfully, and there are a lot of them,” the expert said, adding that the fish prefer the upper part of the canal, where the water moves fastest. The environment is too harsh even for guppies downstream, where the water slows down and pollutants settle.

We’ve heard about aquarium fish being dumped in toilets and winding up in lakes or ponds where they disrupt the local fauna, but to actually settle and thrive in the dirty sewers of an urban capital is something else.