Refugium | News

Traps and Buckets—How We Are Helping Critically Endangered Fish

The crucian carp and the sunbleak are two fish species that used to be very common. Today, due to changes in the landscape and the arrival of invasive species, they’re on the verge of extinction. That is why work is now underway to protect them in the Czech Republic.

At the end of April, when the weather was unseasonably warm, we welcomed visitors from the Biology Centre of the Czech Academy of Sciences to our site near Jakubov in the Doupov Mountains. Three scientists arrived from the Institute of Hydrobiology: Marek Šmejkal and his colleagues Aleš and Kiran.
Left: Aleš Rektor, Marek Šmejkal, and Kiran Thomas.
Marek Šmejkal is one of the authors of the nationwide “Save the Crucian Carp!” project. As the name suggests, it’s dedicated to the systematic rescue of this critically endangered fish, with the help of the public.

Why are things so bad for the crucian carp?

It used to be abundant in village ponds, river arms, and pools close to water systems. It’s an expert at surviving in inhospitable conditions, and so it was often the only species living in stagnant, oxygen-deficient water, or in pools and ponds with plenty of silt.
Crucian carp (Carassius carassius) from Ledviny Pond on our site.
With the artificial straightening of riverbeds and the strengthening of their banks, the drying up of wetlands, the disappearance of small bodies of water, and the increased farming of village greens with ponds, the crucian carp has lost its natural habitat. The situation got worse in the second half of the 20th century when the invasive Prussian carp entered our waters, displacing the crucian carp in several ways and destroying its population by interbreeding with it.

Unfortunately, the fate of the sunbleak is similar.

This short-lived (usually two to three years), very small fish (6 to 8 cm) originally shared the same habitats as the crucian carp in high numbers. Its population has also dwindled, and the situation is further complicated by the invasive stone moroko. Today, the sunbleak can only be found in a few places in the Czech Republic and is perhaps even more endangered than the crucian carp.
The sunbleak (Leucaspius delineatus).
Now that you know the context, you can imagine how happy we were when a few years ago the naturalist Jan Matějů informed us that he had discovered the crucian carp in one of our three ponds in the Doupov Mountains. Genetic analysis soon confirmed that it was indeed a “pure” crucian carp.

Shortly afterwards, we contacted Marek Šmejkal, who visited the site with his team and confirmed that the crucian carp population in our beautiful pond situated in a depression created by an ancient landslide on a slope above the Ohře River is stable and thriving. We could now sample it without endangering it.

Preparations began in fall 2022.

We had already started restoring the second pond on our site. It’s a slightly younger reservoir set in a similar environment. When we took it over as its new owners, however, it was in a state far from “sustainable.“

Apart from the crumbling, illegally built hut, old broken freezer, and dump site on its shore, we were troubled about the abundance of fish that lived there. It was an intensely farmed fish pond full of common and grass carp, which contributed to the poor quality of the water.
The so-called Lower Pond during the drainage process in November 2022.
We drained the pond in the fall of 2022, which proved challenging as it didn’t have a proper drainage mechanism. We got rid of the fish and then left it alone during the winter, summer, and following winter. This resulted in partial de-mudding and once we refilled it we had clean water to work with.

“Save the Crucian Carp… and the Sunbleak!”

A few months later, in April 2024, a key part of our mission with the “Save the Crucian Carp… and the Sunbleak!” work title could begin.

Marek Šmejkal and his colleagues Aleš and Kiran first installed twenty traps on the site. The goal was to study the pond’s fish community with crucian carp in more detail and at the same time capture at least a hundred individuals of this endangered species for transfer. The same approach was used at a nearby sandbar where, thanks to a tip from the locals, we knew of a stable population of sunbleak.
Traps installed in Ledviny Pond during two days in April.
Over the course of two days, we captured 109 crucian carps, both large and small, and over 50 sunbleaks. All of the fish were then transferred in buckets to a prepared, clean pond where we hope they’ll thrive. If nothing goes wrong, by this time we should have the first generation of both species in the pond, which we will confirm by monitoring the situation.

We are working closely with hydrobiologists led by Marek Šmejkal not only on rescuing the crucian carp and sunbleak, but also on the overall management of our two ponds so that fish, aquatic insects, and amphibians can thrive there. As a result, we have removed a total of 12 kg of common rudds from the crucian carp pond during the two days in April. Their absence will create space for the above-mentioned species.
Sunbleaks waiting to be transferred to their new “home.”
In parallel, an entomological survey under the auspices of the Institute of Hydrobiology of the Biology Centre of the Czech Academy of Sciences is underway at the site. A closer understanding of aquatic insects will allow us to provide even more targeted care.

Last but not least, we’d like to make a small but serious request to all nature lovers. If you want to get involved in the protection of sunbleaks or crucian carps, don’t do it on your own. There are plenty of ways to contribute such as pinpointing forgotten sites with possible crucian carp populations, providing water reservoirs for their expansion, etc. However, unskilled interventions, even with the best of intentions, can cause damage. Stories of people accidentally mixing crucian and Prussian carp are unfortunately very common.

So good luck to the crucian carp and sunbleak, but always under professional supervision. 🌱

A large-scale restoration project of natural habitats is underway on our 16-hectare site near Jakubov. Learn more about that here. The patron of the entire site is FlixBus.