PHOTO STORY: The invasive black bullhead catfish threatens Czech fishponds

13. 10. 2025

One of Medek’s fishponds near the Czech town of Kroměříž is a place where the invasive black bullhead catfish reigns beneath the surface. It is a serious threat to native fish and its presence needs to be carefully monitored and controlled. And that’s what researchers from the Institute of Vertebrate Biology of the Czech Academy of Sciences and other institutions are working on. Click on the photos for the photo story, which was first published in the 3/2025 Czech issue of A / Magazine.

Written and prepared by: Markéta Wernerová, External Relations Division, CAO of the CAS
Translated by: Tereza Novická, External Relations Division, CAO of the CAS
Photo: Jana Plavec, External Relations Division, CAO of the CAS

The text and photos are released for use under the Creative Commons license.

On the outskirts of the Moravian town of Kroměříž (Czech Republic) lie two Medek’s fishponds. The invasive black bullhead (Ameiurus melas), or black bullhead catfish, is found in only one of them.

The black bullhead originally comes from North America. It was introduced into Europe in the nineteenth century and spread quickly.

This fish poses a major threat to aquatic ecosystems. It is omnivorous, has few natural enemies, and reproduces rapidly. In fishpond farming, it causes significant economic losses.

“Fish farmers have a serious problem with this species of fish. Financial losses can amount to twenty thousand crowns [app. EUR 850] per hectare,” says research team leader Pavel Jurajda from the Institute of Vertebrate Biology of the Czech Academy of Sciences (CAS).

The presence of this invasive fish needs to be monitored and controlled. Researchers from the Institute of Vertebrate Biology of the CAS are working together with fisheries enterprises and associations. Among other things, they are trying to improve capture techniques for this unwanted American intruder.

Alongside the black bullhead (shown below), the pond is also home to its close relative, the brown bullhead (shown above). Its numbers here, however, are a hundred times lower.

The black bullhead (right) has a greenish skin tone, while the brown bullhead (left) is more brownish or marbled. Their tail and anal fins also differ in appearance.

Bullheads feed on the eggs and fry of other fish. Their diet also includes invertebrates and amphibians such as newts and frogs. This severely disrupts biodiversity in the affected sites. Freshly lifted traps provide the proof: apart from bullheads, they contain no other fish.

One of the traps used to catch the fish.

Keeping, selling, and transporting black bullheads is illegal. It may not be imported into the European Union or released anywhere within it. It is listed on the EU’s official list of invasive alien species, and violations of the ban can result in fines.

On the day we visited the Medek’s fishponds with our photographer, workers and students together hauled in more than 35 kilograms of fish. Over the entire week from Monday to Friday, the total reached one quarter of a ton.

In the Czech Republic, the invasive black bullhead is found in five locations: in addition to Kroměříž also in Pasohlávky, Telč, Mělník, and Prostějov.

Researchers monitor the populations of black bullheads, carry out culling, and at the same time work on designing the most effective methods and principles for regulation.

Apart from a few specimens reserved for laboratory analysis, the week’s catch was taken by staff from the Zlín Zoo in Lešná – as a healthy snack for their otters.

Unlike in countries such as Hungary or Slovakia, where the situation is already beyond remedy, in the Czech Republic, it is still possible to keep this dangerous fish in check. Researchers from the Institute of Vertebrate Biology of the Czech Academy of Sciences will continue to work intensively on the task in the years ahead.

The Czech Academy of Sciences (the CAS)

The mission of the CAS

The primary mission of the CAS is to conduct research in a broad spectrum of natural, technical and social sciences as well as humanities. This research aims to advance progress of scientific knowledge at the international level, considering, however, the specific needs of the Czech society and the national culture.

President of the CAS

Prof. Eva Zažímalová has started her second term of office in May 2021. She is a respected scientist, and a Professor of Plant Anatomy and Physiology.

She is also a part of GCSA of the EU.

PHOTO STORY: The invasive black bullhead catfish threatens Czech fishponds

On the outskirts of the Moravian town of Kroměříž (Czech Republic) lie two Medek’s fishponds. The invasive black bullhead (Ameiurus melas), or black bullhead catfish, is found in only one of them.

The black bullhead originally comes from North America. It was introduced into Europe in the nineteenth century and spread quickly.

This fish poses a major threat to aquatic ecosystems. It is omnivorous, has few natural enemies, and reproduces rapidly. In fishpond farming, it causes significant economic losses.

“Fish farmers have a serious problem with this species of fish. Financial losses can amount to twenty thousand crowns [app. EUR 850] per hectare,” says research team leader Pavel Jurajda from the Institute of Vertebrate Biology of the Czech Academy of Sciences (CAS).

Alongside the black bullhead (shown below), the pond is also home to its close relative, the brown bullhead (shown above). Its numbers here, however, are a hundred times lower.

The black bullhead (right) has a greenish skin tone, while the brown bullhead (left) is more brownish or marbled. Their tail and anal fins also differ in appearance.

Bullheads feed on the eggs and fry of other fish. Their diet also includes invertebrates and amphibians such as newts and frogs. This severely disrupts biodiversity in the affected sites. Freshly lifted traps provide the proof: apart from bullheads, they contain no other fish.

One of the traps used to catch the fish.

Keeping, selling, and transporting black bullheads is illegal. It may not be imported into the European Union or released anywhere within it. It is listed on the EU’s official list of invasive alien species, and violations of the ban can result in fines.

On the day we visited the Medek’s fishponds with our photographer, workers and students together hauled in more than 35 kilograms of fish. Over the entire week from Monday to Friday, the total reached one quarter of a ton.

In the Czech Republic, the invasive black bullhead is found in five locations: in addition to Kroměříž also in Pasohlávky, Telč, Mělník, and Prostějov.

Researchers monitor the populations of black bullheads, carry out culling, and at the same time work on designing the most effective methods and principles for regulation.

Apart from a few specimens reserved for laboratory analysis, the week’s catch was taken by staff from the Zlín Zoo in Lešná – as a healthy snack for their otters.

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The Czech Academy of Sciences (the CAS)