Unclear Behavior

The Mystery of Swarms of Giant Catfish in Lake Constance

Underwater commotion: A video currently circulating on Instagram and YouTube shows a previously little-studied behavior of catfish in Lake Constance.(Bild: Screenshot: YouTube/Seya)

Swiss video creator and angler “Seya” is causing a stir with spectacular underwater footage from Lake Constance: A video posted a few days ago on his YouTube channel “hookedbyseya” shows unusually large gatherings of giant catfish. The reason for this behavior is a mystery to fishermen and researchers alike.

An unusual natural spectacle is apparently unfolding beneath the surface of Lake Constance—one that has long remained hidden from the human eye. Until now. The Swiss video creator “Seya,” who regularly shares insights into the exciting and secret world of fish on his social media channels, has now captured the phenomenon on camera. 

For years, the YouTuber has been observing gatherings of catfish at depths ranging from two to 25 meters. “But I’ve never seen them in such extreme numbers as this year,” he says in astonishment in his YouTube video.

The Largest Predator in Lake Constance
The European catfish (Silurus glanis) is considered the largest freshwater predatory fish species in Europe. Adult catfish can grow several meters long, weigh up to 200 kilograms, and live to a remarkable age of up to 100 years—record-breaking specimens are also regularly caught in Lake Constance. With their long barbels, keen sense of smell, and sensitive lateral line, these predators can locate their prey even in poor visibility.

For the passionate angler “Seya,” however, it is not the impressive size of the fish that is particularly astonishing, but their behavior. While individual catfish or small groups are not uncommon, the Swiss angler has recently observed on multiple occasions how veritable schools of the fish gather at a few, precisely defined locations in Lake Constance: “It almost seems as if a large portion of the catfish from a wider area are gathering in a single location.”

Behavior Not Yet Fully Understood
Science has not yet offered a clear explanation for why the catfish “cuddle up” like this. Studies do show, however, that catfish tend to gather in larger groups, especially as temperatures drop. International research teams have already observed similar behavior in French and Czech waters. There, water temperature and oxygen levels played a particularly important role.

These factors could also influence the animals’ behavior in Lake Constance. This is because climate change is causing water temperatures to rise here as well, and there is less oxygen available in deeper water layers. These large predators are, however, masters of adaptation: catfish cope with such conditions significantly better than many other fish species, and as a result, their populations have grown substantially in recent years. 

Cross-Border Monitoring Project
To learn more about the behavior of these prehistoric animals, the Baden-Württemberg Fisheries Research Institute in Langenargen, together with the Technical University of Munich, Swiss professional fishermen, and the Vorarlberg Rheindelta Fishing Association, launched a cross-border monitoring project in 2020. The goal is to uncover further secrets about Germany’s “Fish of the Year 2026” by next December. 

Whether temperature, oxygen levels, or other, as yet unknown factors are behind the strange behavior of the Lake Constance catfish remains to be seen. One thing is certain, however: The impressive underwater footage that “Seya” has shared with the public offers rare insights into the life of a fish species about which even experts are still far from knowing everything.