Avery, a 12-year-old boy, was fishing in the Ounasjoki River in northern Finland, as he had done so many times before, when something big pulled hard on his rod. With skill and some help from his grandfather, he managed to pull a 5-kilo pike out of the water, his biggest catch so far. But the most impressive thing was not the size of the fish, but what they found inside it.
‘I wondered why it was so fat,’ Avery told Finnish newspaper Ilta Sanomat. The answer came when they gutted it: the pike had devoured a whole small duck. ‘I was a little horrified,’ the young boy admitted after seeing the decomposed animal inside the fish’s stomach.
The find, although unusual, did not entirely surprise experts. According to Anna Kuparinen, professor of aquatic environmental science at the University of Jyväskylä, consulted by the aforementioned media outlet, ‘pike are natural predators, known for their varied diet and ability to ambush all kinds of prey. Finding fish inside a pike is common, but birds not so much,’ Kuparinen explained. ‘Even so, if a young duck swims at the wrong time, it can end up like this.’
Nature has no filters
This case, although anecdotal, is not at all unusual in everyday life in nature. Here, in the Iberian Peninsula, there are aquatic ecosystems that are home to species that are also capable of devouring birds, small mammals, amphibians and reptiles.
One of the main protagonists of this type of behaviour is precisely the European pike (Esox lucius), an allochthonous species —i.e. not native— introduced decades ago for sporting purposes. It has established itself in reservoirs, rivers and lakes in the north and centre of the peninsula, such as the Riaño reservoir in León and the Tormes River in Salamanca.
This fish has a mouth full of sharp teeth and a jaw that acts like a spring trap, capable of capturing and swallowing its prey in seconds. Although its usual diet includes smaller fish, amphibians and crustaceans, it is not uncommon for it to hunt mallard ducklings or common coots swimming alone near the banks. In several reports, the Institute for Research on Hunting Resources (IREC) has documented this type of predation in wetlands in La Mancha.
Even more shocking is the case of the giant catfish (Silurus glanis), another invasive species found especially in the Ebro River, where it has caused significant ecological disturbances. Some studies, such as ‘Ecology of Freshwater Fish’ published in the ecological journal Willey, have confirmed that catfish do not only eat fish: their diet can include ducks, water rats and even pigeons. In fact, catfish have been filmed partially emerging from the water to catch birds perched on the shore.
Natural behaviour in an environment that is not their own
These scenes, although natural from a biological point of view, create significant imbalances when the predators are not native. Introduced species, such as pike and catfish, often lack natural enemies, which allows them to spread rapidly and disrupt food chains.
Various organisations, such as the Ebro Hydrographic Confederation and the Iberian Ichthyological Society (SIBIC), have been warning for years about the spread of these fish and their effects on local wildlife. That is why in some autonomous communities, the release of species such as pike is prohibited, and fishing for them is even encouraged to curb their spread.
