Stage 10 takes us into Albi where the shallows of the Tarn River are stalked by “freshwater killer whales,” a population of European catfish that have developed a taste for pigeon.
Catfish were introduced to the Tarn in 1983 and have firmly established themselves in the river. It has been suggested that the growth of the catfish population depleted the populations of prey fish, pushing the catfish to seek out a new food source. An excellent and abundant prey for any urban predator are the pigeons that inhabit major cities around the world. But how is a bottom-feeding catfish supposed to catch a flying pigeon? This video, taken by the authors of the PLOS ONE paper that reported on this behavior, shows how they manage it.
The pigeons of Albi frequent a small island where they drink and wash up in the Tarn. The catfish have learned that they can thrust themselves out of the water, beaching themselves on the sandy shore, and catch an unsuspecting bird. This behavior looks a lot like a killer whale or a crocodile, but it doesn’t appear to have been learned by catfish populations outside of Albi. The combination of an urban river, an abundant pigeon population that comes close to the water’s edge, and an introduced predator — all of which are direct consequences of human influences on the environment — has created a novel ecological interaction that reveals the tremendous potential for evolution in the built environment.