Killer fish with teeth? Danish swimmers escape waters fearing killer fish

"Discovering whether this fish is a lone wanderer or a new invasive species will be very exciting. And a bit scary. It's the first time this species has been caught in the wild in Scandinavia," says Associate Professor and fish expert Peter Rask Møller of the National History Museum of Denmark, University of Copenhagen.

Caught by hobby fisherman

The frightening fish was caught by hobby fisherman Einar Lindgreen on August 4. As he emptied his nets north of Danish isle Saltholm in the strait Oresund which seperates Denmark and Sweden he saw the red-bellied bigtooth among eels and perch. Back in the harbour the exotic fish caused quite an uproar, as several of Lindgreens colleagues were convinced that they were staring at a South American Piranha. Fortunately the fish was sent for study at the Natural History Museum of Denmark. Here Peter Rask Møller and fellow fish expert Henrik Carl examined the animal. Their calming communique for Danish bathers is that the fish is not a Piranha but a close South American relative; the Pacu.

Though the Pacu, like its cousin the Piranha, belongs in waters like the Amazon it has found its way to other waters around the globe by way of aquariums and fish farms. In large parts of the USA and Asia it is now considered an invasive species. It grows to as much as 25 kilograms and is a popular fish for farming and eating. But how this fish ended up in Scandinavian waters is a bit of a puzzler.

Some have imagined that it escaped from the recently established The Blue Planet -- Denmark's Aquarium. This Copenhagen attraction is situated right next to Oresund and gets its water from there.

But that is flat out impossible assures curator at The Blue Planet Lars Skou Olsen.

"We do exhibit Pacus in our Amazonas exhibition, but ours are a lot larger than the one caught. And even though the distance between our aquariums and Øresund can be measured in meters there are sophisticated filters in place to avoid contamination. So this is not one of our fish," assures Olsen.

The usual suspects

"Amateur aquarium owners and fish farmers are "The usual suspects" when we meet fish where they do not belong," says Peter Rask Møller.

"It is not unlikely that someone has emptied their fish tank into a nearby stream just before a vacation and that the Pacu then swam out into the brackish waters of Oresund. We don't know of any commercial farming of Pacus in Europe. But just like the Piranhas the Pacus are quite easy for amateurs to raise," says Møller.

Only once before has a Pacu been caught anywhere in Europe. That was in 2002, when a sportsfisher hooked one near a power plant on the Odra river in Poland.

"The river Odra empties into the Baltic Sea very close to Denmark," says Peter Rask Møller.

The Pacu is known as the peaceful cousin of the Piranha as it is largely vegetarian. Their powerful teeth are not as sharp as those of the Piranhas, but they are fully capable of severing fishing lines and even fingers.

"In order to be one hundred percent certain of the identification we will now perform a genetic examination, as there are several species of Pacu which are very similar when young. In the aqua culture busines they even produce hybrids between species," explains Peter Rask Møller.

The teeth are used mainly to crush nuts and other fruits, but the Pacu eats fish and small invertebrates as well. Its preference for nuts have had fatal results though. In Papua New Guinea where the species has also escaped it is rumoured to have mistaken nuts for male reproductive organs. So anyone choosing to bathe in the Øresund these days had best keep their swimsuits well tied.