This news smells fishy! The pacu is a freshwater fish. The Oresund Sound is salty sea. Not as salty as the Atlantic next door, but more salty that the brackish Baltic on the other side (many freshwater fish live in the almost-freshwater northern parts of the Baltic, but going south the salt goes up and the population gets more marine). It is also much, much colder than Orinoco or the Amazon.
I think this is a prime example of what we in Finland call a "mätäkuun juttu", a "rotten month's story". It is also the right time of the year for it, the humid tail end of Summer when things start rotting.