The rare catch happened in the city of Norrköping, which is home to the Motala Stream, a 100 kilometer long river system that drains lake Vättern, Sweden's second largest, into the Baltic Sea.
By Rosales' own admission, he and his friends were fishing with roach as baitfish and had several rods in the water, when they saw a shadow moving in the water. Suddenly, the bait was gone. Shortly thereafter, the shadow re-appeared and bit the hook.
"We all had to step in to pull it out. However, it became calmer when we dragged it out of the water," Jonathan Rosales related his unique experience to Swedish national broadcaster SVT.
On the hook, they saw a three kilogram heavy monster of a fish, unusually ugly and with razor-sharp teeth. Since the fish did not remotely resemble anything they had seen before they photographed it and, good-natured as they were, released it back into the water — something they most likely regretted afterwards.
"We're sporting fishermen, so we released it," Jonathan Rosales said.
"It's a shame it was released. Unfortunately, aquarium fish are often released and found in natural waters. Therefore, there is a risk that new species will settle down in this country. Besides, there is an extra risk that it will also spread new fish diseases," county fish consultant Per-Erik Larson told the fishing newspaper Fiskejournalen, stressing that releasing fish without permission is illegal.
Shortly afterwards, however, stubborn Rosales managed to fish out the intruder (or possibly its fellow pacu) again. This time, it stayed on land and was sent to the Östergötland County Administrative Board for examination, SVT reported.
The pacu is an omnivorous freshwater fish related to the piranha. Unlike its notorious relative, the pacu mainly feeds on plant material, yet can reach much larger, uncanny sizes of up to one meter in length and 40 kilograms in weight. It is known to be highly invasive and spreads quickly in favorable conditions.
On Papua New Guinea, where it has been farmed for food, it has the reputation of being able to attack human testicles and has been given the frightening nickname of "ball-cutter," which has subsequently been debunked by scientists.