Silent Assassin

"Jeremy Wade travels to the Paraná River in the remote northeast corner of Argentina to investigate the death of a young girl. Once there, he discovers a second death. The people who live along this river bank fear the creature they call "River Dog." This is a new species to Jeremy, a river monster that somehow got under his radar. His discovery is not only the most alien of all river monsters, it is also the largest totally freshwater river monster Jeremy ever has caught!"

Synopsis
In Argentina's Paraná River, a young girl was killed by a river stingray. Venturing to a country he's never been to before, to face a fish he's never heard of before, Jeremy travels to this river in order to find the killer: the short-tail river stingray, which grows to 6 feet across.

Unlike Thailand's giant freshwater stingray, this fish isn't long or wide. Instead it is thick and heavy, with a short, club-like tail with which it delivers its venomous sting. After hearing many more reports of attacks, Jeremy becomes determined to face this beast, which can glue itself to the river bottom like a suction cup.

Its strength is then proven as Wade hooks into this tank-like fish in what is his longest battle to date.