Hi Andy,
Austria is offering lots of mountain-rivers with large populations of trout-species. Flyfshing is the gentlemens technique and those who "cast the fly" sometimes recognize their fishing as an art. So from very cold rivers with ideal conditions for trouts we also have slow nutricious rivers that join the danube like the river March. In 2014 someone catched a "pacu" there, a vegetarian piraniah... obviously a invasive species i belive. https://www.vienna.at/exotischer-fisch-in-der-march-gefangen-kein-piranha-sondern-pacu/4093172
The danube itself hosts a very special lifeform, the beluga sturgeon, or sometimes called "Monsterstör" or "Donaustör", or "Hausen"
I have never seen a living one myself but we belive they swallow children swimmin to far into the river 😉