https://www.reuters.com/world/europe/mass-fish-die-off-german-polish-river-blamed-unknown-toxic-substance-2022-08-12/
It is currently unclear what is causing it but some tests indicate Mercury poising and it is thus risky for people and animals. Stay away and take care.
Rumors:
Known by the Polish department responsible for over a week but nothing happened until it also hit Germany.
That there are known polluters with so low fines that it amounts to a permit. https://www.reddit.com/r/CatastrophicFailure/comments/wmfk5l/polands_second_longest_river_the_oder_has_just/ijzchdk/