Many pond members will measure their pond temp at different depth daily or at least a couple times weekly. If done over an entire year, you can a good temperature range over the seasons. Although you may have an unusually cool or warm year, you start to see what your temps run and can decide which species may or may not do well in your pond.
I think of wels catfish as the European equivalent to our blue cats but maybe even more aggressive and they get a bit bigger. Seems like predatory fish are really lacking in Europe? Not many options to pick from... Maybe just a couple of pike of the same sex to control them but to prevent reproduction may work? There is a forum member from Austria on here who occasionally posts. He was able to source some North American largemouth bass for his pond. I wonder if they would be legal for you to stock if you could find some?
I would assume carp spawn just as successfully in Europe as they do here and if so, you're gonna have a pile of carp in a hurry. Tench are prolific spawners as well.
Of the animal predators listed, the otters are the worst of the bunch.
Agreed! Are you able to trap or shoot otters in Latvia? The others will take a few fish, but the otters really can wipe a pond out.
