It's a type of fish, and they are making word plays with the name of the fish. Why? I'm not sure of the origin of this one. I imagine it's like the beans or the beef stroganoff memes - the origin doesn't matter much at this point.
I tried my hand on one of those AI image makers... Almost got kicked off twice because apparently "like-a-virgin-era Madonna but a sturgeon" is considered inappropriate somehow.
I actually started with eels. Aal (=> eel) can be inserted basically everywhere by just stretching the "a" a bit, like "Aalle wissen das" instead of "Alle wissen das" (everyone knows that). As far as I know, this started (or at least became popular) via Jan Böhmermann, he had some jokes about that in his show - the same guy who wrote the "goat fucker"-poem about Erdogan that caused massive diplomatic problems and resulted in changed laws.