even with all of webp' faults (of which i've yet to see any), avif is still so much worse. for each AVIF there's like a 50% chance it will load just fine on my browser or image viewer and a 50% chance that it won't for reasons equating more or less to "fuck you"

Unsure if you're actually asking or trolling, but at least in the USA, in some areas, there are fishing limitations, often times requiring a fishing license. These limitations can be that you only keep within a specific size range, a maximum number of total fish, or both. That's what the comic is referencing.

The joke being that society is destroyed and what that means (to that guy) is no more fishing restrictions

It's when a communist and a neocon love each other very much, but sometimes things don't work out: Reykjavík Summit - Wikipedia

What are the two doing in the image?

Penises. It answers both your questions.

Whitefish is pretty delicious though.