Hello and welcome!
Pike, huh? Wow, they get big, and aren’t they loner, carnivorous cannibals?
When you say they don’t have filters in the sea, that’s not really true — the entire body of water acts as a filter. Same for oxygen. The motion of the waves and the vast openness of the sea (or whatever body of water), huge surface area, and plants provide oxygen.
The reason that ponds need help with filtration and aeration has everything to do with the fact that humans are placing fish in an artificial environment. That artificial environment can’t compete with what Mother Nature provides, so to keep our fish healthy and happy, we have to supplement what they would normally get in the wild.
Some fish do well in captivity, some just won’t do well in captivity. I don’t know enough about pike to know what they they require. But my advice would be to do as much research as possible on the particular fish you have in mind — you will want to duplicate its environment as much as possible. But before you even get that far, learn all you can about fish-keeping, including the nitrogen cycle.