Some of them (like the ones in the south Pacific and the one south of India) are exclusive economic zones around islands. EEZ boundaries typically extend 200 nautical miles from shore (or until they run into another EEZ). See this map [1].

Presumably the nations that control these zones aren’t fishing them extensively, but other nations are fishing the international waters surrounding them, which is why you can see certain EEZs as negative space on the map in TFA.

[1]: https://en.wikipedia.org/wiki/Exclusive_economic_zone#/media...