I just saw a video over Cat Goes Fishing and I realized a lot of the mechanics and a lot of the fish remind me very strongly of a Flash game I used to play. It included the same fish, trade in for money and better gear, keep getting bigger fish cycle as Cat Goes Fishing but had a very small plot as well.

I think the basis of the game is that you're trying to rescue your sibling who got stuck on an island. And the way to do that is to get a fishing rod that goes so far that you hook into your sibling and drag the entire island closer to you.

Some other stuff I remember: there were bombs that you could use to kill fish. I think you needed to kill all the sharks that were at the bottom of the ocean before you could save your sibling. The cowfish, grumper, and mustardfish from Cat Goes Fishing were all directly from that Flash game, but I can't find any references that say where CGF got its influences from