I haven't seen this posted anywhere, and I don't remember where I found out about it, but this method can get you ~7000 fish per hour.
Play Barracuda Buffet (Level 36) and finish the stage. Before transitioning to the Bonus Stats screen, position your Orca so that it will die by an oyster while the Bonus Stats screen is up - the oysters continuously open and close in the background. If done correctly, you'll see the screen flash while counting up your Food Bank and Score and it'll reset the screen, doubling your Food Bank and Score (since it thinks you completed the stage twice).
This method will work for any stage, as long as you can find a way to die while the Bonus Stats screen is up and tallying your score. If anyone can find a non-Bonus stage that this will work on where the Food Bank can be easily doubled, please let me know and I'll update the solution.