Cheat mode
Start the game by running the VDSFishdt.exe in the game folder instead of the normal shortcut. Press [F11] to force a fish to bite your hook. Press [F12] when a fish bites to automatically catch it.
Hints
Catch a tiger shark
Press [Forward] + [Back]. If done correctly, your boat will be moving very slowly and the next fish that bites will be a tiger shark.