Actually, with the Mac version there is a chance to recover the game. There is a backup file for each game slot that can possibly be used to recover a game in progress, depending upon the nature of the crash and the damage that it caused.

Check the Fish Tycoon folder for files named FishTycoon*.ldw. There will be 2 files for each player's game (e.g. FishTycoon1.ldw and FishTycoon21.ldw for player slot 1). The file with the single-digit number is the primary savegame file, and the file with the two-digit number is the backup. To see if your game can be recovered, you should move or rename the primary savegame file, then you can remove the "2" from the backup file's name and see if it will load when you run the game. If it's not damaged, you should be able to continue playing.

Good luck!

P.S. For any Windows players out there, this doesn't apply to the Windows version. See this post for information about the Windows version.