The problem that you are having is usually caused by insufficient video memory (VRAM) in your Mac. The Mac version of the game typically requires at least 32MB of VRAM in order to run.

You can find out how much VRAM is installed in your Mac by following this procedure:

Click on the Apple icon in the upper left corner of the display. Click on About this Mac. On the popup window that results, click More info...
The System Profiler will be displayed. In the left pane, there should be listed something like Graphics/Display (may vary depending upon your Mac's model and age). Click on that and you'll see your display specifications in the right pane. Your video memory will most likely be displayed as VRAM (total).

If it's less than 32MB, you will probably not be able to run Fish Tycoon.