Evolved so they *only* hatch after passing through the digestive system of a waterfowl might be a good evolutionary trick to avoid inbreeding.

Your eggs won't hatch until they're well away from your relatives.

In my experience, inbreeding doesn't seem to bother fish very much. Certainly you don't want it to happen too much, but I have several species of fish that have inbred and they look fantastic. I'm sure if I continued this line with absolutely no introduction of new genetics, some problems could arise... But after a few generations, they're doing great.

I will add new genetics to the line soon, for what it's worth. It isn't an experiment to see what will happen. In most cases I never expected them to breed to begin with.

This tends to simply not be that much of a problem for "lower" species.

If it happened just once, yes.

a high rate of mutations would be an advantage in a new environment, though.

Right, as long as you can still get out.