Prawns can be grown out in ponds successfully but I doubt cages are a good option for the reasons you stated. You'd need a pond specifically for the grow out of prawns. Here's a link to prawns:

http://aquanic.org/publicat/usda_rac/efs/srac.htm

Freshwater Prawns

# 483 - Freshwater Prawns: Biology and Life History PDF (78k)

# 484 - Freshwater Prawns: Pond Production and Grow-out PDF (114k)

I see no problem in what you advocate with the cats as long as you get the required license or permit if you need to.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.