Then there is the real issue that forage fish should be made into human food, for third world countries, not ground up to feed carnivores sold to first world customers who can afford this wasteful process of using fish protein to make fish protein. Tut, tut.
The reality is that fish feed made of fish is dramatically on the decline. For example, the jack mackerel stocks off Chile and Peru have been decimated so much by Norwegian fish farms and other companies that stocks have collapsed. While there was only one Norwegian ship in the past year fishing there, the rest of the boats were catching what remained to feed to shrimp in Asian countries.
Not sustainable, and not for human consumption.
By the way, there is so much disease and other issues in tropical fish farms, I would not eat the shrimp and so on and suggest you do the same. One practice has chickens raised in cages above the fish. Their sewage drops into the fish ponds as food.
Here is a good current article on the issues: http://impactalpha.com/what-to-feed-the-fish/.
There are issues with plant based feed like soy, such as GMO problems, deforestation and throwing of indigenous people off their land, and so on.
I will shortly be reviewing the scientific research into gut bacteria of Atlantic salmon. The purpose is to make them able to digest vegetarian diets and thus move to a non-fish diet that has much more scope for greater amount of cheaper feed. A good idea.
Scotland researchers published their work in a recent issue of Nature: http://www.nature.com/ismej/journal/vaop/ncurrent/full/ismej2015189a.html.
It is important work and I suggest you read it. I will tell you more shortly.
"This week, researcher Dr. Martin Llewellyn (Institute of Biodiversity, Animal Health and Comparative Medicine, University of Glasgow) and co-workers in Ireland, Scotland, Canada, USA and Wales took a first step towards understanding the role of salmon gut bacteria in salmon health."