Hi John i appreciate the manner in which you address this subject as it is fair to say that it is a subject of interest to many including myself. i will if possible answer all of your questions if i cannot i will find out for you and answer at a later date if that is ok.
There is reasearch ongoing i think into the use of vegatable proteins to replace or partially replace fish proteins. however i belive that in the fresh water stages of growth that an invertebrate or similar thinking along these lines this bneing my personnel veiw.
As for what the diet is made up of it usually consists of between 45% to 60% of protein 20% to 30% between 20% to 30% ash then the rest is made up of carbohydrates fibre and phosphorus.
The fish used varies a bit details of which i am not 100% of but if you are interested i can find out. and yes feed bills are on the increase.
As for the question of how i see fish farming in say 20 or so years well i would like to think that will still be here and i hope thriving as fish are a good nutritional meal for any one, but as we hear every week that sea stocks are being over fished and that fish stocks are declining.
I dont pretend for one min that its perfect but will say that salmon farming as an industry is only 30 years old so still in its infancy, but farming fish is as old as the hills. It just needs to find the right way to see it into the future.So to answer your question about the future of fish farming? well depends which species you farm who knows?
As for the restocking of our seas well i dont know much about cod and would need some help to first of all to understand a cods natural life cycle but i feel that it is the fish lower down the food chain ie sand eels that need the attention as they are the key to most predatory fish,birds,whales and dolphin and not forgetting man. But its a sad day when even the humble mackerel is far less common than once apon a time in the not to distant past.
I hope this has helped in some way John and would gladly go on discussing these issues furthur. I dont have all the answers but do care enough to at least explore possibilities and try and come up with some answers.