Improving on existing established trends in food production with more technical refinements. More domestication and more efficiency will reduce the footprint and up the productivity.
Scientific american discusses the blue revolution of aquaculture
A cow requires around seven kilograms of feed grain for each kilo of meat, while a carp requires around three kilos or less. Fish farming economizes on feed grain, and of course on the land area needed to produce it
I have an article with more aquaculture stats
In 2004, capture fisheries and aquaculture supplied the world with about 106 million tonnes of food fish, providing the highest apparent per capita supply on record. Of this total, aquaculture accounted for 43 percent.
Aquaculture continues to grow more rapidly than all other animal food-producing sectors, with a global average annual growth rate of 8.8 percent per year since 1970
Adjustments can be made to prevent waste build up problems while still maintaining a smaller ecological footprint. Bioengineering can be performed to make the processes more energy and resource efficient and productive.
In about a decade, meat factories using stem cells will reduce the resource usage needed for cows and other farm animals. The resources used are described at openthefuture with the cheeseburger footprint
also, high rises converted to urban aeroponics, Skyfarming, vertical farming
Roughly 150 such thirty story buildings, Despommier estimates, could feed the entire city of New York for a year.
The promoters of vertical farming were noting that there were enough abandoned buildings which could be converted to the purpose of food supply.
Also, a cooperative city government could zone certain air rights (permission to build X stories high) to encourage the activity of providing the vertical farming. Another way for a city to arrange vertical farming is to require it as part of a larger development.
Just like now the cities require a certain amount of parking and traffic and utility improvements from developers. They could require builders of 100 story building which have a certain profitability to also supply a 30 story vertical farm to go along with the parking and other amenities.
Some of the economic encouragement for this is that supplying food locally would reduce the need for transporting the goods in.
Initially it would make sense to produce more high value product. Supply high end restaurants with fresher food. Provide food for the Whole foods and higher end groceries.
It would also make more sense where the city needed to replace, upgrade or add to sewer treatment. The vertical farms also processed waste water.