Fish are one of the most valuable sources of alimentation for they contain proteins, fat which is helpful for the human body, unlike animal fat. They also contain several minerals and vitamins and are good for function of our heart.

Fifty years ago, the areas away from the sea could hardly have access to quality fish. With the development of fish farming, millions of people were granted access to fresh and cheap fish.

Today, 40% of the fish consumed worldwide come from fish-farming facilities.
http://en.wikipedia.org/wiki/Fish_farming

If fish-farming did not exist, people should catch more open sea fish, setting the long term sustainability of open sea fish population in danger.

In Greece, many fish-farming facilities exist, as it is the major fish-farming country in the Mediterranean Sea. Fish-farming facilities also nowadays strongly developed in Turkey, Cyprus and Croatia.

However, the development of fish-farming facilities create severe environmental problems. For the protection of the installations, fish-farms are installed near the coasts or in gulfs, where there is not much renewal of the sea water.
Therefore, pollution appears, because of food and food supplements which are given to the fish. It has measured that water around fish-farms contains relatively large quantities of zink, copper, nitrogen, coal and phosphorus.
http://www.econews.gr/2014/11/21/psaria-ydatokalliergeies-118940/ (in Greek)

Many people living in the areas near fish-farms complain for the downgrading of tourism, of landscape (because of the buildings needed for the fish farms), but also because of the decrease of conventional fishing, which feeds many families and at the same time is part of the local tradition.

It is interesting that in 2015 there will be a 50 million euros investment in the island of Crete for three new technology offshore fish-farming projects.
http://www.onexcompany.com/50m-investments-fishfarming-technologies/

As new project suggests there will be a movable unit of fish-farm, which will be connected to a ship and will move in certain areas following sea currents. That way avoids possible problems of pollution of seabed and improves the health and development of fish.
Essentially, fish treated in such a way could be compared to wild fish.

This technology called "Mobile Fish Pen" may lead to fish cages free in the oceans, completely automated, led by satellites.
http://www.azorobotics.com/Article.aspx?ArticleID=85
https://www.youtube.com/watch?v=OPs-0LfCEq0

In any case, there should be national and international spatial plans setting the areas of fish exploitation oriented by the protection of the environment and other criteria.

Technology should find solutions for the nutritional problem of the increasing world human population with major criteria the reliable and sustainable development.

http://ec.europa.eu/fisheries/inseparable/en/farmed-eu
http://www.econews.gr/2014/11/21/psaria-ydatokalliergeies-118940/ (in Greek)

I know it's against the nature of fish, as any other animal, to put them in a cage and feed them, however, intensified open-sea fishing would probably lead to worse problems, as fish population would greatly decrease.