Offshore cage farming has potential in Asia
These large cages were designed and developed in Norway and are being used for rearing cobia/pompano in the sea off central Vietnam. Credit: Jorge Alarcon, Marine Farms Vietnam
Cage culture technology developed in Norway is being increasingly used in Asia to farm species such as cobia, tilapia and pompano. It is a far cry from the primitive industry still in use there where small cages made from wood or bamboo with nets hanging from the top are used to rear fish.
Modern cages can be enormous structures made from plastic or steel, which are designed to hold large volumes of fish – as many as 500,000 fish, or about 1,000 tonnes, in a single cage – and to withstand very harsh weather conditions.
The development of modern cage culture is commonly attributed to the emergence of salmon farming in about the 1960s. Since then cage culture technology has developed tremendously and the industry is now a very advanced, science‐based activity.
The main reason to use cage farming, according to fisheries consultant Erik Hempel, is the possibility of having fish in the open sea where water quality is better than close to shore and, if the cages are large enough, the living conditions are close to what the fish are used to.
“Another advantage,” he added, “is that it is possible to move the cage to another location during an algae bloom, or an outbreak of pollution or adverse temperatures.”
Mr Hempel was speaking about modern trends in offshore cage technology at the 4th International Symposium on Cage Aquaculture in Yeosu, Korea, at the beginning of May.
As the size of farming operations grew, technological and engineering challenges increased, Mr Hempel said. “We have had to use technology from marine architecture, materials technology, physics, navigation, and a number of other disciplines to develop tomorrow’s ocean farms.
“Technologies such as submersible cages, closed and semi-closed systems, and tension leg cages have been tried.”
Submersible cages overcome disadvantages associated with floating cages such as current and wave forces which are stronger on the surface. However there are problems with submersible cages, too. Fish need oxygen and during certain times of the year it is an advantage for them to be able to live in warm, nutrient‐rich surface water.
The cages must therefore be flexible so that they can be brought to the surface relatively easily.
Closed systems in the sea are still quite rare, said Mr Hempel. “They have to have impermeable walls and bottoms made of plastics, steel, or concrete. None of the systems tested have proven commercial successes, to my knowledge.
“Some semi‐closed systems in open sea have been installed and are in use. These systems allow free flow of water into and out of the cages.”
The tension leg cage, which is based on the same principles as deepwater oil drilling platforms, is in use in deep water and in truly offshore locations. Many of these cages are in the Mediterranean Sea.
Less than a third of the 60 million tonnes of aquatic organisms produced in 2011 came from marine operations, despite the oceans offering the greatest potential for aquaculture.
Said Erik Hempel: “If we are going to be able to increase food production as much as we need in the future, it is time to look to the ocean to create large scale fish farming,"