Mobile fish farms could soon navigate the oceans

Swimming cages may soon shepherd farmed fish about the ocean, giving them a more natural environment and reducing their impact on natural ecosystems. The first tests of the wandering cages have just taken place in Puerto Rico.

Fish farms in the open ocean offer an alternative to conventional fishing, which is on track to wipe out all commercial stocks by 2050. But there are concerns that installing large, static farms could damage local ecosystems.

"Depending on the size of the stock, large residues of fish faeces could catch under the cages and degrade the seabed," says Cliff Goudey from Massachusetts Institute of Technology. Anchoring cages against the battering of storms would also be a challenge.

So Goudey came up with the idea of wandering cages. These wouldn't stay in any one place long enough to damage local wildlife, and could drift with storm waves to avoid feeling their full force.

High-tech migration

He recently tested the first self-propelled cage at a sea farm in Culebra, Puerto Rico. Existing near-shore farm cages are moved only occasionally, using large tugboats.

To provide the necessary propulsion, two large propellers, 2.4 metres (8 feet) in diameter, are attached to one side of a 19-metre (62-feet) diameter spherical cage.

The cage mostly follows the sea's natural currents, but if it drifts too far off course, the propellers can provide 12.4-horsepower propulsion to guide it back onto the planned route.

The propellers are driven by electricity from a diesel generator on a small boat tethered above the cage. The prototype managed to propel itself at a steady rate of roughly 0.3 metres per second, with good manoeuvrability, Goudey reports.

Future tests will see cages equipped with GPS receivers and route-planning software to autonomously keep the fish on track, perhaps steering themselves back to shore when it is time for harvesting. Before they can set sail the cages would also need technology to communicate with and avoid shipping and other hazards.

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