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Self-Propelled Aquaculture Cage

US - A boat towing a fish cage is business as usual on fish farms. But a cage towing a boat? That novel role reversal was on display recently in Culebra, Puerto Rico, as part of a project led by Cliff Goudey, director of MIT Sea Grant's Offshore Aquaculture Engineering Center (OAEC).

According to Aquafeed, Goudey conducted the tests at Snapperfarm, Inc., an offshore fish farm that has been growing cobia in submerged cages for five years. The spherical cage, developed by Ocean Farm Technologies, Inc. of Searsmont, Maine, was an Aquapod with a capacity of 3,250 m3.

Aquafeed reports that the project is funded by NOAA’s Marine Aquaculture Program, aimed at demonstrating the technology needed to raise fish in the vast portions of the U.S. EEZ that are too deep for conventional anchored fish cages. By operating while submerged and in predictable ocean currents, mobile fish farms need only a modest means of positioning to stay within planned trajectories. There are many locations both in U.S. waters and around the world where oceanic currents and gyres offer useful frameworks for such mobile operations. Though futuristic compared to today’s near-shore fish farming practices, the concept avoids the user conflicts and compromised water quality of coastal waters. However, it introduces technical and logistic challenges, including the need for efficient self-propulsion.

In conventional protected-water fish farming, cages are routinely repositioned when their shallow sites are fallowed to control disease. Stout towboats haul the enormous cages to another site, and both the cage size and typical propulsive inefficiency of boats make such movements very energy-intensive events.

Goudey‘s different approach to moving cages exploits the inherent efficiency of large-diameter, slow-turning propellers. By placing propellers on a cage, Goudey frees it from the normal constraints of a boat. His system uses a pair of eight-foot diameter, electrically powered propellers, with 6.2-horsepower underwater motors. Through a two-stage reduction gear, the propellers turn 42 RPM at full speed. By fixing a pair of the units to the mid-depth of the 62-foot diameter Aquapod, in a horizontal line nine feet apart, Goudey was able to maneuver the cage like any twin-screw vessel.