...Feed Fish Offshore

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Open-ocean aquaculture differs from its coastal water counterparts in many ways. One of the more significant factors is remoteness and the need for automated processes to perform functions normally completed manually in coastal aquaculture (Picture 1). Of these processes, feeding is most important and without cost-effective, reliable unmanned feeding systems, open-ocean aquaculture is commercially impractical.

An innovative approach to unmanned feeding - called Robofeeder (Picture 2; Figure 3) - has been developed by the Center for Fisheries Engineering Research at MIT in cooperation with Ocean Spar Technologies for use on the 600 m3 Sea Station submersible cage. The Robofeeder is meant to serve as an on-cage feed storage and dispensing system. Two Robofeeders, each with a 500-pound capacity, have been built for evaluation and deployed on cages at open-ocean research sites off New Hampshire and Mississippi.

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Employing electronic control and pneumatic actuation, the system relies on gravity dispensing of feed. The on-board timer system allows the dispensing of controlled amounts of pellet feed for up to 24 feeding times per day. Additionally, one battery and air tank are expected to last for greater than 3-months.

The OAC Robofeeder was installed in December 2001 (Picture 4). The silo is designed to specifically fit on top of the Sea Station work platform (Picture 5). Feed is dispensed via a pneumatic-controlled gate-valve that opens to a 2" Y-fitting. All electronic components are currently housed in an on-board Pelican case (Picture 6). The initial configuration had a 2" diameter hose connected directly to the Y-fitting and exiting through 2" holes cut below the structural bar of the spar (but above the spar weld). However, this arrangement resulted in some feed settling in a slight horizontal bend in the hose and eventual clogging. Feed settling was remedied by reducing to 1½" PVC pipe that exited straight through the holes from the Y-fitting and then connected to the 2" hose (Picture 7).

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Throughout development some lessons regarding feed type and size were also learned. Initial thoughts were to move fingerlings offshore at a very small size, 8-10 grams in weight. However, to do so would require the ability to feed a crumble feed which has proven difficult given the high humidity offshore and use of the 2" hose that easily clogged with damp crumble feed. With a feed size limitation, fingerlings must now be moved offshore at a larger size, 40-80 grams, which can be fed pellets. Initially, we used a slow sinking pellet to increase the residence time of the feed in the cage thereby minimizing the amount of potentially wasted feed. However, slow sinking feed absorbs water while in the 2" hose and again resulted in clogging. Now we use sinking pellets to ensure feed is dispensed to the fish in the cage (Pictures 8 and 9). Observations have determined that even sinking feed has sufficient residence time to allow efficient feeding without expensive wastage.

The existing Robofeeder will next be modified to operate in a submerged mode, which may be purposely chosen for operations or periodically occur during times of tropical frontal movement, i.e. hurricanes. Modifications for submerged feeding will include pressure compensation at depth and development of methods to re-supply a submerged feed hopper from a surface vessel (Figure 10).