Subtidal oyster aquaculture: Creating safe and efficient production techniques through sustainable innovations

Air-assisted cage lift system

Summary

By bringing compressed air to the site (in a SCUBA tank or similar apparatus), farmers can attach an airline hose to the tank and begin filling PVC tubes surrounding the cage. As the PVC tubes inflate, the cage will rise to the surface. Since the cage is attached to an anchor on one side, and to the next cage in the line on the other, it also will not move laterally while on top of the water, which means the cage will sink back to approximately the same footprint once tending the shellfish is complete. Once the farmer is done tending to the oysters, he simply opens the valve at the surface and water pressure will push the air out of the PVC, allowing the cage to return to the bottom.

Typical oyster culture in subtidal waters is performed in cages that are constructed of coated wire, similar to lobster cages, utilizing 3” mesh, weighing approximately 75lbs (Fig. 2).

The air-lift system which was designed started with the same cages, and then modifications were made to allow the cage to be lifted using air instead of the typical hauler system (Fig 3).

The proposed design included four, 4” schedule 40 PVC tubes attached on the four sides, open at each end, with a rubber bladder inside the PVC. The tubes were designed to be the same length of the cage (two, 36” tubes, two, 63” tubes) to lift the cage out of the water. The cage also was proposed to have four, 2” air filled balls attached to the upper corners to keep the cage from tipping over while being raised, a weight to prevent lateral movement on the surface and an air line running from the surface down to the cage from which to put air into the system. While some of the proposed elements were implemented as proposed, several other proposed modifications had to be changed as the project went forward. Initially, the four, 4” schedule 40 PVC tubes were attached to the cage, however the cage would not raise to the surface fully (Fig. 4).

The initial 4” float design did not work for two reasons: 1) four 4” floats did not supply enough lift to bring the cages to the surface, and 2) with four air entry points and four air exit points, only one side of the cage would lift at once (Fig. 4). This brought about many iterations of possible solutions to the problems. The first problem of not enough lift was simply fixed by changing out the 4” PVC for 6” PVC. Two 5’ sections and two 3’ sections of 4” PVC provide 2,411.5in3 of displacement which equals 11.16 gallons of seawater, which was very close but not enough lift with oysters and bags in the cage. While only two 5’ sections of 6” PVC provides 3,391.2in3 of displacement which equals 15.70 gallons of seawater, which is more than enough lift to bring the cage to the surface.

The switch in PVC sizes solved the problem of not enough lift, however the more complicated issue was that of getting the cage to rise to the surface evenly and stay on the surface. The initial proposal called for an internal rubber bladder that could be inflated and deflated as needed. The two commercially available options for a rubber bladder is a simple inner tube or a reinforced PVC-coated pad which can be inflated from the surface. The rubber bladder is cost effective ($80 each), however much too brittle to be used in a marine environment. The reinforced material would work excellent as a lifting device, however at approximately $320 in costs for just that part of the system per cage; it is a prohibitively expensive option. The next best option was to use commonly available, relatively cheap PVC tubes. PVC is a robust material, will have a long life in a marine environment, and is available for cheap pretty much anywhere.

Initially two 5’, 6”-PVC sections were attached to either side of a cage with 12-gauge aluminum wire in four spots per PVC tube. Compressed air tubing (3/8”) was attached to a SCUBA tank at one end and brought to a four-way manifold located centrally under the cage. From there ¼” compressed air tubing was distributed to four ¼” barbs located on each end of the tubes, which were tapped and installed into the 6” PVC section. While this mechanism worked to get air into the system, the air would only move to the side that was higher in the water column, and then rush out into the water; therefore only one corner would lift at once. Four valves were then installed onto the manifold (substantially increasing costs at approximately $8 each), and each was only opened ¼ in order to create back pressure on the airline, and cause air to be pushed equally to each air inlet. This worked well to cause both airtubes to lift at once, however the air still went to only one end. The next modification was to cut and cap each 5’ section into two, 2 ½’ sections, each with one air inlet and two ¼” air outlet holes located centrally on each tube. This design allowed each inlet to get air equally, which caused the cage to rise to the surface equally and right itself once at the surface. The valves on the manifold were removed in the final design before implementation by reducing the airline going to each outlet to 1/8”. This allowed the back pressure on the airline to be achieved through smaller diameter aquarium tubing, reducing costs and simplifying the design. Cheaper manifolds were purchased as well with six ports, in order to be able to attach multiple cages in series (Fig 5).

In order to lift the cages, a typical SCUBA first-stage regulator was modified to be able to release air from the tank into a 3/8” compressed air line (Fig. 6). This would allow the 3000psi tank pressure to be lowered to a consistent 150psi which would go to the cages. The air would be slowly turned on with a ball valve, and then the cages would begin to rise to the surface (Fig. 7). Once all of the cages were at the surface and stable, the ball valve on the air line would be closed, sealing the PVC tubes from letting any water back in. The cages would then stay floating at the surface to allow the oysters to be cleaned, graded, etc. Once the work is finished, the air line is disconnected from the SCUBA tank, released back into the water on the buoy to which it is attached, and the air begins to leave the system back through the airline from which it came. The cages sink back to the ocean floor in the same spot where they came up due to anchoring on both sides, and the airline is easy to find next time by just picking up the buoy it is attached to on the surface.

Underwater upweller

Summary

Current subtidal shellfish aquaculture regulations limit structures in the water column to no higher than 18” off the ocean floor. This prevents subtidal farmers from taking advantage of current upweller systems that protect oyster seed and pass large amounts of food by the shellfish to increase their growth rate. A system was designed to mimic the same principles of protection and increased food in typical upwellers, however move the system to the bottom of the ocean below 18” to fit within current regulations. Through a battery powered system located on the ocean floor, subtidal farmers can buy smaller seed and take advantage of the gains in growth and survival without needing the upweller to be located on the surface.

The proposed upweller system consisted of the same wire-coated mesh oyster cage (Fig. 2), and utilized the same method of air-lift to the surface as the air-assisted cage lift system (Fig. 7). The cage was to be surrounded by fiberglass matrix, and epoxy coated to create a hard exterior to the cage, then a window was to be cut and fine metal mesh (2-4mm) was to be inserted. On the opposite end, two small waterproof submersible pumps were to be attached to pump water through the structure, and located in the back of the underwater upweller were to be two waterproof sealed 12V DC batteries with wet-connect power cables to power the submersible pumps.

As work into the airlift system progressed, it became evident that the lift needed to bring a system such as the underwater upweller to the surface would be prohibitively large. Also, by only having one window to pull water through, if there was ever a pump failure then water flow would be so low that high mortalities would be almost guaranteed if the pump was not repaired immediately. Therefore, to make the system more robust the design was modified, while keeping the same basic elements intact and bringing about the same result.

Instead of the large wire-mesh cage to hold the oyster spat bags, a 6’x2’ fiberglass tank was purchased, and large 6” windows were cut along the entire length on both sides. The windows were fitted with 9mm APDI mesh and connected to the tank with 2” strapping to seal the edges (Fig. 8). A vinyl-coated rack was attached to the tank in order to keep the bags centrally located in the water column within the cage. This way water would be able to flow freely around the upweller instead of only from the top down. A lid was then constructed and installed on top of the upweller in order to keep any predators from entering the upweller. The lid was put on hinges and constructed out of the same 9mm ADPI materials and 2” strapping. The lid was then held down on either end of the upweller with shock cord and S clips.

Installed within the underwater upweller were two submersible pumps, individually connected to battery power, so that each could function independently of the other in case of failure. Each pump was wrapped in 1.5mm nylon mesh to act as a crude filter to remove large particles in the water column before they clogged the pump. This filter was cleaned every two days when the bags within the upweller were cleaned. Each pump had a 3/8” plastic hose leading from the pump to a hole drilled in the exterior of the fiberglass tank by which water was removed from the system. The removal of water from the system would cause more water to enter through either the top lid or the windows on the side. The pumps were strategically located below the oyster bags, this way when water was pumped out of the system, the new water would be forced to travel through the oyster bags before getting to the pumps, thereby increasing food passing by the oysters.

The proposed design was to connect each pump to a 12V DC battery located at either end of the upweller system. Once the airlift system was investigated further, it was clear that it made no sense to include the batteries into the system which had to be lifted up and down. Incorporating the batteries within the system would only require more lift to raise the cage up and down, whereas if the batteries were separated from the system, less water would be required to be displaced in order to raise the upweller. Therefore the batteries were moved from inside the upweller to a watertight box on the surface with sufficient floatation to hold the batteries and float above the upweller (Fig. 9). The proposed design was to have two 1.25amp pumps and one standard car battery that could be swapped out and recharged; however as the plan progressed those expectations had to be modified. A standard car battery is 45 amp/hours (a/h), and in order to maintain integrity should only be drained 50% before recharging. Therefore if two 1.25amp pumps were running off of it, the battery would need to be switched out every 9 hours.

Clearly either the pumps had to be downsized, the batteries needed to be bigger, there needed to be a way to recharge the batteries or some combination of all three. In order to purchase solar panels that would have any meaningful impact on battery life, the cost would be many thousands of dollars, and while possibly a long-term solution, definitely outside the scope of this project. The first option was to get bigger batteries. The biggest batteries commercially available are for solar installations and have a maximum capacity of 225a/h. Unfortunately they are $550 each and 120lbs, which brings other complications and unexpected costs (Fig. 9). Even with the increased battery size, the two 1.25amp pumps still would only function for 45 hours before needing to be switched out. If there was any kind of weather event or mechanical breakdown which prevented accessing the site, all of the oysters could be lost with very little room for mistakes. Therefore the pumps had to be downsized as well. The pumps that were eventually purchased are 0.5amps, which meant that the battery still needed to be switched out every 3-4 days at the most, and we switched it out at three days to ensure the batteries would get a full charge. Cables were run individually from each pump to the surface and attached to the battery (Fig. 10). While the battery case was water tight, it was easier to tie it down to a float installed on the site in April 2012, since the batteries needed to be switched out every three days.

Due to the higher than anticipated battery and supplies costs only one upweller was constructed instead of the five that were proposed. However, even if five upwellers were constructed, the insight gained would have been the same. The upweller was then attached to the ocean floor with 18” helical anchors and 7/16” rope. The same airlift system as objective 1 was also used to lift the upweller to the surface and lower it back down after cleaning and sampling the oysters (Fig. 9). Since the upweller was so much lighter and smaller than a cage, 4” PVC was used instead of 6” PVC and functioned excellent (Fig. 11).

• Figure 4. Initial design with four, 4” PVC tubes lifting the cage. Orange line is air being put into the floats from a SCUBA tank.
• Figure 5. Two cages with the airlift system installed connected in series floating at the surface.
• Figure 7. Steps 1-4 in raising a cage to the surface. 1. Air pushes the water out of the PVC tubes, 2. One cage begins to rise, 3. Multiple cages are coming to the surface at once, 4. All three cages are at the surface and stable.
• Figure 10. Jordan Ward (summer research assistant) switching out the 120lb. battery which powered the two submersible pumps within the underwater upweller. Black cable leads from the battery down to the pumps; orange airline also leads to the upweller to lift it to the surface with the same airlift system as objective 1.
• Figure 2. Oyster cage used for deep water oyster culture. Dimensions are 63”Lx36”Wx18”H
• Figure 3. Typical oyster cage with float modifications being lifted using SCUBA tanks on the boat.
• Figure 8. Inside of underwater upweller with lid open. 6” windows fitted with 9mm ADPI mesh across the whole length on both sides (upper part of picture). Vinyl-coated mesh shelf to hold spat bags off the bottom (white mesh in center of picture). Orange 1.5mm mesh over the two pumps with plastic tubing pulling water to the exterior of the upweller (central in picture). Blue and orange lines leading to and from manifold in center of upweller to lift the cage to the surface.
• Figure 9. 225a/h battery in watertight box. Orange airline leading to the system and black battery cable leading to the two submersible pumps in the upweller.
• Figure 11. Underwater upweller floating on the surface with lid open.
• Figure 6. Using SCUBA tanks to put air into the system. 1. Airline with ball valve leading to the cages, 2. SCUBA tank with modified first-stage regulator to lower tank pressure to 150psi.