Normally, catfish producers use tractor power or an electric paddlewheel to move water through the sock, and provide oxygen to the catfish. And while paddlewheel aerators can increase the level of dissolved oxygen they also increase water velocity, adding to the metabolic oxygen demand of the fish.
Charlie Hogue, catfish researcher at the Thad Cochran Warmwater Aquaculture Center in Stoneville, Miss., says the “Sock Saver” provides liquid oxygen to catfish spending hot summer nights awaiting transport to a processing facility. The invention also minimizes problems caused by a loss of electricity, or tractors being moved during the night from one pond to another.
“Fish come out of the sock alive and well. It’s good piece of mind to have that oxygen in there, because it greatly reduces the number of stressed-out fish turned down, or dead on arrival, at the processor,” Hogue told growers attending the East Mississippi Ag Expo in Mayhew.
“After a night of chronic oxygen stress, assuming the fish survive the night, the fish are further crowded to facilitate loading on a transport truck. This additional stress often results in several hundred pounds of dead fish arriving at the plant – weigh-backs for which the farmer is not paid – and also reduced the flesh quality of the processed fish,” says product developer Les Torrans with USDA’s Catfish Genetics Research Unit in Stoneville.
“It can be moved around easily, and we figure it costs about $3 an hour to operate,” says Hogue. “It does not interfere with normal loading of catfish.”
The “Sock Saver” is not currently manufactured commercially, but Torrans and Hogue say it’s a simple do-it-yourself project for producers. “We designed the Sock Saver using a single-axle trailer that could be moved around the farm with either a pickup truck or small tractor.”
They use a single axle trailer with three 50-gallon liquid oxygen (LOX) dewars connected together. The tanks deliver oxygen through eight 20-200 SCFH flow meters, each supplying an eight-foot self-weighted bioweave diffuser through a 100-foot oxygen hose.
The three 50-gallon LOX dewars are connected to the trailer with brackets, and the three dewars are connected to a steel manifold with one-fourth-inch-grade “R” oxygen hoses. One oxygen hose connects this manifold through a single oxygen regulator to a second steel manifold. Pressure in this second manifold is held at 30 PSI with the regulator. Eight acrylic-block air flow meters with individual rated capacities of 200 standard cubic feet per hour (SCFH) are connected in parallel to the second manifold through 12-inch lengths of oxygen hose. Each flow meter is connected to a 6-foot self-weighted Bio-Weave diffuser tube through a 100-foot length of oxygen hose with a quick-disconnect coupling.
In use, the diffusers are placed inside the sock where they sink to the bottom. After use, the diffusers are removed and stored in a box on the trailer. The eight lengths of oxygen hose are stored coiled on hooks welded on the trailer. In tank trials with water 3.5 ft deep at a temperature of 27º C, this diffuser had an oxygen transfer efficiency of over 15 percent. The transfer efficiency would be greater in the 4-to-6-foot water depths where socks are normally positioned.
The entire system, including the custom built trailer, three dewars, flow regulators, diffusers and assorted fittings cost a total of $8,333, according to Torrans. “The system was designed to deliver oxygen at full capacity for nearly 11 hours. In practice, flows have not exceeded one-third maximum capacity.
The system was designed to deliver oxygen at a rate of up to1600 SCFH (13.9 liquid gallons or 132 pounds per hour). With a transfer efficiency of 15 percent, in excess of 20 pounds of oxygen per hour would actually be transferred to the water. In theory, 80 percent of the oxygen demand of 50,000 pounds of catfish could be met with this added oxygen alone.
“One, several or all eight diffusers can be used as needed. The diffusers and hoses are self-weighted, tangle-free, and do not interfere with the normal process of crowding the fish in the sock and dipping them up in the loading basket,” says Hogue.
The sock-saver has so far proven to be simple to operate, durable and virtually maintenance-free, according to its developers. Since it does not require an electrical or mechanical power source, it can be safely set up anywhere. It has a small “footprint” and does not interfere with other equipment normally used when loading fish.
“In our tests, catfish mortality was reduced, and the higher oxygen could result in improved quality of the processed fish,” Hogue says. “While we tested this equipment only with food fish, it should be equally valuable to a fingerling producer harvesting fingerlings in warm water.”