Liquid Oxygen Saves Channel Catfish During Harvest
Fish biologist Les Torrans
(left) and catfish farmer
Sam Pilkinton adjust the
flow of liquid oxygen from
the Sock Saver into the
sock (in pond). Oxygen is
added when dissolved oxygen
in the pond is low.
An improved method of supplying farm-raised catfish with
enough oxygen during a crucial stage of production means more fish could
survive to make it to market.
Les Torrans, an ARS fishery biologist in Stoneville, Mississippi, and his cooperators Charles D. Hogue, Jr., an aquaculture specialist with the Mississippi State University Extension Service, and Sam Pilkinton, a catfish farmer and live hauler with Pilkinton Brothers Farms in Columbus, Mississippi, designed and built equipment to apply liquid oxygen when fish are crowded together during harvest.
Channel catfish are harvested on commercial farms in the United States with long nets pulled by tractor and hydraulic net reels. This method has been used since the 1960s. In the final stages of harvest, fish are concentrated in net "socks" at high densities. They're often held overnight to allow smaller, submarketable fish to escape, or "grade out," through the mesh of the sock. More than 50,000 pounds of food-size catfish can be held in a sock, which measures 10 feet wide and 50 to 100 feet long. To provide oxygen, water is slowly moved through the sock with tractor-powered or electric paddlewheel aerators.
Workers prepare to haul
another basket of catfish
from the sock, where fish
are first gathered and held
overnight before being sent
When the dissolved oxygen level is high, sock-grading works well. But when it is low (as low as 0.5 milligrams per liter in some intensively managed production ponds), many fish can die. This is especially true in the summer when water temperatures are high.
Paddlewheel aerators are used to increase dissolved oxygen slightly while catfish are held in the sock. But the increased water velocity through the sock adds to the metabolic oxygen demand of the fish. After a night of chronic oxygen stress, fish are further crowded as they're loaded onto a transport truck. This additional stress often results in several hundred pounds of dead fish arriving at the plant. The farmer gets no payment for these dead fish, called weigh-backs.
Torrans said he and others had been talking about using liquid oxygen in socks for about 15 years, but it hasn't been widely available until recently. Today, most private live haulers and fingerling producers use liquid oxygen on their transport trucks, and many farmers use it on their farm trucks when moving fish from pond to pond.
Paul Wilson, manager of
Arrowhead Catfish Farm in
Columbus, Mississippi, guides
a basket containing 2,000
pounds of catfish into a
transport truck that will
haul the fish to a
Air contains 21 percent oxygen and 78 percent nitrogen.
When air is compressed and cooled, these gases become liquid. The liquid
is warmed just enough for the nitrogen to turn back into a gasleaving
only liquid oxygen. It's commercially manufactured and delivered in
tanker trucks to on-site storage vessels, much like gasoline is.
The availability of liquid oxygen spurred Torrans to write an article 2 years ago in an industry newspaper, in which he discussed the feasibility of using liquid oxygen in catfish production operations, especially during summer months.
Sam Pilkinton read the article and contacted Torrans. The two met, and together with Hogue, they designed and built the system they named the "Sock Saver."
Torrans says his 30 years of experience working with catfish on commercial farms, universities, and government agencies left him confident that the method would work. He says he knew catfish farmers like Pilkinton had the know-how to help design a successful system.
Mississippi State extension
agent Charles Hogue, Jr.,
takes an oxygen reading from
one of the catfish ponds at
Arrowhead Farm. In the
background are paddlewheel
aerators typically used to
oxygenate the ponds.
"Catfish farmers are out there every day doing research
on their farms. They're constantly developing and adapting new ideas
to fit their needs. They have the engineering experience to develop
a hypothetical concept into a workable unit using available technology,"
A small trailer was designed to hold three 50-gallon tanks
to deliver the liquid oxygen wherever it's needed on the farm. It can
be hauled behind a pickup truck or a small tractor around commercial
ponds. The equipment is maintenance free, small, durable, and simple
to operate, and it requires no power source. Individual flow regulators
allow use of one to eight diffusers as they're needed. It is compatible
with existing harvesting/loading equipment and methods, so farmers could
use it without much training.
"There are no moving parts," Hogue explains.
"It's completely self-contained and can be set up out of the way.
It's very easy to use."
The unit can increase dissolved oxygen in the sock by
as much as 0.9 milligrams per liter, enough to ensure survival of the
fish. The total cost of the complete unit with trailer, tanks, pressure/flow
regulators, and diffusers was $8,400.
The Sock Saver costs less than $3 an hour to operate.
It has been tested on a commercial catfish farm for the past year, and
initial reports are very positive.
Studies of oxygen transfer efficiencies are complete,
and Torrans is analyzing the findings to determine exact measurement
of dissolved oxygen in the socks. But people using the system are reporting
livelier fish coming out of the socks.
"People in the field can see the results just by
looking at the catfish," Torrans says, "and they are saying
that they have fewer weigh-backs."
Torrans says that they are not making recommendations
for using liquid oxygen in production ponds at this timeother
than in the socks. The mechanics of doing this are not worked out yet,
but he predicts they soon will be. Torrans's research shows that increasing
dissolved oxygen in ponds results in faster-growing fish. "Oxygen
is needed for metabolism and food digestion," he says. "Low
oxygen levels greatly reduce the amount of feed the fish eat. The more
they eat, the sooner they make market size. They could reach market
size in 2 years instead of 3, because with higher dissolved oxygen it
takes them only a year, instead of 2, to go from fingerling to food
Core, Agricultural Research Service Information Staff.
This research is part of Aquaculture, an ARS National
Program (#106) described on the World Wide Web at www.nps.ars.usda.gov.
"Liquid Oxygen Saves Channel Catfish During Harvest" was published in the August 2003 issue of Agricultural Research magazine.