Field Notes: Spring 2010

Prawn Shop: Direct Marketing Pond-Raised Freshwater Shrimp

by Wylie Harris

See the Kerr Center's information packet on pond-raised prawns.
See other Kerr Center resources on aquaculture and aquaponics.

Some might say that the closest thing to Oklahoma-grown seafood is a Rocky Mountain oyster. Cashion farmer Jeremy Eaton has other ideas.
Like others all over Oklahoma, Eaton’s humble farm pond performs many useful functions: stock watering, erosion barrier, swimming hole. Nevertheless, he thought his pond was “underutilized,” and worked out a novel way to make it pay – by raising freshwater shrimp, or prawns, in it.
Eaton has been pioneering the idea for two years with help from an Oklahoma Producer Grant from the Kerr Center. This past September, he hosted a crowd of over 50 people to explain what he’s done and learned.

How to Have Your Pond and Eat It, Too

“Probably the best thing I could plant on this land would be houses, if I wanted to make money,” Eaton says.

While he has no objection to making money, he‘d rather do it in a way that will keep the farm in the family for the next generation. “Our kids are the fourth generation on this land,” he says.

So, Eaton wondered, “What can we do as small producers to make a little extra money?”
In other words, “What can we do locally, with a resource most people already have, that’s underutilized?”

From that line of questioning, the pond-raised prawn project was born.

Shrimp Temps (and Other Factors)

Eaton contrasts his pond-based approach to prawn farming with more capital- and labor-intensive commercial operations, which might stock 16,000 larvae per acre and need aeration 24 hours a day.

Eaton stocks his prawns at fewer than 10,000 per acre, in a pen only a fraction the size of his two-acre pond. The rest of the pond acts as a giant buffer, replenishing the oxygen in the prawns’ pen and eliminating the need for active aeration.

The same effect prevents nitrate buildup in the pond water. “You need a big buffer area,” Eaton explains.

Even so, he tests oxygen levels regularly, two feet down, in the morning, when they’re lowest. “You can use a water testing kit or a digital oxygen meter,” he says. “They’re about the same price.”

The prawns are “pretty hardy,” he says, able to survive oxygen levels down to 3 parts per million. They’re less tolerant of some other extremes, preferring a pH between 5.5 and 10, with 7 being ideal.

Temperature is another important factor. The prawns become fatally stressed when the water temperature reaches 85?F. “We learned this June, with 110? days, how cool the water stays, even just a foot deep, because of all the mud,” Eaton says.
Low temperature can be just as troublesome, with the prawns ceasing growth at 68? and dying at 55?. On the positive side, that sets a natural harvest date, in mid-September. It also keeps the prawns from spreading and becoming invasive.

A Prawn’s Life

Prawn season in Eaton’s pond begins in late May or early June, when he buys sixty day-old larvae from a hatchery in Weatherford, Texas.

Technically called “post-larvae” at this stage, the prawns are the length of a nickel, and half as wide as a pencil. Eaton buys them at seven cents apiece.

To haul the prawns back to his farm from Texas, Eaton used an old chest freezer filled with water. A compressed air tank connected to a regular aquarium-type air stone provided plenty of oxygen for the duration of the trip.

In the first year of his producer grant, Eaton stocked his pen with 1,000 prawns.
“The prawns have density-specific growth,” Eaton explains. “They’ll grow smaller if there are more in a given space.” That means that raising the stocking rate only increases harvestable yield up to a certain point.

Eaton’s pen is a circular net 100 feet around and 8 feet deep. “Pool noodles” float the upper edge, with T-posts to hold it up during periods of low water. The net, made of 1/16-inch mesh, is sewn together at bottom.

“Nothing gets in or out, though some catfish eggs did hatch in there,” Eaton says.

Care and Feeding

Eaton checks his prawns at least once a week, using a seine net, or sending his kids in for a cooling dunk in the pond.

“You’re farming blind – you can’t see what’s in there,” he says.

One of the best ways he’s come up with for checking the prawn’s numbers is to shine a flashlight over surface of the pond at night. The prawns’ eyes throw back a golden reflection.

That may not yield a very precise count, but not much precision is necessary. “If any die off, they all will,” Eaton jokes
.
“Our net’s set out at least ten feet in the water,” he says, “to keep four-legged critters from damaging the nets.”

He also needs to watch what he and his two-legged neighbors are spraying in the pond’s drainage catchment. Many pesticides are also piscicides, harmful or fatal to fish.
Compared to this routine prawn surveillance program, the feeding regimen seems like simplicity itself. The prawns aren’t picky about their diet. As omnivores, they’ll eat catfish feed, alfalfa flakes – even, unlike saltwater shrimp, each other.

Eaton feeds regular sinking catfish feed once a day from the first day onward. “It cost $26.50 for a 50-pound sack this time last year,” he says. “It’s $13.50 now.”

To keep up with the prawns’ growth, he doubles the daily ration every month. “You can check the feeding rate just by scooping it up off the bottom,” he explains. “If there’s a layer of food three or four inches deep, you’re feeding too much.”

“You have to broadcast over whole surface, because the prawns are territorial,” he advises. Dumping all the food in one part of the pond would fatten a few prawns and starve the rest.

Greening the Waters with a Home-grown Shrimp Revolution

Eaton’s project aims for farm-based prawn production to turn the green of pond scum into the green of cash, but the idea also has a green side in the ecological sense.
As oceanographer Sylvia Earle points out, simply choosing shrimp over other seafoods can be a sustainable seafood decision. Some other popular food fish, like tuna and salmon, are carnivores; Earle likens using them as a food source to raising herds of lions or tigers for the table.

In contrast, marine shrimp eat low on the food chain, requiring fewer resources to grow a calorie’s worth of human food.

However, as Eaton has witnessed, freshwater shrimp are happy to eat animal protein, even from their own species. In common practice, commercially farmed freshwater shrimp fatten on an energy-intense diet of other fish.

Shrimp’s popularity increased as its price fell, after researchers worked the last kinks out of pond-based production. In 1980, at the beginning of the global shrimp-farming boom, U.S. shoppers each put away a pound and a half of shrimp per year, on average. By 2006, the figure had risen to 4.4 pounds.

But while shrimp farming cheapened the economic side of the production equation, its ecological and health costs climbed.

Unique coastal ecosystems, mangrove forests possess high biodiversity, flood-buffering roots, and carbon-storing biomass. All these ecosystem services are lost when mangroves forests are cleared for shrimp ponds – a process that still continues in some areas of the coastal tropics.

Mangrove forests declined in area by 35% worldwide during the 1980s and ‘90s. As much as a third of that loss resulted from new shrimp farmers’ clearing them, mostly in Asia and South America.

Intensive shrimp farming also makes heavy use of synthetic fertilizers, pesticides, and antibiotics. Some of these substances, along with antibiotic- resistant strains of bacterial pathogens, end up in the shrimp served at tables all over the world.

Switching from freshwater to saltwater prawn farming just adds soil salinization to this list of problems. Catching shrimp from the ocean dodges all of them – but shrimp trawling, while generating two percent of the global fish catch by weight, also causes over a third of the total “bycatch,” or unwanted species also caught and killed, including whales and dolphins.

Ocean-Front Marketing in Oklahoma

Thanks to the boom in cheap but environmentally costly forms of shrimp farming, shrimp now flies off the shelves faster than any other seafood in the United States, but this country doesn’t produce nearly enough of it to satisfy that demand. The U.S. imports about sixteen pounds of shrimp for every one pound raised here – a $4 billion market ripe for capture by domestic producers.

Eaton harvested 50 pounds from last year’s initial stocking of 1,000 larvae. With all the kinks worked out of his system, he estimates that he could up that figure to 80 pounds – an 80% survival rate.

By tripling his current stocking rate, and charging $10 a pound for live shrimp, Eaton projects a potential yearly net income of over $1,800 from the enterprise.
“That’s not enough to sustain a family or farm,” he acknowledges, “but it’s a significant supplemental income.”

As Maura McDermott wrote in Oklahoma Today magazine, “With 250,000 ponds to call its own, Oklahoma has a shoreline for almost every one of us.” Combined with Eaton’s small-scale approach to prawn farming, that fact translates into ample room for additional farmers.

If every one of those quarter million ponds raised 240 pounds of shrimp a year, the total catch would 60 million pounds of shrimp – about four times the amount that Oklahomans eat in a year, but still only about five percent of total U.S. shrimp imports.

This Pond is Your Pond, This Pond is My Pond…

Such thinking seems to be catching on. According to the 2005 Census of Aquaculture, U.S. aquaculture generated farm-level sales of $1.1 billion, a growth of 11.7% over the preceding seven years. Still, when it comes to putting food on tables, aquaculture both in Oklahoma and in the nation has barely scratched the surface of what’s possible.

Of the $672 million worth of food fish that U.S. aquaculture does produce, Oklahoma contributes only $421,000 – roughly half of it catfish, and the other half grass carp. Most of the country’s farmed prawns come from Kentucky, Tennessee, and Ohio, with Mississippi alone producing half of the catfish.

Commercial catfish monoculture, making up half the value of Oklahoma aquaculture, averages 2,000 pounds of fish per acre, but requires heavy external inputs of feed and fertilizer.

The bass-bluegill biculture common in southern farm ponds averages 200 lbs. per acre – a much lower production level, but without feed or fertilizer. By adding caged shrimp to that setting, Eaton achieves around 120 additional pounds per acre.
Some of the oldest and most productive farming methods in the world integrate aquaculture with terrestrial crop and livestock farming to attain much higher fish production.

Traditional intensive Asian polycultures, for instance, integrate grain and vegetable crops, poultry, pigs, and several different fish species to achieve aquacultural harvests of up to 7,000 lbs. per acre, with few to no off-farm inputs.

Even without such phenomenal yields, Oklahoma and U.S. aquaculture could still make great strides in local fish production with little additional investment.

According to aquaculture researcher William McLarney, the U.S.’s estimated two million half-acre ponds could, without feeding or fertilization, yield 100 million pounds of edible fish every year.

That works out to one-sixth the 2005 yield of U.S. catfish farming (this country’s most productive aquaculture industry), and 1.5% of all food fish landed in the U.S. in 2008, “all at a fraction of the cost of conventional commercial fishing or aquaculture.”

In McLarney’s words, “What are we waiting for?”