by PAUL HOLLIS
Auburn University research aimed at minimizing a deadly disease in catfish has, in a short time, prompted Alabama catfish producers to significantly change what they feed their fish.
In the study they launched in 2015, researchers are examining whether a fish-feed additive can help break the cycle of disease caused by a virulent strain of the bacterium Aeromonas hydrophila. The disease can wipe out up to three-quarters of the fish in an infected pond in a few days. Initial trials indicate the additive can improve catfish health and production.
“It’s exciting to see how quickly the industry has adopted what we’re doing,” says Eric Peatman, associate professor in Auburn’s School of Fisheries, Aquaculture and Aquatic Sciences and lead scientist in the investigation.
Since it began, the project has been awarded two USDA grants totaling $511,000. Peatman’s co-investigator in the study is Ben Beck, research physiologist with the USDA Aquatic Animal Health Research Unit in Auburn.
What makes the research unusual is that it focuses on stopping the disease cycle without the use of antibiotics or vaccines.
“When working with vaccines or antibiotics, it’s a long process to go from hypothesis to where you can actually do something out in the industry,” Peatman says.
The research is exploring the relationship between the use of plant-based diets for farm-raised catfish and the effect that undigested phytic acid and myo-inositol generated from the catfish feed has on the bacteria.
“This disease can use myo-inositol to grow, and there is a large quantity of this substance in catfish feed,” Peatman says. “If we more efficiently break down the phytate, fish can absorb those substances and use them for growth. If not, it passes through the fish and accumulates in the pond. That’s when we get poor water quality due to phosphorus pollution.”
The phytate issue is not new. Peatman says the poultry industry began looking at it three decades ago because it was under pressure to deal with phosphorus pollution caused by runoff.
“Since fish are in a static pond, it wasn’t as critical from an environmental standpoint for the catfish industry, but the more phosphorus you put into water, the more you’re promoting algae blooms that can cause off-flavor as well as spikes and dips in pond oxygen levels,” Peatman says.
What’s more, phytate may foster the growth of the lethal strain of A. hydrophila.
“This disease came out of nowhere in 2009,” Peatman says. “It had a different profile from diseases we had encountered before, hitting the larger-sized fish more and damaging production just when a pond is ready to be harvested. It hit farmers in the pocket to a magnitude that we haven’t seen in some of these other diseases that usually kill the juvenile fish right after stocking.”
Up until a couple of decades ago, fish meal or animal protein was the key ingredient in fish feed. When the feed industry shifted to less-costly plant-based components, however, phytate levels in ponds started increasing.
“Catfish is often a low-dollar enterprise, and producers can’t afford to pay much for feed,” Peatman says. “As commodity prices, along with foreign competition, increased, we changed to a more plant-based diet. As a result, certain feed mills have put different levels of phytate in their products.”
In initiating the study, researchers worked with a major feed mill in Alabama to spray the enzyme onto the feed. The additive has to be added after feed goes through the extrusion process. They also tested phytase-containing feed on three catfish farms in west Alabama while conducting lab work and control studies in Auburn and in Arkansas.
“In our pond studies last year, fish grew faster and about 20 to 23 percent larger,” Peatman says. “And they had a better feed-conversion ratio. They were retaining more minerals, particularly iron. That was interesting because we have periodic problems with anemia in our fish, and the addditive almost doubled the amount of iron in the liver of fish. They had more red blood cells, which may have contributed to their growth.”
The results appear to be consistent in this year’s harvest, too, with gains in performance and improved blood work.
For the 2016 growing season, almost all feed that was fed in Alabama catfish ponds had phytase in it for the first time, Peatman says.
“Both major feed mills in the state paid for professional spray equipment to apply it, and the price per ton is pretty low,” he says. “The average price per ton for catfish feed varies between $375 and $400. Depending on how much the mills add for the actual enzyme cost and to recoup some of their equipment cost, the cost of the additive is about $4 to $6 per ton. That’s a small price compared to the gains we’re seeing.”
Peatman doesn’t believe the feed additive will be a silver bullet for getting rid of A. hydrophila in catfish ponds, but it’s a step in the right direction.
“It looks to be a disease impacted by the environment,” he says. “As the environment degrades, the producer is feeding heavily, water quality goes down, and the disease shows up. If we can clean up the water by putting less phosphorus into it and improve the feeding efficiency, we can decrease the waste in ponds.”
Continuing research and industry trials in the coming months will seek to answer the following questions:
- What’s the most effective level of phytase inclusion in the catfish diet?
- Does phytase addition reduce A. hydrophia outbreaks or their severity?
- How does a 28-percent protein diet with phytase inclusion compare in performance with a 32-percent protein diet without phytase?
- What is the effectiveness of the addition of phytase versus high levels of dietary iron in combating anemia?
