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Oxygen-Depleted Ponds Can Threaten Crawfish Crop

Foggy swamp

By Patrick D. Bonin

BRANCH, LA. –  While the foggy, overcast days of winter can be depressing or bothersome for us, those same conditions can sometimes spell more serious trouble for crawfish.

That’s because, under some conditions, they can lead to low levels of dissolved oxygen (DO) in crawfish ponds, resulting in weak, lethargic mudbugs with a significantly higher mortality rate than normal.

Mark Frugé, co-owner of Frugé Aquafarms, said oxygen levels are usually more of a problem as the fields are initially flooded in the fall when the crawfish are very small.

“When you first pump water in the fields, all the straw from the rice harvest starts decaying and it can suck all of the oxygen out of the water,” Frugé said. “As small as the crawfish are at that point of the season, they’re very susceptible to lower oxygen levels. But once they reach marketable size, all of your water quality issues are typically behind you.

“The only other time we sometimes have problems is much later in the season, when the temperature is hot and the water gets really warm: we’ll sometimes see crawfish crawling up on the stubble and putting their gills out of the water to breathe air.”

Those are typically the two times during the season when oxygen levels can become an issue, unless Mother Nature throws a curve ball and strings together several days of foggy, overcast weather. Under those conditions, lower levels of dissolved oxygen could once again spell trouble.

Aquaculture Specialist Mark Shirley with the LSU Ag Center said persistent clouds or foggy conditions over several days can prevent photosynthesis from occurring, which helps to replenish the oxygen level in water. Over even a relatively brief time frame, if calm, overcast or foggy conditions persists, the oxygen level in a pond can sometimes drop low enough to become problematic.

And according to the Crawfish Production Manual from the LSU Ag Center, warm water can’t hold as much oxygen as cold water. And rising water temperature increases biological activity, so what oxygen remains is consumed at an even faster rate.

This “double whammy” effect is pointed out clearly in the manual: when the water temperature increases from 70 to 80 degrees F, the rate of oxygen loss caused by decomposition of plant matter in the ponds doubles!

Despite all the potential trouble with DO, Frugé said it’s never created a huge problem on the farm in Branch. But the crop is closely monitored during harvest for signs the crawfish may be struggling under less than optimal oxygen conditions.

“We can definitely tell when they’re weak,” he said. “You can very easily see them being lethargic right out of the ponds.  At that point, what we try to do is harvest only what we have sales for and not carry any additional inventory. Their shelf life is drastically diminished when they’re weak.”

The good news, according to Frugé, is that the conditions that cause the problem don’t usually persist for long.

“Mother Nature normally takes care of that issue on her own,” he said. “The sun eventually comes out and everything gets back to normal.”

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