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'Free' water could be costly for crawfish

BATON ROUGE -- Crawfish farmers who elect to fill their ponds with rainwater from the recent storms may be risking at least the early part of their crops, says LSU AgCenter aquaculture expert Dr. Greg Lutz.

Fall is the season when farmers flood ponds to bring crawfish out of their burrows and into the water, where they can grow and develop. That process of filling the ponds usually involves pumping water from underground wells or other sources, which can be expensive, so rainwater poses temptations, Lutz says.

"Free water falling from the sky certainly is tempting," Lutz says. "It looks as though it could save money, but if ponds are filled at the wrong time, it actually can cost a lot more."

Lutz explains that crawfish ponds rely on the gradual breakdown of vegetation grown during the summer to provide a natural food chain for the crawfish. But the natural microorganisms that cause the vegetation to break down can consume large amounts of oxygen from the water.

"If the oxygen levels in a pond get too low, the crawfish present (including the newly hatched young) may be lost, eliminating any chance of harvesting an early crop," Lutz says, adding, "Knowing how and when to flood really is both a science and an art."

The LSU AgCenter expert explains that crawfish farmers generally flush water through their ponds early in the season to overcome the problem of low oxygen.

"A common mistake made by many crawfish farmers, however, is to partially or completely fill their ponds only to find that they can't replace the water as quickly as it goes bad," he says, stressing, "This can equate to a major economic loss, in unharvested crawfish and in pumping costs."

That could be possible, for example, if farmers take advantage of recent rainfall but don't have adequate sources of additional water readily available - such as underground wells or other surface sources.

"Temperature is the main factor in determining when to flood a crawfish pond, because temperature directly affects oxygen levels in several ways," Lutz says, explaining that also is a consideration with flooding ponds right now.

The demand for oxygen will always be higher in warmer water because of the increased activity of the microorganisms that break down the vegetation, and the oxygen requirements of the crawfish also will be higher in warmer water, he says.

In addition, the warmer water is, the less oxygen it can hold in solution, Lutz points out.

"Taken together, these factors explain why early flooding of crawfish ponds during September is generally discouraged, and later flooding under cooler conditions, in mid- to late October, usually produces better results for growers," he says.

Lutz says a crawfish pond that has a good stand of green rice plants and not much dead grass will have very low oxygen demand and not require much flushing, while a previously-harvested rice field with plenty of straw still on the ground will have a very high oxygen demand.

"For many crawfish producers, the temptation to fill their ponds a few weeks early with all those inches of free rainwater that hurricanes can provide may be too much to resist," the LSU AgCenter specialist says. "Often, within a few days after the storm has passed, oxygen levels in newly flooded ponds will drop to unacceptable levels, and the water will have a brown, dingy appearance.

"In situations where water quality cannot be maintained after a crawfish pond has been filled, it is often preferable to lower the water to a depth of only several inches and then start pumping new water into the pond. In this way, energy is not wasted trying to dilute a pond full of stale water, the young crawfish will still have sufficient water depth to avoid most predators, and an early harvest may still be possible."

Contact: Greg Lutz at (225) 765-2848 or [email protected] Writer: Tom Merrill at (225) 578-2263 or [email protected]

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