Managing Water for Fall Flood-Up

When To Flood?

Flooding should usually coincide with peak egg hatching by females in burrows, which occurs in September and October. We recommend that most farmers flood ponds or fields when daytime air temperatures cool to a consistent 80-85^oF in the afternoon and 60-65^oF in the morning, which traditionally is early October. The October flooding recommendation takes into consideration not only peak crawfish reproduction but also the ability and willingness of farmers to manage water quality. Because the type and maturity of vegetation in the pond affects water quality, your forage crop should be taken in consideration when deciding on an appropriate flooding date.

Fields with young, immature rice (green rice planted in late July or August) with few weeds or grass can be flooded in mid- to late September. September flooding can potentially give some producers an edge in producing an early crop, but be advised: the earlier one floods, the higher the risk of damaging the crawfish crop from poor water quality, specifically low oxygen.

Ponds with cut-over rice stubble, natural vegetation (weeds) or sorghum-sudangrass as forage crops should not receive a full-flood before October. Ponds with these forage crops almost always have poor water quality if flooded to full depth in September. Most crawfish producers do not have sufficient pumping capacity to correct oxygen deficiency when it occurs, and this is why we discourage September flood-up for many farmers. Oxygen deficiency slows the growth of newly hatched crawfish and possibly holdover crawfish, and under extreme conditions will suffocate and kill crawfish.

Also, the earlier one floods, the higher the rates of water evaporation and the need for pumping just to maintain proper water level in the pond. Although none of us can predict the weather, September and October are historically dry months in Louisiana.

Another consideration to remember is that just because a pond is flooded early, this does not mean that many females will be emerging from their burrows and releasing their young at that time. Many females that burrowed high in the levees do not emerge from their burrows with eggs or young until flushed out by rainfalls occurring from October through December. The key point is that early flooding in September does not speed up the spawning cycle, and thus it will not assure you of an early crop.You may see a few female crawfish with babies crawling around after a heavy rain in September, but many more will emerge with young in October.

Water Depth

A maximum water depth of 14 to 18 inches is sufficient for crawfish production, but not all the water needs to be added at once. There is no need to bring the pond up to full flood as quickly as possible. It is usually best to raise the water level in stages, with the goal to have a full flood by November or early December. If rice was planted only for crawfish forage (July/August planting), a shallow, partial flood of several inches in September is sufficient to keep the rice growing, and this shallow flood will not prematurely flush out females burrowed above water level. By early October when the daily temperature begins to cool and the rice is 18 inches or taller, the water depth can be increased gradually in stages to assist in flushing females with young from their burrows. Consider filling your pond to a depth several inches below the top of the drain pipes and allow late autumn and early winter rains to raise the water level gradually to full flood. This should save you some pumping expense.

Flushing to Improve Water Quality

Flushing, or replacing water in a pond, is often used to try to alleviate oxygen deficiency. Ideally, water management and pumping decisions should be based on oxygen measurements. No benefit to crawfish health and survival is gained by exchanging water if oxygen levels are satisfactory. Although pond water lacking in oxygen is often clear and dark (the color of tea, coffee or cola), one should not rely on visual observation or smell to determine oxygen concentration. Dissolved oxygen can be measured with an oxygen meter or a chemical test kit. A dissolved-oxygen meter is best if you have many ponds to check. Most producers, however, choose oxygen test kits because the kits are inexpensive and easy to use. The easiest kit uses a vacuum ampoule that draws in a water sample. The oxygen concentration is determined by matching the color of the sample in the ampoule to a chart. A list of supplier of test kits and meters, and advantages and disadvantages of each are available in the publication “Measuring Oxygen in Crawfish Ponds,” which is available on the LSU AgCenter crawfish webpage:

www.lsuagcenter.com/crawfish

Or send us an e-mail or give us a phone call and we’ll mail you a copy of the article.

Ideally, dissolved oxygen should be maintained above 2 parts per million (ppm) for good crawfish production, but this level of oxygen content can be difficult, if not impossible, to sustain in the first 2 to 6 weeks following flood-up, particularly when flooding over rice stubble or in ponds with sorghum-sudangrass or volunteer vegetation. When oxygen levels remain consistently below 1 ppm throughout the day for several weeks, crawfish become sufficiently stressed that they may stop feeding. Growth will slow, and some death may occur. When oxygen levels remain below 0.5 ppm throughout the day for a week or more, newly hatched juveniles and molting crawfish may die. Larger crawfish stressed by low dissolved oxygen climb to the surface on vegetation or traps and expose their gills to higher oxygen levels at the surface. If you see large crawfish at the surface, clinging to vegetation or traps while lying on their side, this is a sure sign that oxygen is very low.

 

BOD -- What is it and Why is it Important in Crawfish Production

BOD stands for Biochemical Oxygen Demand or Biological Oxygen Demand. Scientists use this laboratory test to measure how fast oxygen is consumed in water. If the BOD is high, that means many organisms of all types are present in the water, consuming the oxygen at a rapid rate. All crawfish ponds have large amounts of decaying (rotting) vegetation after flood-up, and the bacteria and other organisms associated with decomposing vegetation consume large amounts of oxygen. The warmer the water, the more rapidly oxygen is consumed. That is why the oxygen level in ponds gets critically low, usually 2 to 6 weeks after fall flood-up, and why the earlier in the year one floods over large amounts of vegetation (such as early September), the more severe oxygen depletion is likely to be and the greater the danger to damaging the potential crawfish crop. Ponds with “volunteer” terrestrial vegetation like grasses and sedges usually have a high BOD and low oxygen because these forages usually die and decompose fairly quickly after flooding.

Crawfish and all the other organisms in the pond need oxygen to survive. Low oxygen levels in the pond will either slow the growth of the young crawfish or kill them in large numbers. This is why there is some potential benefit to flushing oxygenated water through the pond, especially early in the season when temperatures are warm, to overcome the problem of high BOD and low oxygen. Cool and cold temperatures reduce the BOD -- and also oxygen problems. Dissolved oxygen is seldom low in late December, January and February.

Ponds with green, immature rice and few terrestrial grasses and weeds at flood-up will have the lowest BOD and the fewest oxygen problems. Ponds with rice stubble and abundant straw remaining from rice harvest will have a high BOD and low oxygen. Cut-over rice fields can benefit by holding several inches of water for a few weeks over the cut straw to get a head start on decomposition and to reduce the BOD before most of the females have emerged from their burrows with juvenile crawfish. It is probably best to drain most of that water before the deeper, permanent flood is held for crawfish.

Flushing water through a pond to reduce the BOD is expensive, but the loss of a significant number of juvenile crawfish because of low dissolved oxygen is also expensive. Many farmers do not have adequate pumping capacity to flush water quickly enough to effectively prevent low oxygen, so this is why we suggest most producers flood ponds in October.

If you do have adequate flushing capacity, the cost of putting oxygen in the pond is determined by the quality of the water you have to work with and the efficiency of your pumping system. A pond that can be flooded in 3 to 5 days after the bottom is wet should have adequate flushing capacity. The important thing to focus on is distributing oxygenated water throughout the pond so crawfish have access to good-quality water. In most ponds this is best accomplished by draining as much as ½ or even more water from the pond, and then re-filling the pond with fresh, oxygenated water to the original depth. Don’t worry about the loss of crawfish when draining off some of the bad water – it is minimal. This procedure is more effective than flushing the pond by draining bad water and pumping good water at the same time.

The oxygen content of the source water is important, whether that water is pumped from a surface source (canal or stream) or from a well. If, for example, canal water has low dissolved oxygen, which is not uncommon in some small surface canals with a good amount of organic matter (leaves, grasses, etc.) and low flow rates, you may be spending money pumping water for nothing unless you effectively aerate the water prior to it entering the pond. Well water has no oxygen so it must be aerated prior to being added to the pond if it is to have any benefit to the crawfish.

Pumping surface water with a high BOD or low oxygen through an aeration screen to add sufficient oxygen for the crawfish is important. Mixing water with air by passing it through an aeration screen will raise the dissolved oxygen level to an acceptable level of 5 or 6 ppm. But because many surface water sources in Louisiana have high BOD from all the organic matter they contain, the oxygen in water you just added can drop rapidly to critically low levels in just a few days, regardless of what type of vegetation is in the crawfish pond. Even though ground water (well water) has no oxygen, because it has so little organic material in it, it has a BOD that is near zero. Aerating well water through an aeration screen can raise oxygen levels to over 7 ppm, but more importantly, because of the well water’s low BOD, it will retain the oxygen for a longer time before it is depleted. Of course, how long the water retains the oxygen before it is depleted to critically low levels will depend on the type and amount of vegetation and the water temperature.

Aeration screens used to oxygenate your water supply don’t have to be pretty, but to be effective they have to break the water into tiny droplets to absorb oxygen from the air. Water that is pushed through a pipe and out a riser without being broken up into tiny droplets picks up very little oxygen. Splashing water on tractor tires, cans, concrete piles, or old crawfish traps around the water discharge usually adds only a little oxygen to the water, so you are spending money on fuel or electricity but not getting full benefit from it. Discharging either surface water or well water over a properly designed aeration screen will nearly saturate the water with oxygen and give you the most oxygen for your pumping dollar.

Trash fish, such as bullhead catfish or bream, can eat a tremendous number of juvenile crawfish if they are pumped into the pond from a surface water source. If you are pumping from a surface water, put a fence or basket on the top of the aeration screen to filter out the larger trash fish. If you need plans for a properly designed aeration screen, contact us and we'll send you specifications.