A perspective on ecological considerations in crawfish pond aquaculture.
KEY WORDS: crawfish, aquaculture, ecological services, ag wetland habitat
Freshwater crawfish, Procambarus spp., are cultivated in shallow-water impoundments by establishing self-perpetuating populations in permanent culture systems or mature crawfish in rotational culture systems (McClain & Romaire 2004). Most crawfish culture is located in Louisiana where 52,000 ha are devoted to crawfish production (LCES 2004). Most of the shallow-water impoundments are used to cultivate rice in warm months and are reflooded for crawfish culture in the cool months following rice harvest. These so-called agricultural (ag) wetlands are managed in such a way to follow the region's natural wetland cycle with alternate wet dry cycles. Crawfish mature in 4-6 mo and retreat to burrows during the heat of the summer when natural wetlands are either dry or water is too hot for survival and growth of the crawfish. The two important species are the red swamp crawfish, Procambarus clarkii and the white river crawfish, Procambarus zonangulus, whose basic biology and size are similar. See McClain and Romaire (2004), Huner (2002a) and Avery and Lorio (1999) for information about basic crawfish biology and fisheries and aquaculture management considerations.
Because crawfish are cultivated by simulating natural hydrological cycles in what amounts to be seasonally flooded wetlands, it should not be surprising that crawfish ponds provide ancillary ecological services (Huner 2002b). Furthermore, crawfish culture, as practiced with Procambarus spp. is probably the most sustainable form of pond aquaculture practiced in North America. That is, agricultural waste, rice stubble serves as the base of the food web that generates substantial crops of crawfish with minimal environmental impacts while providing economic benefits to communities (Huner 2002b, Caffey et al. 1996). This brief review discusses the various ecological services, foremost being waterbird habitat, provided by the agricultural community through cultivation of crawfish in shallow water, earthen ponds.
Huner et al. (2002) discuss at length the management of agricultural wetlands in the coastal regions of Louisiana, which ultimately benefit resident and migrating waterbirds. Over 280 bird species including at least 75 waterbird species have been documented utilizing agricultural wetlands and adjacent riparian areas. This discussion is appropriate, however, for the South in general. Rice and crawfish farming operations are the main agricultural wetlands found in Louisiana. The following are water management practices associated with permanent and rotational crawfish ponds:
Permanent Crawfish Ponds
These systems produce crawfish on a continuing basis. When rice is planted as a forage base for crawfish in permanent ponds, grain is usually not harvested. Where rice is harvested, such systems are considered to be a type of rotational management--see later. Alternatives to rice as forage bases most commonly encountered include sorghum-sudan grass hybrid or volunteer vegetation including perennial forbs such as alligator weed, Alternathera philoxeroides, smartweeds, Polygonum spp., duck potatoes, Saggitaria spp. and annual wetland grasses and sedges. These ponds are usually "open" in the sense that they are not wooded. However, there is some acreage associated with impounded forested wetlands (swamps). These so-called swamp ponds are usually very large compared with most ponds, regardless of type, that rarely exceed 20 ha in size. They typically have very poor crawfish production compared with all other crawfish pond systems.
Rotational Crawfish Ponds
In general, these systems produce crawfish in alternate years. However, some farmers harvest rice and crawfish from the same ponds year after year and are rarely restocked with crawfish after the first crawfish crop is harvested. In most rotational ponds, crawfish are stocked after the planting of rice and the establishment of a shallow "flood" of water once the rice has developed permanent leaves. Rice is managed for grain production and harvested. After rice harvest, the ponds are refilled in the fall and managed for crawfish production into the following spring. Once crawfish harvesting ceases, farmers may drain the fields and plant a crop such as soybeans or leave the fields fallow until rice is planted the following spring. Crawfish are then stocked and the cycle is repeated. If a crop like soybeans is not planted, farmers often choose to not drain fields until the following autumn to control weeds. They do not add water to the fields and water is lost to evaporation and percolation with rate being related to rainfall patterns. (See McClain & Romaire 2004, McClain 2005 and Romaire 2005 for more information about management of permanent and rotational crawfish pond systems.)
Bird Feeding Niches and Behaviors
The presence of water in the crawfish ponds provides feeding habitat and loafing habitat for all types of waterbirds including waterfowl, grebes, pelicans, cormorants, wading birds, shorebirds, coots, rails, gulls and terns. Furthermore, adjacent riparian areas and nearby wetlands attract numerous nonwaterbird species (Huner & Musumeche 1999. Musumeche et al. 2002). Each suite of birds has a particular feeding niche. Waterfowl and coots feed largely on rice left in the fields intentionally (permanent ponds) or as a consequence of harvesting processes (rotational ponds). They also benefit from seeds, green plant materials and tubers associated with volunteer vegetation especially where that is the crawfish forage base. Ducks (and coots) will also prey on zooplankton "blooms" concentrated along shorelines by winds especially in the spring and benthic invertebrates including crawfish when animal protein is required for egg formation. Wading birds and rails feed along pond margins when water is deep or throughout shallow areas present as ponds are filled or drained or because of untended water loss from evaporation or seepage. Hovering birds such as gulls and terns take prey from the water surface. These birds are especially adept at gleaning molting crawfish from emergent vegetation where they concentrate during daylight hours. Soft crawfish have very low mineral content and, being soft, are very easy to eat. Diving birds such as grebes and cormorants feed within the water column often in water so shallow that they cannot submerge. Pelicans. specifically American White Pelicans, form feeding lines and drive prey, including crawfish, toward shores where they are concentrated and easily scooped up. Shorebirds feed wherever shallow water is available--see later.
With the exception of shorebirds, all other species use crawfish traps that extend above the surface for resting places. Cormorants and wading birds are often seen to remove cut fish from traps when it is used for bait. Cormorants, wading birds and gulls regularly take crawfish that climb to the water surface inside and outside of traps.
There is no set time that ponds are drained and flooded. Because target flooding and draining dates are midOctober and mid-June (Avery & Lorio 1999), management considerations generate wide windows of flooding dates from midsummer to midfall and draining dates from early spring to late summer. Food organisms are most abundant when ponds are drained. Draining can take place as early as February or March even if the crawfish crop is too poor to justify harvesting, but, even though the crops are not worth harvesting, prey concentrates to desirable levels as water levels drop generating excellent feeding opportunities for carnivorous birds. In rotational rice-crawfish ponds, draining may be delayed until midfall to control weed growth prior to preparing soils for planting spring rice crops. The moist soils that are ultimately exposed are of special significance to shorebirds including wintering species or migrants. Such soils are sources of high densities of invertebrate prey especially insect larvae, crustaceans and annelid worms that provide the mainstay of shorebird diets.
Table 1 provides general information on the seasonal utilization of crawfish pond systems by waterbirds. See Huner et al. (2002) for more details.
In the case of rotational crawfish ponds, a practice that is especially important to waterbirds involves leaving water in the pond once crawfish harvesting ceases. Water is not drained but allowed to evaporate. Because the bottoms of most of these ponds have been leveled carefully with laser technology, there is very little slope and variation in depth. Water remains across most of the pond bottom until it finally dries or it is drained to permit tillage for an agronomic crop the following spring. The presence of water reduces problems with terrestrial weeds when pond bottoms are tilled. In practice, water depths of 2.5-5.0 cm generate ideal habitat for shorebirds migrating through the area during the "fall" migration beginning in July and ending the following October. Likewise, large concentrations of post nesting wading birds frequent these sites and good numbers of early fall waterfowl such as Blue-winged Teal, Northern Shoveler and Northern Pintail are attracted to this shallow water/moist soil habitat.
Wetland Habitat Considerations
Rice and/or crawfish are cultivated in over 300,000 ha of shallow earthen impoundments within 161) km of the Gulf of Mexico from the central Texas coast eastward to southeastern Louisiana. The region includes the Gulf Coastal Plain and the Lower Mississippi River Valley. Annual loss of 4,475 ha of coastal wetlands over the past 50 y has significantly reduced desirable natural freshwater habitat in the region (Coreil 1993, Raynie & Beasley 2000).
The significance of agricultural wetlands as waterbird habitat becomes very clear when one notes that over 600,000 ha of adjacent coastal wetlands have disappeared since the 1950s (Coreil 1993, Raynie & Beasley 2000). What was once ideal waterbird habitat has become open water with no chance that it will be restored in the foreseeable future. Therefore, "artificial" wetland habitat has replaced those lost wetlands.
Crawfish are cultivated mainly in two distinct geographical regions in Louisiana--coastal prairie areas with light, loessal soils and floodplain areas with heavy, alluvial soils (Huner 2004a, 2004b). An impervious "hard pan" develops in cultivated prairie areas under several centimeters of topsoil and holds the water permitting the cultivation of rice and crawfish. The heavy clays in floodplain areas naturally hold water tenaciously and permit rice and crawfish culture. Interestingly, much of the area where crawfish is cultivated in floodplains was very low, flood prone, marginal land cleared in the 1960s to permit cultivation of soybeans. Within 5-10 y soybean cultivation became unprofitable and, at least in southern Louisiana, such fields surrounded by levees for flood control, were converted to crawfish ponds.
During the drought period of 1999 to 2001, there was a very real reduction in crawfish yields in the prairie regions of Louisiana, whereas crawfish yields in alluvial regions did not suffer as much (Huner 2004a, 2004b). To be sure, Icon, a systemic rice pesticide, was associated with considerable loss of crawfish in prairie regions. However, scrutiny of crawfish production from alluvial areas, especially where treated rice was not used, suggests strongly that drought resulted in a much greater reduction of crawfish production in the lighter prairie soils than in alluvial soils. Therefore, it is apparent that farmers need to manage soil moisture much more carefully in the prairie soils than the alluvial soils. This, then, benefits waterbirds as there is more moist soil/shallow water habitat available to them.
Threats to Waterbird Habitat Provided by Ag Wetlands
Farmers will cultivate rice and/or crawfish only as long as it is economically feasible to do so (Huner et al. 2002, Westra et al. 2005). Rice farming would not exist today were it not for farm subsidies. Crawfish was once considered to be a providential "cash" crop. Today, it is a principal source of income in a very troubled agricultural economy (McClain 2005). If crawfish, for whatever reason, fails to sustain the rice component of an integrated rice-crawfish enterprise then the enterprise will close and the wetland habitat will not be sustained. Farmers have been evaluating sugar cane as an alternative to rice in southwestern Louisiana. Sugar cane must be kept reasonably dry so cane fields lose shallow water/moist soil functions when a rice crop is replaced with a sugar cane crop.
In the case of absentee landowners, farmers leasing rice fields cease to farm rice if the practice based on rice values and lease costs exceed income. Fields are abandoned and are quickly over grown by exotic tallow trees that form thick, impassable stands of trees in less than 5 y. The costs of clearing such trees are such that such properties are unsalvageable in the current agricultural economy.
Farmers are generally of the opinion that predaceous waterbirds negatively impact their crawfish crops. This issue is discussed at length by Huner et al. (2002). In the absence of replicated, controlled predation studies, ancillary observations suggest that birds such as wading birds, gulls, terns and pelicans probably do not negatively impact crawfish production directly when farmers practice recommended management procedures. However, larger birds using crawfish traps as perches do dislodge them causing loss of bait and catch and, in that case, do have a negative impact on the crawfish operation. Waterbirds largely considered to be nonpredaceous, such as geese and coots, can adversely impact crawfish production indirectly by destroying emergent vegetation, especially rice stubble. Aside from providing fuel for the detrital system, the emergent vegetation serves as critical substrate for crawfish dispersal, provides access to the surface and atmospheric oxygen when dissolved oxygen levels are very low and cover from the myriad of predators, including birds, associated with all wetlands, natural or artificial, in the region.
Use of water, especially ground water, by farmers for irrigation of all crops is being carefully scrutinized as demands for water escalate for industrial and residential activities (Lutz et al. 2004, Huner et al. 2002, Lovelace 1994). In 2000 (DOTD 2002), Louisiana "rice" farmers used approximately 42% of the ground water used in the state. Increases in the cost of "energy" force farmers to be much more guarded in their use of water, regardless of source, because they must pay for the energy required to pump the water into pond systems. However, farmers will continue to use a very high percentage of the water consumed in Louisiana.
If resource managers do not recognize the critical wetland habitat created by rice and crawfish farmers, they could make water allocations that negatively impact waterbird populations with staggering implications especially when hemispherical shorebird populations are taken into account.
OTHER ECOLOGICAL/SOCIETAL SERVICES
All impounded agricultural areas hold water for varying lengths of time following rainfall events. Water levels in rice and crawfish systems are rarely held at drain level so all units have some freeboard that equates to water storage capacity (Huner 2002b). However, because drains cannot accommodate the volume of water falling into ponds during major rainfall events, ponds store the water for varying periods of time until the drains can discharge water temporarily stored in the ponds. Even empty ponds hold water until such time as it can drain through the openings cut in levees. As a result, rice fields and crawfish ponds hold water and reduce the impact of floods in lower reaches of receiving bayous, streams, and rivers, a definite benefit to homes and businesses. Furthermore, even when such locations drain completely, soils remain damp for various lengths of time creating landscape level moist soil vistas. There are at least 240,000 ha of land used for rice production and surrounded by levees in Louisiana during nonrice production seasons (LCES 2004). No more than 20% to 25% of this land is flooded for crawfish production then.
Water is discharged from crawfish ponds for aeration purposes, to eliminate excess rainwater, and to drain ponds during the spring and summer (Lutz et al. 2004, Huner et al. 2002, Lovelace 1994). Concerns about discharge of pollutants, especially nutrients and suspended solids have been addressed by the development of Best Management Practices (BMPs) (Lutz et al. 2004). In general, water quality is not generally so poor as to cause excessive concern as long as the BMPs are followed (Parr et al. 2004).
The waters discharged from crawfish ponds benefit the receiving waters in 2 ways (Huner 2002b). First, water is often discharged during very dry periods in the mid-late fall and late spring/early summer when receiving waters exhibit seasonally poor water quality. Therefore, the increase in volume and movement of water generated by discharges actually, it can be argued, improve water quality. Second, fish food organisms are discharged with effluent providing feeding opportunities for fish and other predaceous aquatic organisms comparable, it can also be argued, to discharges from natural wetlands.
Crawfish culture, as practiced in the southern USA, represents a form of highly sustainable aquaculture that has very real benefits to wildlife and fisheries resources. Crawfish impoundments are artificial wetlands, comparable in many ways to natural wetlands in the region. Resources managers should encourage crawfish aquaculture wherever it is economically feasible and seriously consider managing wetlands on state and national wildlife management properties in a similar manner.
Incorporation of crawfish management into government-sponsored land conservation programs should encourage landowners to sustain standing water habitat outside of program mandated fill/drain requirements. Farmers could adjust the times when their ponds are filled or drained to maximize benefits to many species, especially migrating shorebirds. The issue of possible damage done to crawfish crops by predaceous birds remains unresolved but, if such birds are proven to be deleterious, compensation to encourage farmers to continue to cultivate crawfish would certainly be justified on the basis of the ecological value of their ponds to wildlife.
The research that led to the development of this manuscript has been supported for a number of years by grants provided by the Louisiana Crawfish Promotion and Research Board. A recent grant provided by the Coypu Foundation provided supplemental funding.
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JAY V. HUNER
Crawfish Research Center, University of Louisiana at Lafayette, Lafayette, LA 70504
Corresponding author. E-mail: firstname.lastname@example.org
TABLE 1. Seasonal abundance of major waterbird taxa in crawfish pond systems in southern Louisiana (USA). Season Taxon SP SU F W Ducks C R C C Grebes R U U R Pelicans -- -- -- R Anhingas R R R -- Cormorants U C C U Bitterns R R R R Herons/egrets C C A C Night-herons U C C U White ibises A C A A Dark ibises A C A A Spoonbills U U C U Storks -- U U -- Coots U X -- C Shorebirds C U A U Gulls C U U A Terns U R U U Legend. Seasons: SP = spring, March to May; SU = summer, June to July; F = fall, August-October; and W = winter, November to February. Seasonal abundance: A = abundant, widespread and easily found in proper habitat in large numbers: C = common, widespread and easily found in proper habitat, but generally not in large numbers: U = uncommon, widespread and present, but in low numbers at proper season: R = rare, found infrequently at proper season, but a general pattern of occurrence is evident.
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|Author:||Huner, Jay V.|
|Publication:||Journal of Shellfish Research|
|Date:||Aug 1, 2006|
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