Crawfish Burrows and Drought

Linda Benedict, Mcclain, William R.  |  8/16/2006 7:34:54 PM

Sealed entrance of a crawfish burrow, showing the typical “chimney” and dirt plug. (Photo by Ray McClain)

Crawfish extraction from burrows begins with slow, careful excavation. John Sonnier, research farm specialist, is doing the extraction. (Photo by Ray McClain)

Burrow excavations are difficult and time consuming, but necessary to obtain data critical to the understanding of burrow ecology of crawfish in aquaculture. (Photo by Ray McClain)

Live crawfish were mostly associated with burrows containing free water. (Photo by Ray McClain)

A burrow after crawfish extraction showing the simple, nearly vertical tunnel and slightly enlarged terminal chamber. Note that no water was present in this burrow at the time of excavation. Cracks observed in the upper portion of this burrow may have compromised the integrity of the burrow allowing water, which normally is trapped inside, to evaporate, compromising the survival and reproduction of the occupant crawfish. (Photo by Ray McClain)

Table 1. Aspects of burrow ecology relating to crawfish extracted from burrows following pond draining in experimental crawfish production ponds. Information is organized by crawfish burrow occupancy number and survival status.

Table 2. Observations obtained from burrows with female crawfish only, segregated by location of burrow and burrow depth.

Ray McClain

Crawfish yields for the 2005-2006 production season were abnormally low. This is especially true for those using the production strategy of culturing crawfish following rice in a field rotation. This strategy, used on most crawfish production acreage in Louisiana, is most susceptible to crawfish population adversities because population densities are typically lower than in ponds permanently dedicated to crawfish production. When crawfish are not cultured in the same field (pond) during consecutive production seasons, there is little opportunity for populations to build up and develop a wide range of reproduction cycles – both aspects that tend to mitigate adverse effects on populations. Therefore, unfavorable weather patterns or other environmental conditions that affect crawfish broodstock survival and reproduction generally have a greater effect on crawfish production systems that employ field rotation. By all perspectives, the 2006 drought in Louisiana began during the summer of 2005 and persisted through the fall, with the exception of some short-lived rains associated with one or both hurricanes.

Prolonged summer drought when crawfish are confined to burrows, where they reproduce, can hamper reproduction if residual water within the burrows is lacking. If the drought is severe enough and burrows completely dry out, massive broodstock mortality can result. Drought during the fall, at a time when crawfish are emerging from burrows with young, can also hamper production by preventing or delaying emergence from burrows. Crawfish remain trapped in burrows until the hardened dirt plug at the entrance of the burrow is sufficiently softened by pond flooding or rainfall. For crawfish burrowed in the levees above the normal waterline of the flooded pond, timely rainfall is critical for the crawfish’s emergence, and limited rainfall amounts are often not enough to adequately soften the hardened plug.

Soil type, burrow depth, burrow location and amount of water inside the burrow at the time of initial burrowing may play a role in how crawfish respond to prolonged drought conditions. Because little research exists regarding the burrow ecology of crawfish, especially as it relates to crawfish aquaculture, research at the LSU AgCenter is being focused in this area. One such project examined crawfish burrows in 2005 at the Rice Research Station in Crowley, approximately one month after pond draining. Two hundred sixty burrows were excavated, and crawfish were retrieved, water volumes and burrow depths were recorded, and locations of the burrow entrances were noted.

Average burrow depths and water volumes, by crawfish survival and number of burrow occupants, are presented in Table 1. Burrows containing only female crawfish are further organized according to burrow location and depth in Table 2. It was surprising to observe that 45 percent of the burrows contained no living crawfish after such a short time following pond draining. Dead crawfish were generally associated with burrows containing no measurable water, and a strong positive relationship exists between the amount of water present and burrow depth. There was also a marked difference in average water volume, the percentage of burrows with no measurable water and crawfish survival between burrows initiated at or above the pond’s water line (likely pre-drain burrows) and those made on the pond bottom (post-drain burrows). Though the average burrow depths were similar, burrows found on the pond bottoms (often near the base of the levees) generally contained much greater volumes of water, had fewer burrows without free water and contained more living crawfish than burrows found at or above the waterline on levees.

Though the implication of these findings with regard to burrow location is unclear, in the Crowley silt loam soil, burrow depth and water volume appear to be highly correlated; and the volume of water found in burrows soon after burrowing may play a significant role in the ultimate survival of reproductive female crawfish within the burrow. These findings strongly implicate summer drought as a significant factor in negatively affecting crawfish reproduction and subsequent yield, at least on silt-loam soils, and may help to explain the reduced yields reported following the unusually dry conditions during 2005 in Louisiana.
Ray McClain, Professor, Rice Research Station, LSU AgCenter, Crowley, La.

(This article was published in the summer 2006 issue of Louisiana Agriculture.)
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