Linda Benedict, Sonnier, John J., McClain, W. Ray | 9/22/2011 8:38:51 PM
W. Ray McClain, John J. Sonnier and Ricardo O. Ramalho
Louisiana is the largest producer of crawfish in the United States, where some 1,200 farmers harvest in excess of 110 million pounds of crawfish annually from about 184,000 acres of ponds. A large majority of the annual crawfish production is marketed as live or wholeboiled crawfish, with less than 20 percent processed for the tail meat. Large volumes of live crawfish are delivered to metropolitan areas within and outside of Louisiana, such as Baton Rouge, New Orleans, Shreveport, Little Rock, Dallas, Houston, Mobile, Jackson, Memphis and Atlanta. Marketing issues for live crawfish include significant variation in sizes, with the larger crawfish being the most valuable.
Although size grading of crawfish is initially accomplished by the mesh size of the wire-mesh traps, further grading is sometimes accomplished by large volume dealers at dock-side. There are logistical and cost issues with this additional step in handling, however. Some crawfish farmers have begun employing a culling (grading) operation on the boat at the time crawfish are emptied from traps. Harvesting boats are fitted with a bar grader that will effectively cull smaller crawfish from the catch and allow those to immediately re-enter the pond. The retained crawfish are more uniform in size and easier to sell because of the greater appeal with buyers. The common assumption is that the culled crawfish will resume growth and be recaptured later at a larger, more valuable size.
This practice of using on-board graders has increased dramatically in recent years and often renders further grading at dockside unnecessary. However, the efficacy of this practice is largely unknown because the rate of recapture has not been determined. Little is known about the efficiency of trap harvesting as a means of removing crawfish from the population once they reach market size. Therefore, a series of studies was conducted at the LSU AgCenter’s Rice Research Station in Crowley to estimate the percentage of recapture and to better understand aspects of trap harvesting as well as crawfish movements within the pond.
A mark/recapture technique was used in a small commercial crawfish pond and, subsequently, in the experimental pond complex of the Rice Research Station, to ascertain capture efficiency rates using baited traps. Captured crawfish were marked with a long-lasting, waterproof marker and released back into the pond within two hours of their capture. Bold marks were made on the carapace of crawfish that could be quickly and easily observed. Various colors and/or markings distinguished the date and/or site of release. Subsequent trap catches were observed for marked individuals, which were noted for date and location of capture.
The initial trial was conducted in a 28-acre commercial crawfish pond where 200 mature and 270 immature individuals were marked and released at a central location in the pond. The subsequent harvest was monitored for one week, and only 54 marked individuals (11.5 percent) were recovered. While there was a possibility that molting occurred in some immature individuals (i.e, lost marks), the rate of recapture for mature (nonmolting) individuals was noted at 20 percent – still a low recovery. Some crawfish traveled up to 589 yards, surely passing baited traps, before being recaptured, although the average distance traveled before recapture was 147 yards. Trap spacing typically ranges between 50 and 70 feet between traps.
Trials 2 and 3 were conducted in a contiguous arrangement of 1-acre experimental ponds where “mature-only” crawfish (489 crawfish for Trial 2 and 200 crawfish for Trial 3) were released in selected ponds, and recaptures were monitored from the entire pond complex. Those trials lasted for nine to 12 weeks with an average total recapture rate of 49 percent in Trial 2 and 45 percent in Trial 3. Of those crawfish recaptured, 83 percent in Trial 2 and 97 percent in Trial 3 were caught from the same pond they were released into. Most of the others were captured in adjacent ponds, indicating some movement across a levee. Recapture rates were greatest within the first two weeks of capture, with few crawfish recaptures occurring after four weeks.
For Trial 3, recaptured crawfish were marked with a different identifier and released after each capture. Also for that trial, additional marked crawfish were held in cages within the ponds from the beginning of the trial to estimate natural mortality in the marked population during the experiment. While 45 percent of the crawfish released in Trial 3 were captured only once, 17 percent were captured twice, and 5 percent were captured three times. Adjusted for natural mortality, the recovery rate for first-time recaptures remained at slightly less than 50 percent.
Precise causes for the consistent low recovery (less than 50 percent) in these trials are unclear. Factors that contributed include natural and predator-induced mortality, attrition from the pond population because of burrowing, and individuals exiting the pond. However, these results also seemingly illustrate the inefficiency with which baited crawfish traps remove market-size individuals from the pond population.
In conclusion, based on the results of this study, it appears that only half (or less) of the crawfish released back to the pond are ever recaptured. These findings do not bode well for the practice of releasing less-desired sizes of crawfish back to the pond with the expectation of recapturing them later at a larger, more valuable size. With recovery rates below half, one has to closely examine the economics of such a practice. Thus, it may be far better to sell all crawfish at the point of first capture, even if a lower price is received for the smaller size grades.
In addition to market-economic considerations, there may be a biological reason to avoid releasing crawfish back to the pond once captured. Those crawfish would only compete with other crawfish for food and space. That would only exacerbate the problem of overcrowding if pond populations were high and food resources were scarce, contributing to reduced growth and lower production.
W. Ray McClain, Professor, and John J. Sonnier, Research Assistant, Rice Research Station, Crowley, La.; and Ricardo O. Ramalho, Researcher, Institute of Marine Research, Évora, Portugal
(This article was published in the summer 2011 issue of Louisiana Agriculture magazine.)