Ž .
Huner et al. 1994 review some of the commercial depuration systems that can be Ž
. classified as either immersion types crawfish are totally immersed in water or spray
Ž .
types non-inundated crawfish exposed to water spray or mist . Few studies have
Ž evaluated the purging process
Lawson and Drapcho, 1989; Lawson et al., 1990; .
McClain, 1994; McClain et al., 1994 , and the scope and findings of those studies were limited. Many aspects of crawfish depuration remain undocumented. This study was
conducted to evaluate several aspects of the purging process. The main objectives were to examine the efficiency at which crawfish evacuate the contents of the hindgut under
‘‘typical’’ purging conditions and to compare mortality and the efficiency of gut evacuation in two types of depuration systems at a common loading rate. Evacuation
efficiency was examined at 12-h intervals, in lieu of 24-h intervals commonly used in the industry, to obtain more accurate assessments of gut evacuation rate.
2. Materials and methods
Ž .
Red swamp crawfish P. clarkii , the most abundant commercial species, were used for this study and were produced at the Louisiana Agricultural Experiment Station’s
Rice Research Station, Crowley, LA. During 1997 and 1998, crawfish were harvested Ž
. 0800–0900 h from commercial-type rice-field ponds using baited wire mesh traps.
They were immediately placed into hard plastic crates and put into one of two Ž
depuration systems within 2 h after capture. Crawfish harvested on four dates April to .
June in 1997 were subjected to an immersion system of depuration, and those harvested Ž
. on seven dates February to May in 1998 were depurated in immersion or spray type
systems. The 11 test periods encompassed a wide array of environmental and biological conditions typical of the crawfish harvest season in Louisiana.
In the immersion system crawfish were fully immersed in water under conditions of profuse aeration and a low rate of water exchange. The spray system consisted of a
Ž .
non-immersion spray with high rate of water exchange. The immersion system Fig. 1 was a smaller version of a commercial type, developed by the author in 1981 and
subsequently adopted by many that commercially purge crawfish. It consists of an Ž
. oblong, convex-bottom tank 2570 l with suspended baskets that are placed crosswise
inside the tank and hold crawfish in the water column. The tank is equipped with a solid partition in the center to provide two similar, but separate, units supporting six baskets
each. The basket frames are semi-circular in shape and made from aluminum tubing
Ž .
covered with perforated aluminum sheets 4.8-mm mesh . Crawfish cling to all surfaces Ž
2
. of the baskets and occupy the entire submerged surface area
1.2 m within a
three-dimensional space. A loading rate of 14.5 kg crawfishrm
2
surface area was chosen to obtain maximum use of available surfaces and was within the 10–24 kgrm
2
Ž range of densities commonly used in commercial purging systems Lawson and Drap-
. cho, 1989 . Profuse supplemental aeration was supplied via a rotary vane compressor
and diffused through two 15.2-cm ‘‘airstones’’ placed directly beneath each basket. Flow-through water was added to each half of the tank at a rate of 0.6 lrmin. The tank
was cleaned and filled with clean water prior to each test date.
Ž The spray system consisted of twenty-two, 38-l plastic containers bottom surface
2
. area s 0.076 m . Each container was equipped with a center drain that maintained 1.27
Fig. 1. This immersion system for purging crawfish utilizes low water exchange and profuse aeration and is designed with hinged baskets for efficient delivery of the washed and purged product.
cm of standing water and a spray nozzle 51 cm above the bottom. Crawfish were partially immersed with only the lower one-half of the animal under water. A fine mist
Ž .
of water 0.3 lrmin was continuously directed into the container, resulting in high Ž
2
. humidity and a steady exchange of water. The crawfish loading rate 14.5 kgrm area
was the same as that of the immersion system; however, crawfish were confined to a Ž
. single plane bottom in this system.
Both systems used a common source of well water filtered through a commercial water softening system. The quality of water from this source was previously determined
to be suitable for purging crawfish; influent and effluent characteristics of the water are Ž
. Ž
. presented in McClain 1994 and McClain et al. 1994 . Water temperature during this
Ž .
study was monitored in the ponds at harvest data loggers and in the depuration systems Ž
. during purging minimumrmaximum thermometers .
Ž .
Randomly selected crawfish were allocated at the prescribed loading rates to two Ž
. baskets one in each tank half of the immersion system for each test period in 1997. In
Ž .
1998, crawfish were assigned to eight baskets four in each tank half of the immersion system and to 22 containers of the spray system at each test period. Samples of crawfish
were removed from the baskets every 12 h in 1997 for hindgut content weight, and mortality was assessed for the remainder of the crawfish after 48 h. In 1998, two of the
baskets in the immersion system were exclusively used for 12-h samplings for gut content, and three other baskets were each emptied at 24 and 48 h to determine
mortality. In the spray system, due to the relatively few crawfish per container and the potential impact of decreasing density on subsequent results, samples of crawfish were
taken for gut content weight from two different containers at each 12-h interval. Seven additional containers were each assayed for crawfish mortality after 24 and 48 h.
For assessment of hindgut content weight, five mature and five immature crawfish were collected prior to depuration and then from designated replicates at each 12-h
sample period. Equal numbers of males and females were collected when possible. Ž
. Crawfish were immediately and individually frozen y18
8C and at a latter date thawed, weighed, and dissected. The intestine, or hindgut, was separated at its junction to the
midgut and left attached to the anus. The telson, with intact anus and attached hindgut, was removed from the abdomen and centered over a pre-weighed aluminum pan.
Contents of the hindgut were collected in the pan by flushing the gut with 3 ml of distilled water dispensed from a small syringe through the anus. Gut contents from each
of the five mature and five immature crawfish were pooled, and dry weight was recorded after drying overnight at 75
8C. Dry gut content weight was computed as a percent of whole wet crawfish weight, wet abdominal muscle weight, and dry abdominal
muscle weight. Gut evacuation rate was determined as the mean percentage loss for each 12-h duration, and net incremental loss was defined as the percent of the mean total loss
Ž .
after 48 h that occurred at each 12-h increment. Mortality that occurred during purging was determined by closely examining every animal after removal from designated
containers in each purge system. Sexual maturity of dead and surviving crawfish was Ž
. also determined as described by McClain 1994 .
The experimental approach consisted of a randomized incomplete block design with test date as the blocking factor. Data were analyzed with the General Linear Model
Ž .
procedure in a factorial arrangement of treatments system and time interval using the Ž
Micro SAS Statistical Software System Statistical Analysis Systems version 6.10, SAS .
Institute, Cary, NC . Main effects were tested for interactions and analyzed indepen- Ž
. dently if no significant interaction P F 0.05 occurred. For the independent analyses, if
significant differences were present, Duncan’s multiple range test was used to separate treatment means at P F 0.05. The relationship between water temperature and gut
Ž .
evacuation rate or crawfish mortality was determined by simple linear correlation analysis.
3. Results
