Environmental Engineering Reference
In-Depth Information
completely is important in the early fall when water is flooded on to the vegetation. During
the spring, warm weather causes rapid plant decay which causes a high demand for
dissolved oxygen in the water. Low oxygen levels in the water cause high mortality rates
and stress which reduces growth. The ability of the farmer to flush the ponds of oxygen
deficient water in a timely manner is important. Few crawfish farmers have the capacity to
supply water in the ideal quantities, therefore following an intense management plan can
make up for lack of pumping capacity. Filling ponds to one-half normal depth will enable a
pump with a smaller pumping capacity to flush the pumps quickly enough when necessary.
However, if less than optimal water depth is utilized, more intensive monitoring of water
quality in necessary because smaller volumes of water can change characteristics quicker
than larger volumes of water.
7.3 Quality of water
Water quality variables include: 1) total hardness, 2) total alkalinity, 3) pH, salinity and
other dissolved materials, and 4) dissolved oxygen. Dissolved oxygen is by far the most
important because low oxygen is responsible for more crawfish mortality than any other
factor. Temperature of the water is a factor because warm water cannot hold as mush
oxygen as cooler water. In water that increases from 21º to 26º C the rate of oxygen use due
to decomposition doubles. The source water after aeration should have a pH in the 6.5 to 8.5
range and total hardness and alkalinity should range from 50ppm to 250 ppm in calcium
carbonate. Most water and soil in Louisiana that is used for crawfish production meets these
parameters. Crawfish are fairly tolerant to salt water, however areas along the gulf coast
that are subject to salt water intrusion should not be used as crawfish farms. Young
hatchlings will die at 15 ppt salinity and juveniles die at 30 ppt salinity after a week of
exposure.
8. Harvesting
8.1 Overview
The harvesting of crawfish utilizes baited traps that are periodically emptied. Trapping may
begin as early as November and continues through the harvest season which usually ends
by late June. In Louisiana, two-thirds of the crop is harvested from March through early
June. In 2008, the most recent year in which accurate numbers are available, approximately
58 million kg of crawfish were harvested from aquaculture operations in Louisiana. The
value of 2008 crawfish farm harvest was 115 million US dollars (Isaacs and Lavergne, 2010).
Trapping is responsible for over half of the production expenses. The cost of bait and labor
are the major harvesting costs. Crawfish yield within a pond can vary greatly from day-to-
day and is governed by many factors. Water temperature, crawfish density, mass molting
and weather are some of the variables that cause fluctuations in harvest.
8.2 Traps and baits
The “pyramid trap” is currently the industry standard in Louisiana. (Figure 8.1) Traps are
constructed of 3/4-inch or 7/8-inch mesh wire formed into a three-sided pyramid. A 15 cm
plastic collar on the opening at the top acts as handle. Traps are 60 cm wide and from 1.1 m
to 1.4 m tall with one-way openings in the three bottom corners of the trap. Crawfish can
Search WWH ::




Custom Search