68In this case, cropping was performed using gravity, which only affected particularmicrobial constituents. Future studies may examine the effectiveness of various othercropping techniques such as using herbivorous fishes or macroalgae to remove particlesand excess nutrients from zero-exchange systems. Mechanical filters such as foamfractionators or bead filters should also be explored further.Cropping with a settling chamber is a simple, inexpensive, and reliable techniquethat appears to have a potentially mixed outcome from a systems managementperspective. The procedure was successful at removing potentially harmfulcyanobacteria. That observation along with the fact that very few free-livingcyanobacteria were seen during the visual observations of the experiment suggests thatthese organisms predominately reside on and within biofloc particles.Having a reliable method of removing cyanobacteria may serve the shrimpfarming industry very well. Reducing cyanobacteria could increase the profitability ofthe business by eliminating some of the risk associated with these organisms. Shrimpflesh quality and flavor could improve as a result of decreased cyanobacteria prevalence(Ploeg and Boyd, 1991). Furthermore, the risk of shrimp mortality or even human healthrisks associated with cyanobacteria toxins can be reduced using this simple methodology.The removal of bacteria reduces the biological oxygen demand in zero-exchangesystems. This may reduce the need for extensive aeration, which could substantiallylower energy costs. The reduction of the risk of bacterial infections in shrimp mayreduce human health risks for workers as well as consumers.Rotifers and nematodes were also removed by cropping suspended solids. It isunclear whether these groups offer a net benefit or liability for zero-exchange systems.
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