64 JUNE 2014 • WORLD AQUACULTURE • WWW.WAS.ORG As an indicator for the availability of zooplankton for fish, the abundance of large-bodied zooplankton (>1 mm), especially cladocerans, is used. Daphnids are the most important food source for carp (Šusta 1887, Wunder 1968). During summer, cladocerans represent most of the zooplankton biomass in Lower-Austrian fishponds (Schlott-Idl 1991). Compared to other components of the carp diet, large-bodied cladocerans can be sampled quickly and easily with a 500-μm mesh net. Inexpensive equipment, built with commonly available materials, was developed (Fig. 2). The accuracy of this equipment was verified in comparison to professional scientific equipment (Spearman’s rs=0.87, n=105). Rather than counting zooplankton with a microscope, the volume displaced by zooplankton was used to estimate the abundance of large-bodied zooplankton. Displacement volume is well correlated with the abundance of Daphnia sp. (Spearman’s rs=0.96, n=261). To measure displacement volume, four 5-L water samples were taken near the outflow of a pond at different locations at 0.5 m depth and filtered through a 500-μm mesh plankton net. Retained zooplankton were transferred to a beaker and fixed with a small volume of 20 percent formalin (Fig. 3). After one minute the formalin was washed out using a small cylinder with a 500-μm mesh net and the sample was transferred to a graduated test tube using a funnel and a wash bottle. After a short period, the volume of settled zooplankton — the displacement volume — was noted. A Daphnia density of 20-40/L is considered sufficient for a stable population (Schlott and Schlott 2001). This density of cladocerans was equivalent to a displacement volume between 0.2 and 0.55 mL (Fig. 4). Knowing displacement volume, carp growers can adjust feeding strategy (Table 1). Under normal conditions, the amount of cereal feed was 2-4 percent of fish weight per day, three times per week. Results of studies in three ponds (0.18-0.38 ha) in 2009 and 2010 indicated that feeding according to the schedule in Table 1 FIGURE 4. Displacement volume of zooplankton as a function of the density of large-bodied zooplankton. Approximately 10 percent of samples were discarded due to pollution. Spearman’s rs=0.96, n=261. TABLE 1. Interpretation of the zooplankton displacement volume and action to be taken. Displacement volume (mL) Situation Action < 0.2 Lack of Protein-rich natural resources industrial feed 0.2 – 0.55 Stable Feed as population calculated > 0.55 – 0.8 Increase Half feed predation > 0.8 Increase Stop feeding predation FIGURE 3. Determination of displacement volume of collected zooplankton. (a) Sample is transferred to a glass jar. (b) Formalin (min. 20 percent) is added to sample. (c) Formalin is washed out after a short time. (d) Sample is transferred to a graduated test tube. (d) The displacement volume is read after zooplankton settle to bottom of test tube.
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