WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2014 63 (CONTINUED ON PAGE 64) In traditional carp aquaculture in Austria, naturally available zooplankton is an important component in the diet of common carp Cyprinus carpio. Fish growers endeavor to increase zooplankton, and thus carp production, by pond fertilization. Because of the complex relationships among nutrients, elements of the food chain and fish stock, no sustainable increase in fish production could be achieved. Energy transfer efficiency is low for ponds with high nutrient levels and high fish densities (Potužák et al. 2007). Additional feed, mostly cereals, can complement the natural diet, but carp growth should not depend on cereals alone. Zooplankton provides fish with protein and essential fatty acids (Tocher 2003), important for growth and fish health (Schäperclaus 1979). Therefore, the feeding rate of grain should be dependent on the availability of zooplankton in the pond. Grazing pressure on zooplankton by carp can be controlled and a stable zooplankton population throughout the production cycle can be maintained. In practice, feeding with grain follows a rather inflexible plan. Feeding starts late in spring and the amount increases with water temperature (Fig. 1). Zooplankton is not considered in this approach. This feeding strategy often leads to an overexploitation of large zooplankton in spring (Fig. 1). The impact of fish on the zooplankton population in ponds is well known (Hrbaček 1962). In many cases, cladocerans, such as Daphnia, are not able to Zooplankton Displacement Volume — A Feeding Indicator in Extensive Carp Culture Markus Böhm, Christian Bauer, Karin Schlott, Martin Fichtenbauer, Günther Gratzl and Günther Schlott recover during the summer from overexploitation in the spring. As a result, an important part of the carp diet is not sufficiently available. The aim of the fish grower should be to maintain a stable population of zooplankton, which can be monitored by sampling the pond. To control grazing on zooplankton by carp, growers adjust the amount of supplemental feed relative to the available amount of zooplankton. It is tedious for carp farmers to estimate zooplankton density in ponds during routine work. Common scientific equipment is not suitable and too expensive for carp farmers. Therefore, a simple and inexpensive method to estimate zooplankton density was developed. FIGURE 2. Inexpensive (US$ 170) and easy-to-build sampling equipment was developed for carp farmers. FIGURE 1. Feeding with cereals starts late in spring, leading to an overexploitation of large zooplankton.
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