Demand-oriented Feeding in Carp Pond Farming - The Settling Volume of Zooplankton

Demand-oriented Feeding in Carp Pond Farming Introduction Common carp (Cyprinus carpio L.) bred in extensive carp pond farming in Austria rely on the natural food (zooplankton and benthic invertebrates) occurring in the ponds. The natural food, especially the zooplankton, provides the fish essential fatty acids and proteins (Dabrowski & Rusiecki 1983; Tocher 2003; Steffens 2011). In good managed carp ponds, the zooplankton genera evolve coherent with the life-stages of the carp. Carp fry mainly feeds on ciliates and rotifers, while adult carps feed on larger zooplankton genera, such as Daphnia sp. (Šusta 1887; Wunder 1968; Adámek et al. 2023). For good water quality and economically management, it is advisable to use additional feed, such as grains, as needed instead of following feeding plans given in the literature. As it is very difficult for carp pond farmers to assess the amount of zooplankton in the pond, we have been working on an easy way for measuring the zooplankton in situ. The Settling Volume To determine the right amount of additional grain feed it is essential for the carp farmers to measure the given amount of zooplankton (Schlott-Idl 1991). Thus, they can adjust their feeding strategy in such a way as the carp get a balanced diet and both, insufficient utilization and overexploitation of zooplankton are avoided. It is commonly known that fish has a major impact on the zooplankton populations in the pond (Hrbaček 1962). By balancing the amount of grain feed, predation on the zooplankton can be regulated (Steffens 2011). The first experiments to use the settling volume (SV) of large zooplankton (>1 mm) as measurement technique for the natural food amount for carps have been conducted in Austria in 1984. Daphnia can be sampled relatively easy, by using a 500 µm net. As they are especially important for adult carp diet, it makes sense to sample only larger zooplankton and disregard smaller ones. To assess the SV, 5 L water samples on four different places at 0.5 m depth are taken from the pond with a Schindler trap. As already mentioned, we use a plankton net with a patch of 500 µm net on the bottom to catch only daphnia. After flushing the smaller plankton out, the daphnia are concentrated in the cod end. The zooplankton is killed with 20% formaldehyde and flushed into a volumetric tube with distilled water. After a short settling time the SV can be easily read from the scale on the tube. Schlott & Schlott (2001) showed that an abundance betwe