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Recent developments in European recirculating aquaculture systems suggest expanding potential for this extremely water-efficient technique. However, the technology still faces challenges due to concerns over economic efficiency and system stability-both essential in minimizing the risk of financially and environmentally expensive failures. One key factor in maintaining stable production conditions in a recirculation loop is the effective removal of solid waste, i.e. fish faeces. This study tested a novel approach for solid control and demonstrates the value-adding potential of floating faeces under commercial conditions in a semi-recirculating fish farm in Germany. A commercial control diet was compared with an experimental diet in which the addition of 2.5% cork granules led to the production of floating faeces. Physiological assays indicated no pathologic tissue alterations associated with the experimental feed, and growth, survival and feed conversion were unaffected. Average single-pass removal by a specially developed surface separator accounted for 78.3% of floating solids, which accounted for 35.4% of total system solids. Total ammonia nitrogen concentrations in production water were roughly halved, from about 0.95 mg l −1 in the control to 0.47 mg l −1 using the cork diet, an improvement that in practice allowed a doubling of production on the same available water flow. This study shows that the application of floating faeces facilitates rapid and cost-effective removal of suspended solids, resulting in a considerable decrease of nutrient load in system and discharge water of the investigated farm.

In the present study, groups of ruffe Gymnocephalus cernuus, reared singly, were exposed to defined numbers of Anguillicoloides crassus or Camallanus lacustris under controlled laboratory conditions. Infection took place orally through feeding G. cernuus with axenically cultured and laboratory infected copepods, in which the parasites had developed to the infective third stage (L3). Mean prevalence (94Á3%) and infection probability (38Á5%) for the established C. lacustris were significantly higher than for the neozoic A. crassus (14Á3 and 1Á0%, respectively). Peripheral blood leukocytes were significantly increased in infected fish, apparently independent of exposure level, parasite species or intensity of infection compared to the controls. In infected fish, the gonado-somatic index (I G ) was significantly reduced by c. 50%, and the spleen-somatic index (I S ) was significantly increased compared to controls. Both parasites raised similar physiological and immunological responses in G. cernuus, which was able to effectively reject the neozoic A. crassus.