Reduction in the make-up water in semi-closed recirculating aquaculture systems

Three feeding trials with rainbow trout (Oncorhynchus mykiss) were run in a closed recirculating aquaculture system with a nitrification unit and a sedimentation tank. The used trout feed had first a protein content of 47.9% (no. 1) and afterwards a protein content of 38.5% (no. 2). The effects of different protein contents of the feeds (trial I and II) and different sludge removal intervals (trial II and III) on the nitrogen loading of the culture water were examined. The nitrogen content of the culture water was measured photometrically. In trial I and II, comparing both feeds, the sludge was removed daily from the system. In trial III using the lower proteinuous feed as in trial II, the sludge was not removed from the system for a period of 17 days. In all trials, fish showed a high FCR of 0.8 (trial I) and 0.9 (trial II and III). In order to compare the trials, the results were calculated on a common basis of 1 kg gain in weight. In doing so the make-up water was reduced to 60% by comparing trial I and II and 77% comparing trial I and III, using the NO3 content of the culture water as limiting factor. (C) 1998 Elsevier Science B.V. All rights reserved.