The effect of dietary water content on Dicentrarchus labrax growthparameters and resistance to Vibrio anguillarum infection wasinvestigated using commercial pellets with identical energy contents and differentmoisture levels. The first experiment hypothesis was that moisturizing pelleted ration canhave an impact on Dicentrarchus labrax growth performance by theosmoregulation energy cost reduction. In a second time, the experiment explores the effectof water addition in pellets on the fish resistance to a disease. A specific device wasbuilt to uniformly moisturize dry pellets to different moisture levels, i.e. 8%, 20%, 40%and 60%. After an acclimation period and a 54-day rearing period, the control fish hadgrown from 72.7 ± 17.9 g to133.3 ± 29.4 g. Nosignificant differences were recorded for fish growth parameters. After the growth period,the tagged fish were mixed and challenged by bath exposure to live Vibrioanguillarum in triplicate. After 7 days, mortality was significantly lower inthe group of fish fed with pellets containing 60% water. Adding water toDicentrarchus labrax feed did not affect fish growth parameters butincrease its resistance to a Vibrio anguillarum infection. The moisturizing processcould be used to add specific compounds (such as probiotics or vitamins) to pellets justbefore fish feeding and could have a positive effect on fish rearing.

Conventional, low-yield aquaculture methods use earthen, standing water fish ponds that typically are stocked with 0.5 to 2 fish/m2 and produce 3 to 10 tons of fish/ha/year using refined feeds. At higher fish stocking densities and fish yield, production of metabolites and residues exceeds such a pond's natural recycling capability, so that a higher rate of flushing is required. This wastes water and valuable nutrients and creates a potentially polluting effluent In an alternative system, by increasing the rate at which the metabolites and feed wastes are microbially processed within the culture system, production rates exceeding 500 tons of tilapia fish/ha/year have been attained with less than 0.5 percent water change daily. In preliminary experiments in this closed bioconversion system, we have replaced all formulated feeds with agricultural residues and fertilizers, which are converted into protein by intense microbial processing. The rate of fish growth indicated that much or possibly all the feed requirements of tilapia grown in this type of closed, densely stocked system can be satisfied by intense cultivation of heterotrophic microbes.