In recent years, aquaculture become one of the fastest developing food sectors of the world with an effect on environment. An increase of phosphorus rich effluents from culture system creates consequences on environment sustainability. Phosphorus (P) is present in fish diets, which are the essential source of P in aquaculture. I mportance of P nutrition on bone development and energy kinetics in the cells are well studied (Strauch et al , 2018). Although, it is considered as biologically important for fish, it is as well established that excess phosphorus in fish feed can promote eutrophication of aquatic environments (Antony Jesu Prabhu et al, 2016).
The excretion of indigestible P leads to an accumulation of P in closed water systems, like recirculation aquaculture systems (RAS). Previous studies show that freshwater tilapia species can absorb P from the water through the gills and gastrointestinal tract (Al-Kholy et al., 1970) and in other species, it has even been shown that the accumulation of this mineral in water can be beneficial for growth and normal skeletal development (Strauch et al., 2019; Van Bussel, C et al., 2013). Normal developing skeleton is a prerequisite for sustainable production and animal welfare, but vertebral column deformities are a persistent concern for farmed Atlantic salmon (Fjelldal et al., 2012a).
The major objective of this study is to understand and quantify the differential impact of water- borne phosphorus and dietary phosphorus content on P retention juvenile Atlantic salmon. The final goal is to find an equivalence ratio between P-water and P-feed to reduce the dietary phosphorus content by recycling the P accumulated in water. Differences will be assessed by quantifying fish growth, P-accumulation in the whole body and opercula and comparing the skeletal development and the total fat content.
Materials and method
Atlantic salmon juveniles weighing on average 2.71 g were reared in this trial. This experiment includes three identical RAS systems with 48 freshwater tanks (each 50L) . Three water treatments with different phosphorus level were applied in these systems: no phosphorus addition ; supplemented to 30 mg P/litre and supplemented to 60 mg P/litre. Mono sodium phosphate was used to maintain phosphorus level in water. 8 different diets were tested in this experiment with different phosphorus levels in feed: 0.00%, 0.21%, 0.42% ,0.63%, 1.25%, 1.88%, 2.50% and 3.13% . E ach diet was tested in triplicate groups at 120 0 C in freshwater. Growth performance of each tank was measured after 8 weeks. At the end of the trial, phosphorus content in whole body and opercula was analysed and skeletal changes were assessed.
Growth performance was significantly impacted by P-water and P-diet. High water-P depressed growth performance. At the end of 8 weeks, phosphorus content in opercula and whole-body of fish has been analyzed . Analyses of skeletal abnormalities will support chemical analyses. A recommendation will be given how far water-borne P can replace dietary P in a sustainable manner.
3. Fjelldal, P.G., Hansen, T., Breck, O., Ørnsrud, R., Lock, E.-J., Waagbø, R., Wargelius, A., Witten, P.E., 2012a. Vertebral deformities in farmed Atlantic salmon (Salmo salar L.) – etiology and pathology. J. Appl. Ichthyol. 28, 433–440.
4. Strauch, S.M., Wenzel, L.C., Bischoff, A., Dellwig, O., Klein, J., Schüch, A., Wasenitz, B., Palm, H.W., 2018. Commercial African catfish (Clarias gariepinus) recirculating aquaculture systems: assessment of element and energy pathways with special focus on the phosphorus cycle. Sustain 10. https://doi.org/10.3390/su10061805.
5. Strauch, S. M., Bahr, J., Baßmann, B., Bischoff, A. A., Oster, M., Wasenitz, B., & Palm, H. W. (2019). Effects of ortho-phosphate on growth performance, welfare and product quality of juvenile African catfish (Clarias gariepinus). Fishes, 4(1).
6. Van Bussel, C. G., Mahlmann, L., Kroeckel, S., Schroeder, J. P., & Schulz, C. (2013). The effect of high ortho-phosphate water levels on growth, feed intake, nutrient utilization and health status of juvenile turbot (Psetta maxima) reared in intensive recirculating aquaculture systems (RAS). Aquacultural engineering, 57, 63-7.