Introduction
Replacing fishmeal with plant ingredients coincides with decreased bioavailability of dietary phosphorus (P) in fish feeds, since phytate bound P is poorly absorbed by fish. In aquafeeds that contain more plant-based ingredients, inorganic P needs to be supplemented in order to fulfil the P requirements for growth. However, using P from (mined) phosphate rock deposits is unsustainable, given that natural reserves of phosphate are limited (Prabhu et al., 2013; Obersteiner et al., 2013). It is essential that sufficient levels of bioavailable P are present in aquafeeds, as P plays a fundamental role in the synthesis of ATP and nucleic acid, and the formation of bones and scales (Fontagne et al., 2009) . By improving the bioavailability of phytate bound P from plant ingredients, the dependency of aquaculture on inorganic mined P sources and the level of dietary P in aquaculture effluent can be reduced. In recirculating aquaculture systems (RAS), minerals can accumulate in the water (including P), which can lead to waterborne mineral uptake by fish.
Whether or not fish take up P (and other minerals) from the water is most likely related to the amount of available minerals in the feed, as well as the concentration of available minerals in the water (Rodehutscord et al., 2000; Prabhu et al., 2017) . When denitrification occurs, phytate can be partly broken down (when faecal waste is used as carbon source) and P becomes soluble in water
(Goddek et al., 2016; Goddek et al., 2018); this, in combination with low water exchange, can lead to accumulation of P in the water. The use of phytase in aquafeeds is getting more attention as a wa y to break down phytate and improve the dietary P availability (Kumar et al. 2012). Th e presented experiment evaluated two potential ways to increase the P availability in aquafeeds (high in phytate): 1) increasing the dietary availability of P through the diet with the use of phytase, and 2) increasing the P availability in the water using a system low in water refreshment with the use of a sludge denitrification reactor.
With the experimental setup, the contribution of water born P uptake to growth was determined, and whether the effect of water born P uptake is dependent on the availability of dietary P.
Materials and Methods
A plant-based diet high in phytate and non-starch polysaccharides (carbon source denitrification) was used. The effect of available dietary P level was tested by supplementing phytase (PHY) versus a control without phytase (CON). The contrast in water available P was achieved by using a system with a high water refreshment (HR, low in water P) versus a system low in water refreshment (LR, high water P), using RAS with a denitrification reactor. The four treatments were studied over a period of 56 days according to a 2 x 2 factorial design. Tanks in the HR system were stocked with 30 and the LR with 40 Tilapia (mean initial weight 30 g). Fish were restrictively fed twice a day (aimed feeding level: 17 g/kg0.8) . For each tank growth performance was measured and the P utilisation calculated/estimated. The digestibility and the effect of phytase on the digestibility were only tested in the LR system (the faecal waste in the HR system was used as carbon source for the denitrification reactor).
Results
Phytase improved the dietary P availability with 56% (to 4.5 g P/kg DM), in both water P levels, resulting in higher growth (g/d). Increasing the water P levels using a denitrification reactor, resulted in higher growth (g/d). The effect of phytase on growth (SGR, %/d), was larger in the low water P treatment (interaction effect, P<0.05). For both diets, water P uptake occurred. However, the water P uptake was larger for the control diet, low in available P (figure 1; P<0.05). For the control treatment in the high water P treatment (HR-CON), at least 47% of the P retained was taken up from the water. Improving the P retention by increasing the P availability in the diet and/or increasing the water P levels, correlated with improved growth (figure 2).
Conclusion
Water P utpake under high water P conditions can partly compensate a P deficiency for P retention. The effect of increasing P retention on growth suggest that the P availability was limiting growth (figure 2) for the low water P and low dietary P levels. This study shows that at least 6.5 g P retention is needed per kg weight gain to sustain maximal growth. Meaning the P availability in the diet should be above 6.5 g P/kg DM when the diet is the only P source (and has a FCR of 1).
References
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