Introduction
Waste management is a critical issue in aquaculture. Waste accumulating in the recirculating aquaculture system (RAS) is a function of amount of waste produced and their efficiency of removal. Since most solid waste is of faecal origin, manipulating the dietary factors may result in attributes desirable for their easy and efficient removal. In earlier studies, increasing the starch level in diet improved the faecal removal efficiency in Nile tilapia, Oreochromis niloticus (Amirkolaie et al., 2006), but reduced it in yellowtail kingfish, Seriola lalandii (Horstmann et al., 2023, under review), and African catfish, Clarias gariepinus (Phan et al., 2022). Similarly, NSP level in diet was shown to improve the faecal quality in Rainbow trout, Oncorhynchus mykiss (Meriac et al., 2014). Hence, in this study, we investigated the effect of starch and NSP levels in the diet and their interaction on nutrient digestibility, faecal waste production (digestibility), and faecal characteristics of rainbow trout.
Materials and Methods
Four diets were formulated according to a 2 × 2 factorial design. The first factor, starch was tested by including 0% gelatinized wheat flour (LS- low starch) or 20% gelatinized wheat flour (HS- high starch) in a plant based basal diet. The analysed starch content of the LS- and HS-diets was 40 and 240 g/kg respectively. The second factor, NSP was tested by adding 0% soya hull (LNSP- low NSP) or 10% soya hull (HNSP- high NSP). Tanks (experimental units) were stocked with 25 rainbow trout (initial weight, 81 grams) and the experiment was run for 6 weeks. Fish were fed restrictively (12 g/kg0.8/d, expected FCR 0.9) twice a day. For each tank, faecal waste production and faecal removal efficiency was determined by sedimentation. To get insight into the faeces stability, part of the collected faeces (non-stressed) were exposed to mechanical stress and the PSD was measured.
Results
The amount of faecal waste produced was influenced by the interaction effect of starch and NSP supplementation (P<0.001; Figure 1). At low NSP levels, starch inclusion did not alter faecal waste production. Inclusion of NSP increased waste production, being the highest at the HS-HNSP diet (P<0.001). For most other faecal waste characteristics (faecal removal efficiency; PSD of non-stressed faeces; amount of non-removed faeces) the interactions effect was not significant. Increasing dietary starch content, reduced faecal removal efficiency by 11.3% (P<0.001). NSP inclusion in the diets increased the removal efficiency by 4.4% (P<0.05), which fully compensated for the increased faecal waste produced. The amount of non-removed faeces was unaffected by NSP inclusion (P>0.05), whereas dietary starch inclusion resulted in a doubling of the non-removed faeces (P<0.001). Both starch and NSP inclusion significantly altered PSD of non-stressed faeces (P<0.05). Starch inclusion reduced the size of faecal particles, while NSP inclusion increased it (Image 1). Exposing faeces to mechanical stress reduced the particle size, but this reduction seemed to be affected by an interaction effect between starch and NSP inclusion. After stress exposure, the amount of large faecal pellets (>1600 µm) was lower at the HS-diets, but the difference between LS and HS-diets was largest at the diets without NSP inclusion (P<0.05).
Conclusion
Non-removed faeces by settling is determined by dietary starch content and not by NSP. Dietary starch negatively affects faecal removal efficiency while NSP improves it. Dietary starch content reduces the faeces stability but this effect is dependent on the NSP content.
References
Amirkolaie et al., 2006. https://doi.org/10.1016/j.aquaculture.2006.06.039
Phan et al., 2022. https://doi.org/10.1016/j.aqrep.2022.101051
Meriac et al., 2014. https://doi.org/10.1016/j.aquaculture.2013.11.018