Nutrient digestibility and faecal characteristics in rainbow trout, Oncorhynchus mykiss reared at different salinities and fed diets with varying protein to energy ratios

Knowledge on quantity and characteristics of faecal waste produced by cultured fish species is important to develop appropriate waste management strategy in aquaculture. The current study aimed to evaluate the impact of dietary protein-to-energy ratio (P:E) on faecal waste production and faecal characteristics of rainbow trout (Oncorhynchus mykiss), reared at different water salinity. Two diets (HPE diet- high protein to energy ratio diet; LPE diet- low protein to energy ratio diet) varying in protein-to-energy ratio (P:E) were produced while keeping the fat-to-starch ratio constant. Each diet was allocated to 6 replicate tanks and feeding was done restrictively. Experiment was divided into two phases: in the first phase, fish were reared at 1.5 ppt salinity for 4 weeks, followed by a second phase where salinity increased weekly to 6, 12, 18 and 24 and 30 ppt. Nutrient digestibility, faecal waste production, faecal characteristics were measured at 1.5 ppt, 12 ppt, 24 ppt and 30 ppt, respectively. Both salinity and diet influenced the digestibility of macronutrients, including crude protein, crude fat, and starch. No interaction effect between diet and salinity was observed for crude protein and crude fat digestibility. Both crude protein and starch digestibility were higher at HPE diet than at LPE diet, whereas for fat digestibility, the case was opposite. The amount of faecal waste produced and their removal efficiency was influenced by the interaction effect between diet and salinity. The amount of faecal waste produced was higher at the LPE diet than at the HPE diet. At both diets, the amount of waste produced increased with salinity, but more so with the LPE diet. Visual observation and analysis of particle size distribution (PSD) data showed greater presence of faecal fines at LPE diet, while the impact of salinity on faecal PSD was less distinct. The amount of non-removed faeces was more than double at LPE diet compared to HPE diet, with no clear trend observed with increasing salinity. In conclusion, the amount of faecal waste produced and removal efficiency are influenced by diet and water salinity, but also by their interaction.