Introduction
Minimising the quantity of waste produced and its sustainable management are major concerns in aquaculture. The type of production system, species cultured, the salinity of the rearing medium, the composition of the diet and the waste removal technique used interact to determine the removal efficiency of waste produced and its subsequent accumulation in the production system. In the current study, we first investigated the impact of dietary starch, NSP and their interactions on solid waste production, its removal efficiency and the size and stability of faecal particles in three salmonid species: rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar) and Arctic charr (Salvelinus alpinus). Following this, another trial was conducted with rainbow trout to understand the impact of the rearing medium’s salinity and the diet’s protein-to-energy ratio on the characteristics of faeces produced and its removal efficiency.
Materials and Methods
In the first trial, four diets were formulated using a 2 × 2 factorial design. The first factor, starch, was tested by including 0% gelatinised wheat flour (LS- low starch) or 20% gelatinised wheat flour (HS- high starch) in a plant ingredient basal diet. The second factor, NSP, was tested by adding 0% NSP source (low NSP) or 10% NSP source (high NSP). High NSP level was achieved by adding an equal mixture of soya hull (5%) and wheat bran (5%). Diets were tested in triplicates for each species, and feeding was done restrictively. The experimental duration was 42 days for rainbow trout and Atlantic salmon and 49 days for Arctic charr. Fish were fed restrictively twice a day.
For the second trial, two diets varying in protein-to-energy ratio were formulated. Protein to energy ratio (P/E ratio) was varied so that the starch-to-fat ratio remained constant, indicating that higher starch levels also correspond to the higher fat content in the diet. High and low P/E ratio diets corresponded to P/E ratios of 25.5 and 15.8 mg/KJ, respectively. Each diet was randomly allocated to 6 replicate tanks and fed restrictively twice a day. The salinity of the rearing medium was kept at 1.5 ppt for an initial 4 weeks. In subsequent weeks, salinity was raised to 6, 12, 18, 24 and 30 ppt. Fish were maintained for three weeks at 30 ppt. Read-out parameters were measured at 1.5 ppt, 12 ppt, 24 ppt and 30 ppt.
In both trials, nutrient digestibility, faecal waste production and faecal removal efficiency were determined by sedimentation. Faecal particle size distribution (PSD) was measured pre and post-exposure to mechanical stress, giving insight into faecal stability.
Results
Visual observation of faeces collected in bottles indicated that most faeces consisted of fine faecal particles in Arctic charr in contrast to the dominance of faecal pellets or strings in rainbow trout and Atlantic salmon (Image 1). The amount of faecal waste produced was influenced by the interaction effect of starch and NSP (P<0.05). High dietary starch levels increased the proportion of smaller-sized particles, while high NSP content increased the ability of faecal particles to withstand mechanical stress in all three species. In rainbow trout and Atlantic salmon, high starch levels in the diet lowered faecal removal efficiency (Fig. 1a, P<0.001) but increased by high NSP content (Fig. 1a, P<0.05). The highest and lowest faecal removal efficiency averaged over all 4 diets was recorded for Atlantic salmon (91%) and Arctic charr (56%), respectively. The amount of non-removed faeces accumulating in the system was increased (Fig. 1b, P<0.001) by the high starch levels in rainbow trout and Atlantic salmon diet but remained unaffected (Fig. 1b, P>0.05) by the NSP content of the diet across the species. Among the three species investigated, Arctic charr had the highest (96 g) amount of non-removed faeces per kilogram of dry matter feed, followed by rainbow trout (17g) and Atlantic salmon (29 g) averaged over all four diets.
For the second trial, we are still awaiting the results from 30 ppt. Results presented below are based on our findings for 1.5 ppt, 12 ppt and 24 ppt. Preliminary results indicated that the removal efficiency of faeces was influenced by the interaction effect between the salinity of the rearing medium and the P/E ratio of diet (P<0.05). For a high P/E ratio diet, removal efficiency declined as the salinity was increased, while the effect of salinity was the opposite for a low P/E ratio diet. Due to significantly higher faecal waste production (P<0.001) and lower removal efficiency (P<0.001), the amount of non-removed faeces was higher at a low P/E ratio diet than at a high P/E ratio diet. The amount of faecal waste production and the non-removed faeces increased with the increase in salinity of the rearing medium.
Conclusion
Considerable differences exist between rainbow trout, Atlantic salmon, and Arctic charr regarding faecal waste production and faecal characteristics. High starch levels in the diet reduced faecal stability and removal efficiency in salmonids. Non-removed faeces by settling is determined by dietary starch content and not by NSP. Salinity of the rearing medium has potential to impact the quantity of faecal waste produced and its removal efficiency. To conclude, solid waste management measures for salmonids , should keep in consideration the differences in species, diet composition and rearing medium salinity.