FEEDING RAINBOW TROUT (Oncorhynchus mykiss) WITH GUAR MEAL PROTEIN CONCENTRATE: WHICH IS THE BEST PERCENTAGE OF CONVENTIONAL PROTEIN SOURCE REPLACEMENT?

Introduction Since rainbow trout (Oncorhynchus mykiss) is one of the most significant freshwater fish species reared, efforts to improve this industry through genetic selection and sustainable diets are being made (Garcia-Ballesteros et al., 2021; D’Agaro et al., 2021). Soy, rapeseed, wheat gluten and corn gluten are commonly used as protein concentrates in aquafeeds, due to their high digestibility (Parisi et al., 2020). Guar meal, deriving from the endosperm of an Indian cluster bean, the galactomannan polysaccharide Guar gum (Cyamopsis tetragonalobus), has already been used in aquafeed, increasing faecal stability (Janphirom et al., 2010). In the last years, it has been purified to reduce saponin, tannin, phytates and protease inhibitor concentration, that are considered to negatively affect growth of salmonids (Pach F. and Nagel F., 2017). In this trial proprietary guar meal protein concentrate (MYCOPRIME®, Panghea SPA, Milan, Italy) replaced different percentages of conventional proteins in feed for rainbow trout, to investigate its effects on zootechnical performances during the fattening phase of rainbow trout. Materials and Methods A total of 2700 rainbow trout (mean body weight 50±1.4g) were reared at the initial stocking density of 15 kg/m3 in 12 concrete tanks (6x1x0.5m). The principal water physical-chemical parameters (temperature, dissolved oxygen and pH) were recorded daily. Total Ammonia Nitrogen (TAN), nitrites (NO2-N) and nitrates (NO3-N) were determined weekly using a spectrophotometer (Hach mod-2005, Hach Company, Loveland, USA) following the American Water Works Association and Water Pollution Control Federation of American Public Health Association (APHA) standard methods (1995). Trout were fed twice a day ad libitum; each diet was administered to fish of three tanks. The Control diet (CD) was a growing feed available for trout with 43% of proteins and 25.3% of lipids (% as it is). The two experimental feeds (D5 and D15) were formulated isoproteic and isolipidic; a partial replacement with guar meal concentrate (5% in the D5 and 15% in the D15) of fish meal, chicken meal and soybean meal was applied. The trial was performed during a standard zootechnical cycle and lasted 90 days. Final mean weight (g) and Final mean length (cm) were recorded and Palatability was calculated. Water physical-chemical parameters, fish biometric parameters and final productive traits were submitted to one-way analysis of variance (ANOVA) with p<0.05. Results and discussion The water physical-chemical parameters showed a very similar trend in terms of temperature, dissolved oxygen and pH during the whole trial. The nitrogen compounds demonstrated significant differences in TAN, having the highest ammonia concentration in D15 tank (0.44 ± 0.01 mg/L) compared to CD (0.24 ± 0.09 mg/L) and D5 (0.22 ± 0.06 mg/L). Nitrites ranged between 0.01 ± 0.001 mg/L (D15) and 0.02 ± 0.002 mg/L (CD). Nitrates varied between 0.9 ± 0.1 mg/L (D15) and 1.1 ± 0.4 mg/L (CD) without significant statistical differences. Brinker et al. observed negative effects on fine solid particles using diets with guar meal replacement, that clogged biofilters, in a recirculating water system (Brinker A. and Fredrich C., 2012); this condition occurred also in tank D15. Concerning the biometric parameters, the final mean weight of trout receiving D5 (201.00 ± 3.7 g) and CD (198.8 ± 3.8 g) showed similar results, significantly major in comparison with D15 (171.2 ± 5.1 g). Final mean length didn’t show significant differences. Feed palatability resulted higher in CD and D5 diets in comparison to D15; in fact, presumably, trout fed with D15 reached the satiation before and this condition affected also the final zootechnical performances as shown by the lowest final mean body weight. This negative output could have been caused also by the reduction of the apparent digestibility of dry matter, crude protein, and crude lipid due to the highest inclusion of guar gum (Liu et al., 2022). Conclusions Considering the zootechnical performances, the 5% inclusion of guar meal protein concentrate in fish feed resulted to be positive. Further analysis concerning fish welfare status and feed efficiency are going to be performed to confirm the satisfactory results.