Introduction
The rapidly growing aquaculture sector is recognized as critical to supplying global protein needs, and protein-rich ingredients are required to fill the protein gap in feed production . S ingle cell proteins (SCPs) such as microalgae, yeast or bacteria are novel ingredients that have all been tested as alternative protein sources for aqua feeds (Glencross et al., 2020) . Among the SCPs, bacterial SCPs are some of the most popular and sustainable ingredients, with a high protein content and an amino acid (AA) profile resembling that of fishmeal (Pereira et al., 2022) . At the same time, SCPs derived from microorganisms grown on gas offers a sustainable high-quality protein, which can be intensively produced without having a negative impact on the environment (Glencross et al., 2020). The objective of the present study was to assess the digestibility of a bacterial SCP as a viable alternative protein source in the diets of Atlantic salmon ( Salmo Salar).
Materials and methods
The diets were formulated based on a 70:30 digestibility model, with the test ingredient, Proton (crude protein = 87.6%), composing 30% of the experimental diet . The control feed was formulated to resemble a commercially available salmon feed for parr to pre-smolt stage. The experimental diets were fed to Atlantic salmon with initial mean weight of 76.18 ± 4.61 g for 49 days in a freshwater recirculating aquaculture system. Before starting the digestibility trial, a random sample of 20 fish were subjected to gross pathology health checks, to ensure a healthy population for the trial. Fish were acclimated to the trial system and fed a commercial diet for 10 days, after which the fish were acclimated to the experimental diet for 6 days, before digestibility collections were initiated . Fish were fed to apparent satiation across three meals per day, and after each feeding round the tanks were cleared for any material (uneaten pellets or faeces). After 15 minutes, the collection of faeces commenced, and continued periodically until no further material was present, or the next feeding round began.
Results
There was a statistically significant higher average individual weight gain of the salmon (p = 0.018) in the experimental group compared with the control group after 49 days of feed ing (53.90 ± 2.72 % and . 45.75 ± 5.95 % , respectively). Additionally, the experimental group had a higher feed intake (2.43 g/day per fish vs. 2.34 g/day per fish) and significantly less faecal matter per tank (p = 0.015) compared with the control group (9.55 ± 0.59 g and 12.52 ± 1.73 g DM , respectively ). Pairwise comparison of ADC between the diets demonstrated that the average ADC values for protein, GE and AAs were statistically higher for the experimental diet compared to the control diet ( p < 0.003). Furthermore, Proton demonstrated a protein ADC of 94.1% and average amino acid ADC of 95.6%.
Discussion and conclusion
Digestibility is a critical factor when evaluating novel protein sources for aquafeeds, particular in juvenile Atlantic salmon that require a high-quality protein for optimal growth. The high protein ADC from Proton demonstrated in this trial suggests a superior nutrient utilisation compared to many alternative protein sources tested in salmonid diets (Øverland et al., 2013; Gong et al., 2018). The high protein ADC is even comparable to other high-quality proteins like fishmeal with ADC values ranging from 80-90% (Øverland et al., 2009) . At the same time fishmeal typically provides high digestibility coefficients of essential AAs exceeding 90% (Storebakken et al., 1998) , which Proton in the present study could compete with at an average AA digestibility of 95.6%.
Over the duration of the trial, fish receiving the experimental diet had a slightly higher feed intake compared with the control group, which may indicate an improved palatability even at high inclusion levels of Proton. On the other hand , differences in nutrient composition due to the unbalanced diets, must be considered when interpreting the data. The experimental diet had a higher protein content due to a 30% inclusion of Proton, which resulted in not fully reliable growth data. However, the growth results give an indication of how Proton might perform in a dedicated growth trial, and it supports the high digestibility claim. The study indicates that Proton is a highly digestible alternative protein ingredient for juvenile Atlantic salmon, with strong ADC values for protein and AA. The preliminary performance also demonstrates a good palatability of a Proton diet. The findings highlight Proton’s potential as a sustainable and effective protein source in salmonid diets.
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