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Practical low-fishmeal diets for rainbow trout (Oncorhynchus mykiss) reared in RAS: Effects of protein meals on fish growth, nutrient digestibility, feed physical quality, and fecal particle size

Practical low-fishmeal diets for rainbow trout (Oncorhynchus mykiss) reared in RAS: Effects of protein meals on fish growth, nutrient digestibility, feed physical quality, and fecal particle size

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Written by the Editorial Team of Aquaculture Magazine

To fulfil the future demands of seafood protein, aquaculture must promote its sustainable development. Consequently, it was evaluated the effect of the inclusion in feed formulation of commercially available protein meals alternative to fishmeal on fish growth performance and digestibility of diets, and the dietary effect on the quality of feces, with the general aim of optimizing the efficiency of waste management in RAS for rainbow trout.

In the landscape of aquafeed formulations, several protein meals (such as processed animal proteins – PAPs – or vegetable protein meals) have been tested as alternative or supplementary protein sources to satisfy the dietary requirements of targeted farmed species.

Among PAPs, poultry by-product meal is one of the most promising alternatives to fishmeal due to its high protein content, relatively cheap price, and broad availability (Galkanda Arachchige et al., 2020). Along with poultry by-products, feather meal is another protein-rich and cost-effective ingredient obtainable from the poultry rendering industry (Pfeuti et al., 2019).

Practical low-fishmeal diets for rainbow trout (Oncorhynchus mykiss) reared in RAS: Effects of protein meals on fish growth, nutrient digestibility, feed physical quality, and fecal particle size

In RAS, aquafeeds must also be of excellent physical quality for the most efficient handling, storage and administration as well as produce large fecal particles for a rapid removal from the system. Changes in feed ingredients can affect the technical quality of the feed, such as oil leakage, durability, and its stability in water as well as waste production.

However, knowledge about the performance of diets based on alternative raw materials in the aquaculture environments as for waste production is still scarce.

Thus, it was evaluated the effect of the inclusion in feed formulation of commercially available protein meals alternative to fishmeal, i.e., poultry by-product meal, hydrolyzed feather meal and rapeseed meal, on fish growth performance and digestibility of diets, the physical properties of feed pellets, and the dietary effect on the quality of feces, with the general aim of optimizing the efficiency of waste management in RAS for rainbow trout.

Materials and methods

To evaluate strategies for optimiz ing waste in recirculating aquaculture systems, 1,020 rainbow trout (initial live weight 17.2 ± 7.50 g/fish) were distributed into 12 tanks after 21 d of acclimation and fed during 84 days with four practical diets (crude protein: 49% dry matter (DM); crude fat 26% DM; gross energy: 23 MJ kg-1) containing different rates of fishmeal and alternative protein meals, i.e. Diet FM (307 g kg-1 fishmeal, 61.2 g kg-1 poultry by-product meal); diet PBM (183 g kg-1 fishmeal, 168 g kg-1 poultry by-product meal); diet FeM (198 g kg-1 fishmeal, 61.2 g kg-1 poultry by product meal, 76.5 g kg-1 hydrolyzed feather meal); diet FeM+RM (171 g kg-1 fishmeal, 61.2 g kg-1 poultry by product meal, 76.5 g kg-1 hydrolyzed feather meal, 60.4 g kg-1 rapeseed meal). High structural integrity of ex truded pellets and low oil leakage were measured in all diets.

Results

Diets did not affect fish final live weight (191 g, on average), daily weight gain (1.71 g d-1), specific growth rate (2.16% d-1), feed conversion ratio (1.05), feed intake (1.65 g DM fish-1 day-1), and fish survival (97.7%) (Table 1).

Practical low-fishmeal diets for rainbow trout (Oncorhynchus mykiss) reared in RAS: Effects of protein meals on fish growth, nutrient digestibility, feed physical quality, and fecal particle size

Fish viscerosomatic index (24.6% on average) and carcass yield (75.4%) did not differ among the four diets. The oil leakage in feeds ranged between 1.10% and 1.50%. The durability test showed that the shares of small and large fractures in the feed were always below 1%, with no differences between diets.

At a sieve mesh interval of 0.3 – 0.5 mm, fish fed diet PBM produced a higher percentage of retained feces (64%) compared to the other diets (59.3%; p < 0.001). Regarding apparent digestibility coefficients of nutrients in the diets the ADC of protein were different among the four diets (p < 0.001) with the lowest value associated to diet FeM (79.6%) and the highest value to diet PBM (86.0%).

More over, ADC of protein significantly increased over time (p < 0.001), from 80.3% at 28 days of trial to 86.6% at 84 days of trial (Figure 1).

Practical low-fishmeal diets for rainbow trout (Oncorhynchus mykiss) reared in RAS: Effects of protein meals on fish growth, nutrient digestibility, feed physical quality, and fecal particle size

The highest production cost per ton of diet was measured for diet FM followed by diet PBM, diet FeM and diet FeM+RM (Table 2).

Practical low-fishmeal diets for rainbow trout (Oncorhynchus mykiss) reared in RAS: Effects of protein meals on fish growth, nutrient digestibility, feed physical quality, and fecal particle size

When costs were expressed per kg of pro duced fish, the highest values were recorded for diet FM (1.01 $ kg-1) and the lowest for diet FeM+RM (0.92 $ kg-1), with diet PBM and diet FeM showing intermediate and simi lar values (0.96 and 0.95 $ kg-1).

Discussion

The results showed that the inclusion of 76.5 g kg-1 of hydrolyzed feather meal with 61.2 g kg-1 of poultry by product meal and 198 g kg-1 of fish meal (diet FeM) did not affect rain bow trout growth performance and feed conversion ratio while reducing aquafeed production costs by 8%.

In fact, feather meal composition and digestibility remain extremely variable (Pfeuti et al., 2019b) and a blend of hydrolyzed feather meal with other protein meals is suggested to better balance dietary amino acid profile (Yu, 2019).

Based on the results and previous literature (Bureau et al., 2000), hydro lyzed feather meal could be effectively incorporated from 76.5 to 150 g kg-1 in low-fishmeal (≤ 200 g kg-1) diets for juvenile rainbow trout, whereas higher inclusion levels seem to be possible only through additional en zymatic pre-treatments of the feath ers and amino acid supplementation (Pfeuti et al., 2019).

Practical low-fishmeal diets for rainbow trout (Oncorhynchus mykiss) reared in RAS: Effects of protein meals on fish growth, nutrient digestibility, feed physical quality, and fecal particle size

It was measured the highest protein digestibility in the diet with the highest inclusion of poultry by-products (diet PBM). High digestibility of poultry by-product meal has already been reported in studies on rainbow trout (Badillo et al., 2014; Galkanda Arachchige et al., 2020).

Finally, in RAS, the formation of large fecal particles is desirable, as they are more rapidly and easily removed from the system than smaller ones (Brinker, 2009; Unger and Brinker, 2013). Overall, compared with previous studies on rainbow trout, it was found a similar proportion of fine particles in feces (58–64%), but a lower percentage of mid-large fecal particles (5 – 7% vs. 21 – 38%).

These differences might be likely related to the different methods used to mea sure fecal size, with laser diffraction (Welker et al., 2018) or microscopic analysis (Welker et al., 2020, 2021) being more accurate than using the percentage of retained feces at different sieve mash sizes. In the study, the diet PBM, containing poultry by-product meal and fishmeal as main protein sources, showed the highest protein digestibility and was associated to a higher size of feces particles.

Conclusion

Under study conditions, the partial substitution of fishmeal from fish by-products with alternative protein meals did not affect trout growth performance and feed conversion ratio. The digestibility of protein resulted satisfactory (> 80%) for all diets, whereas improvement for lipid digestibility is desirable in diet containing hydrolyzed feather meal and rapeseed meal.

The inclusion of hydrolyzed feather meal reduced water turbidity during the first thirty minutes after feed administration and the inclusion of poultry by-product meal induced the production of larger particles offeces, which are considered positively in view of the reduction and collection of wastes in a RAS system.

A comprehensive evaluation of aquafeeds, by considering not only their nutritional aspects but also their physical characteristics and effects on waste production together with economic impacts, would be paramount for future feed formulations, especially when designed for sustainable, highly controlled, and resource-efficient environments like RAS.

This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “PRACTICAL LOW-FISHMEAL DIETS FOR RAINBOW TROUT (ONCORHYNCHUS MYKISS) REARED IN RAS: EFFECTS OF PROTEIN MEALS ON FISH GROWTH, NUTRIENT DIGESTIBILITY, FEED PHYSICAL QUALITY, AND FAECAL PARTICLE SIZE” developed by: FANIZZA, C. – University of Padova and Universitat Politècnica de València, TROCINO, A. – University of Padova, STEJSKAL, V., DVORÀKOVÀ PROKESOVÀ, M., ZARE, M., QUANG TRAN, H. – University of South Bohemia, BRAMBILLA, F. – NaturAlleva (VRM s.r.l.), XICCATO, G., BORDIGNON, F. – University of Padova.
The original article was published, including tables and figures, on DECEMBER, 2022, through AQUACULTURE REPORTS.
The full version can be accessed online through this DOI 10.1016/j.aqrep.2022.101435.

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