Using Supplemental Amino Acids to Reduce Dietary Protein Levels of Nile Tilapia Gives Economic and Environmental Benefits

World finfish aquaculture production has been progressively\r\nrising, and now accounts for almost 47 percent of total fish production. On a\r\nglobal scale, tilapia are the second most cultivated finfish group, with Nile\r\ntilapia (Oreochromis niloticus)\r\naccounting for eight percent of total finfish produced in 2016. The species is\r\nparticularly popular due to its fast‐growing rates, disease resistance,\r\nrobustness and ability to adapt to different farming systems.

In aquaculture, feed accounts for 50 percent of total\r\nproduction cost. A major challenge is to find new strategies for precision diet\r\nformulation that minimise feed cost, while increasing sustainability. Feed cost\r\nis largely determined by dietary protein sources and inclusion levels.

In the past few years, advances in the knowledge of tilapia\r\nnutrition, and commercial availability of supplemental amino acids have allowed\r\nfeed producers to be flexible in utilising plant sources to formulate balanced\r\ndiets. In addition to enabling industry to implement zero fishmeal diets,\r\nsupplemental amino acids open windows to reduce the dietary protein levels\r\nwhile balancing the diet for amino acid levels.

In the swine and poultry industries, the low protein concept\r\nwith the use of supplemental amino acids has been a common practice for a long\r\ntime. In aquaculture, this concept is not as common, and protein quantity is\r\nstill used as an indicator of diet quality. However, diet quality is rather\r\ndetermined by the protein quality and not quantity.

This requires additional attention on quantitative and\r\nqualitative amino acid levels. Nile tilapia's response to dietary protein\r\nlevels has been widely studied and is dependent on fish size/age, dietary amino\r\nacid profile and digestibility.

Typically, Nile tilapia starter or fry diets contain 45\r\npercent crude protein, while the diet for fingerlings and advanced juveniles'\r\noptimal growth performance contain 35 percent crude protein. For adults, 25-30\r\npercent protein is commonly used. Diets with high levels of protein but with an\r\nimbalanced amino acid profile will result in increased amino acid catabolism\r\nand consequently higher nitrogen losses.

Given the production volume of tilapia and the expected\r\ngrowth of aquaculture as a strategy to feed nine billion people by 2050, it is\r\nessential to find diets that are cost‐effective and environmentally\r\nsustainable.

We conducted a study to reduce protein inclusion levels in\r\nplant protein‐based diets for juvenile Nile tilapia, through adequate amino\r\nacid supplementation, in order to minimise dietary environmental impact while\r\nmaximising biological efficiency. Furthermore, metabolic trials were performed\r\naiming to obtain an in vivo snapshot of protein utilisation in Nile tilapia\r\njuveniles as a function of dietary protein content.

Materials and methods

Experimental diets

Five isoenergetic diets were formulated with differing\r\nprotein levels (36%, 34%, 32%, 30% and 28% diet), using plant ingredients as\r\nwell as meat and bone meal as protein sources. Diets were formulated to meet\r\nthe minimum requirements of amino acids, on digestible basis, for Nile tilapia\r\njuveniles according to AMINOTilapia (a tool developed by Evonik for the amino\r\nacid recommendations of Nile tilapia).

Apparent digestibility coefficients (ADC) of amino acids for\r\nthe ingredients used were taken from our review report (Konnert and Masagounder\r\n2017). Diets were supplemented with increasing levels of selected indispensable\r\namino acids and di‐calcium phosphate with the decreasing levels of dietary\r\nprotein to avoid amino acid or mineral imbalances.

Growth trial

Nile tilapia juveniles with an average body weight of 5.91 ±\r\n1.66g were used and the experiment was conducted at CCMAR in Portugal.

Triplicate tanks were randomly assigned to one of the five\r\ndietary treatments (D36, D34, D32, D30 and D28). Fish were fed to visual\r\nsatiety by hand, three times a day (09:30, 12:30 and 16:30 hours). Water\r\nquality parameters were monitored daily: temperature averaged 25.2 ± 0.1°C,\r\ndissolved oxygen in water was maintained above 80 percent of saturation, pH was\r\nmaintained between 7.70 and 8.20 and the concentration of unionised ammonia and\r\nnitrites in water was 0 mg/l during the whole experimental period. Fish were\r\nmonitored daily for any mortality and feed intake was recorded daily for 59\r\ndays.

Metabolic trial

After the growth trial, fish from the higher, intermediate\r\nand lower protein dietary treatments (D36, D32 and D28) were randomly selected\r\nand transferred to the nutrient flux laboratory. The experimental diets were\r\nlabelled with [U-14C]-L-amino acid mixture

Tube-feeding was performed on anesthetised fish, which were\r\nthen transferred into individual incubation chambers connected to CO2 traps\r\n(Rust et al. 1993; Rønnestad et al. 2001). Each chamber was hermetically sealed\r\nand supplied with a gentle oxygen flow during the 24 hours of incubation. At\r\nthe end of the incubation period, each fish was weighed and filleted for\r\ndetermining radioactivity in the muscle.

Results and\r\ndiscussion

Growth performance\r\nand feed utilisation

All fish had a fivefold increase in body weight at the end\r\nof the experiment, independently of the diet, and no significant differences (p\r\n> .05) were found at the end of the experiment, with mean values ranging\r\nfrom 29.34 to 31.49g.

Fish weight gain was not influenced (p > .05) by the\r\ndifferent dietary protein levels. Feed conversion ratio (FCR) increased with\r\ndecreasing levels of dietary protein but differed significantly (p < .05)\r\nonly between the group fed D28 (1.30 ± 0.05) and those fed the D36 diet (1.16 ±\r\n0.05). Protein efficiency ratio (PER) increased with decreasing levels of\r\ndietary protein; therefore, the D28 group presented the highest PER (2.60 ±\r\n0.09) and the D36 presented the lowest (2.27 ± 0.09).

The groups fed D28, D30 and D32 diets exhibited no\r\nsignificant differences among them (p > .05) and were significantly\r\ndifferent from the group fed the D36 diet (p < .05). No differences were\r\ndetected among treatments concerning the daily voluntary feed intake. There\r\nwere no significant differences (p > .05) in survival among fish fed the\r\nexperimental diets, which in overall was 98 ± 3 percent.

Dietary protein\r\nutilisation

Fish fed the D30 diet exhibited higher body protein\r\nretention than those fed the D36 diet (41 vs. 36% of intake, p < .05). The\r\nretention of the majority of the amino acids followed a similar pattern to that\r\nof protein retention, with fish fed the D30 diet presenting a tendency for\r\nhigher retention values than those fed the D36 diet.

The exception to this trend was methionine, which presented\r\nthe highest retention in fish fed the D36 diet, although no significant\r\ndifferences were found between the D36 and the D30 treatments (p > .05).\r\nThis is because as the protein level decreased in the diets from 36 to 28%,\r\ncysteine (Cys) level declined from 0.53 to 0.44 percent, which resulted in\r\nMet+Cys (1.33-1.35%) being more limiting than Met (0.82-0.89%) per se.

Due to the limitation of Cys in the low protein diets, fish\r\nmore likely used Met as a precursor for Cys production to meet other metabolic\r\nneeds than for direct protein synthesis, explaining the reduced Met retention\r\nin fish fed the low protein diets.

Daily nitrogen gain was similar among treatments, but there\r\nwere significant differences concerning the values of daily nitrogen losses.\r\nFish fed low‐protein diets, D28 and D30, presented the lowest daily nitrogen\r\nloss although only significantly different from the D36 group (p < .05).

The results of the metabolic trials gave an in vivo snapshot\r\non how dietary protein was being utilised by the fish. The highest values of\r\namino acid catabolism were found for fish fed the D36 diet and the values\r\nshowed a declining trend as the dietary protein level declined. Mirroring N\r\ngain, relative amino acid retention in muscle (mg / g fish) was quite similar among\r\ntreatments and no significant differences were found.

The present work indicates that the excessive dietary\r\nprotein content ultimately results on the use of amino acids as energy source\r\nand consequently on higher environmental impacts, due to increased nitrogen\r\noutputs.

Conclusions

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In conclusion, the present study demonstrates that it is\r\npossible to reduce protein levels in juvenile Nile tilapia diets to 30-32\r\npercent without compromising fish growth and FCR, while reducing nitrogen\r\nlosses to the environment. Using appropriate amino acid supplementation in\r\ntilapia feed seems an advisable strategy to minimise dietary protein levels,\r\nand guarantee economic and environmentally sustainable tilapia production.

Source: Aquafeed