Fishmeal is still one of the main protein sources used in commercial feeds for trout and salmon. But its availability is shrinking and its cost is increasing year by year. The sustainability of the aquaculture industry depends largely on its capability to replace fishmeal with alternative sources of protein, and to reduce the currently excessive protein levels commonly applied in the formulation of commercial diets. At the same time, feeds must be formulated to be effective in covering the nutrient requirements of specific species in order to maximise growth.

1. I N C O R P O R AT I N G
f i s h far m ing t e c h no l og y
January | February 2014
Current challenges and opportunities in
amino acid nutrition of salmonids
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3. FEATURE
Current challenges and opportunities
in amino acid nutrition of salmonids
by Cláudia Figueiredo-Silva and Andreas Lemme, Evonik Industries, Germany
F
ishmeal is still one of the main
protein sources used in commercial
feeds for trout and salmon. But its
availability is shrinking and its cost is
increasing year by year. The sustainability of
the aquaculture industry depends largely on
its capability to replace fishmeal with alternative sources of protein, and to reduce the
currently excessive protein levels commonly
applied in the formulation of commercial
diets. At the same time, feeds must be
formulated to be effective in covering the
nutrient requirements of specific species in
order to maximise growth.
While this great challenge has long been
foreseen and recognised by both industry
and academia, a large number of feed manufacturers are still struggling to adapt, and are
relying on their own ability to turn challenges
into opportunities on a daily basis. Despite a
growing body of evidence showing that fishmeal can be replaced by alternative protein
sources (reviewed by Kaushin and Hemre,
2008; Tacon and Metian, 2008), and that
opportunities may exist to reduce crude
protein level in aquafeeds (Yamamoto et al.,
2005; Gaylord and Barrows, 2009), scientific
research is not always made widely available.
The feed industry is delayed from applying
the most important scientific achievements in
its field. Here, we will try to summarise the
major achievements in the development of
more sustainable diets for salmonids, whilst
not forgetting to point out the existence of
limitations for their fully practical application.
We have reviewed some of the most relevant studies focusing on the replacement of
fishmeal with alternative protein sources, and
on the possibility of reducing crude protein
in diets for salmonids. In addition, we found
it important to highlight that the optimisation
of amino acid nutrition goes beyond meeting its requirements for protein synthesis,
and may constitute a promising approach to
improve, among other things, animals’ immunity response to environmental stresses.
Alternatives to fishmeal: feasible?
Because protein is typically the most costly
nutrient in a formulated feed, it is important to accurately cover protein, or to be
more precise, to accurately cover amino
acid requirements of animals (NRC, 2011).
This has become a priority given the current
constraints on fishmeal – in cost and availability – and the consequent need to replace
it with plant protein sources limited in one or
more of the essential amino acids. The supplementation of low fishmeal or plant protein
based diets with these essential amino acids
was proven to improve voluntary feed intake
and whole body protein accretion in several
fish species, including rainbow trout (Gomes
et al., 1995; Cheng et al., 2004; Gaylord and
Barrows, 2009; Kaushik et al., 1995; FigueiredoSilva et al., 2012) and Atlantic salmon (Espe et
al., 2006, 2007, 2008; Torstensen et al., 2008;
Kousoulaki et al., 2009).
Rainbow trout seem, however, less sensitive than Atlantic salmon to the replacement
of fishmeal with plant protein sources. In
a comparative study, Refstie et al. (2000)
showed that rainbow trout but not Atlantic
salmon grew equally well with soybean meal
based diets, suggesting that rainbow trout are
less sensitive than salmon to anti-nutritional
factors (see Table 1). But that there is much
latitude for the reduction of marine resources
in salmonid diets is clearly shown by the pos-
16 | InternatIonal AquAFeed | January-February 2014
sibility of including fishmeal levels as low as 0
percent in rainbow trout feeds (Kaushik et al.,
1995) and 5 percent for Atlantic salmon (Espe
et al., 2007, 2008), provided that their amino
acid profile is balanced with supplemental
amino acids. These achievements would not
have been possible if free amino acids were
not utilised as efficiently as the protein-bound
kind in meeting the essential amino acid
requirements of fish.
The 100 percent bioavailability of free
amino acids has in fact been demonstrated
in several fish species (reviewed by NRC,
2011), including rainbow trout (Rodehutscord
et al., 1995a,b, 1997; Rollin et al., 2003) and
Atlantic salmon (Espe and Lied 1994; Epse et
al., 2006, 2007, 2008). However, one might
and indeed should ask why some studies have
failed to successfully replace fishmeal with
alternative protein sources even when diets
were supplemented with limiting essential
amino acids. Although the answer is not yet
entirely clear, it seems to lie in the obvious
differences between the nutritional value of
fishmeal and of alternative protein sources.
When replacing fishmeal protein, in particular
with plant sources, we must keep in mind that
we are not only affecting amino acid availability and utilisation, but also that of fatty acids,
vitamins and minerals. In addition, being rich
in anti-nutritional factors and carbohydrates,
plant protein inclusion may significantly impact
on diet palatability and thereby voluntary feed
intake, and on the availability and utilisation
of energy.
This might partly explain why, for example,
the 2006 study of Espe et al. – undertaken
in collaboration with Evonik Industries – was
not completely successful in replacing fishmeal
with plant protein sources. In the study with

4. FEATURE
Atlantic salmon, voluntary feed intake and
growth decreased, even though the dietary
amino acid profile had been balanced with
supplemental amino acids (see Table 1). In
a following study by Espe et al., again in collaboration with Evonik Industries, fishmeal was
successfully replaced in Atlantic salmon diets
by a mixture of plant proteins, provided that
their amino acid profile was duly balanced
with a mixture of supplementary amino acids,
and also retaining a low inclusion level of
fishmeal (5 percent). While without amino
acid supplements, this low level of fishmeal
would not have allowed the requirements of
salmon to be satisfied, the inclusion of 5 percent fishmeal in addition to 3 percent squid
hydrolysates (already applied in the first Espe
study) proved an effective strategy in securing
a similar feed intake level and growth rates
between the fishmeal-based diet (49 percent)
and fishmeal-replaced diets (5 percent).
It seems clear, therefore, that although supplementation with amino acids is a crucial and
effective strategy in keeping a similar protein
accretion between fishmeal-based diets and
plant-based diets, replacement of 100 percent
fishmeal with alternative protein sources in
salmon (as well as other species) still depends
on formulations that cover requirements for
all essential nutrients, including fatty acids,
vitamins and minerals. A complete and accurate evaluation of the differences between
the nutritional value of
fishmeal and alternative protein sources
will shortly allow the
total replacement of
fishmeal in salmonid
feeds.
Lowering
the crude
protein level
Crude protein level
is calculated by multiplying the amount of
nitrogen by the empirically derived conversion factor of 6.25,
which is based on the
estimation that protein
contains 16 percent
nitrogen, although in
reality it varies from 12
to 19 percent. Although crude protein value
offers a good estimate of protein level, it does
not allow the scientist to distinguish between
nitrogen originating from amino acids and
nitrogen originating from non-protein sources.
Crude protein is therefore useless in evaluating the amino acid profile of ingredients and
diets.
Lowering the dietary crude protein level
and supplementing diets with certain essential
amino acids is a well-established method of
formulating diets for farm animals to achieve
an ideal amino acid pattern, and it has been
demonstrated by Verstegen and Jongbloed
(2003) to reduce nitrogen excretion for
pigs and poultry. These findings have even
been incorporated into subsequent legislation. Although aquaculture seems more ecoefficient than pig or poultry production in
providing nutrients for human consumption
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5. FEATURE
table 1. the effect of on voluntary feed intake, growth, feed conversation ratio (FCr) and protein retention efficiency (Per) in atlantic salmon and
rainbow trout fed high plant protein diets compared to a FM control group.
Protein sources (PS)
results
other
marinePS %
Plant-PS
%
Supplemental aa
Feed
intake
Growth
FCr
Per
32.0
0
29.6
no
Similar
reduced
reduced
-
0.0
5.0 to
10.0
41.0 to
43.4
aa Mixture,
including l-lys,
l-arg, Dl-Met,
l-trp, l-thr, l-His
reduced
or similar
reduced
Similar
Similar
Espe et al., 2006
327g
5.0
5.0 to
10
36.5 to
39.6
aa Mixture,
including l-lys,
l-arg, Dl-Met,
l-trp, l-thr, l-His
Similar
Similar
Similar
Similar
Espe et al., 2007
300g
30.0 to
12.0
2.5 to
5.0
29.6 to
43.0
l-lys, Dl-Met,
l-His
Similar
Similar
Similar
Similar
Torstensen et al., 2008
137g
5.0
5.0
48.4 to
48.9
l-lys, Dl-Met,
l-thr
Similar
Similar
Similar
Similar
Kousoulaki et al., 2009
Body
weight
FM %
references
atlantic salmon
200g
300g
Refstie et al., 2000
rainbow trout
55g
20.0
0.0
30-80
l-lys, l-Met
Similar
Similar
Similar
Similar
Gomes et al.,1995
55g
0.0
0.0
80.0
l-lys, l-Met
reduced
reduced
Similar
reduced
Gomes et al.,1995
83g
0.0
0.0
62.0
l-Met
Similar
Similar
Similar
Similar
Kaushik et al., 1995
100g
32.0
0
29.6
no
Similar
Similar
reduced
-
(Ytrestøyl et al., 2012), the move towards a
lower nitrogen output is a major sustainability
drive in fish farming, both for environmental
and economic reasons.
Furthermore, the contribution of amino
acids towards meeting the energy requirements of fish are considered high (NRC,
2011), and thus efforts must be taken to
reduce surplus protein supply whilst improving amino acid and non-protein energy utilisation. A decrease in the digestible-protein-todigestible-energy ratio (DP:DE) –achieved by
reducing the dietary digestible protein levels
with or without an associated increase in the
dietary non-protein digestible energy supply – has proven to be extremely efficient in
improving nitrogen utilisation and decreasing
nitrogenous losses in numerous farmed species (reviewed by NRC, 2011).
Studies including that of Yamamoto et al.
(2005) show that rainbow trout diets supplemented with all the limiting essential amino
acids allow for the reduction of protein level
from 45 to 35 percent, without compromising performance and even improving protein
retention efficiency from 35 to 50 percent,
reducing nitrogen loading into the environment. In their 2009 rainbow trout study
Gaylord and Barrows also showed that by
keeping a similar dietary energy level and sup-
plementing diets with methionine, lysine and
threonine on an ideal protein basis, dietary
crude protein level can be reduced from 46
to 40.9 percent without affecting growth and
even improving protein retention efficiency.
Again, these achievements would not have
been possible were free amino acids not
utilised as efficiently as protein-bound amino
acids in meeting the requirements of fish.
Also very important to bear in mind is
that although the DP:DE ratio is a more
rational way of expressing protein requirements than dietary crude protein requirements, it must not be taken as a fixed or
accurate value. At low DP:DE ratios, fat
was shown to constitute a more effective
source than digestible starch in improving
protein utilisation efficiency in rainbow trout,
underlining the importance of non-protein
energy sources (lipids and starch) in maximising amino acid utilisation in salmonids
(Figueiredo-Silva et al., 2013). Furthermore,
the efficiency with which essential amino
acids are used by rainbow trout for protein
deposition is not constant, but affected by
their concentration in the diet (the law of
diminishing returns) and intake of digestible
energy (Encarnação et al., 2004). Efficiency
is also known to decrease significantly with
increase in live body weight (reviewed by
18 | InternatIonal AquAFeed | January-February 2014
Refstie et al., 2000
NRC, 2011). The development of more
sustainable diets requires adjustment of their
digestible AA and energy content according
to the different stages of production..
Improving health
through nutrition
Several studies demonstrate that supplementation of aquaculture feeds with “functional” amino acids such as arginine and
tryptophan constitute a promising approach
to improve, among other parameters, animals’
immune response to environmental stresses.
This does not constitute a surprise, since
amino acid function goes beyond meeting the
requirements for protein synthesis.
Physiological response to stress and
anxiety involve the serotonin signalling system that responds significantly to the availability of serotonin precursor tryptophan.
The connection between tryptophan and
serotonin explains why supplementation
of the essential amino acid has been
found to reduce aggressive behaviour and
stress-induced anorexia in several fish, as
well as in terrestrial animals and humans.
Interestingly, it was recently shown that
supplementing the diets of salmon smolts
with tryptophan above the recommended
level can suppress their cortisol response

6. FEATURE
after being exposed to confinement stress
(Basic et al., 2013).
Fish also have particularly high requirements for dietary arginine because it is abundant in protein and tissue fluid (as phosphoarginine, a major reservoir of ATP), and
its de novo synthesis is limited or even completely absent. The dietary supplementation
of arginine and glutamate in combination had
positive effects on feeding rate and growth
among Atlantic salmon during the first autumn
after sea transfer (Oehme et al., 2010).
Dietary arginine has also been shown to
improve disease resistance and modulate the
innate immune mechanisms of fish (Costas
et al., 2011). Furthermore, increased dietary
arginine seems to activate polyamine turnover
and β-oxidation in the liver of juvenile Atlantic
salmon, and may act to improve the metabolic status of the fish (Anderson et al., 2013).
Although this requires further confirmation,
supplementation of aquaculture feeds with
functional amino acids such as arginine and
tryptophan constitutes a promising approach
to reduce the stress associated with aquaculture practices, and ultimately to improve the
growth performance of fish.
Additional considerations
Evidence is accumulating that balancing the
dietary amino acid profile with supplements
can be regarded as a cost-effective strategy in
reducing fishmeal inclusion level, and a prom- fed under practical conditions. Nutritional
ising one for reducing the crude protein level Requirements for Fish and Shrimp, published
in salmonid diets. Such findings confirm not by the US National Research Council (NRC),
only the effectiveness of supplemental amino determines requirements for amino acids,
acids in covering the nutritional requirements fatty acids, vitamins and minerals based on
of fish, but also contribute to the increas- diets containing purified and chemicallyingly well-understood perception that animals defined ingredients which are highly digestdon’t have requirements for ingredients or ible to the organism. When formulating diets
crude protein levels per se,
but instead for nutrients,
including amino acids.
"Are the currently recommended
Although
Atlantic
dietary essential amino acid
salmon is the most successfully farmed salmonid,
levels (NRC, 2011) effective in
the nutrient requirements
of the species throughout
maximising performance of animals
the entire production cycle
fed under practical conditions?"
still need to be completely
defined. Besides, there is a
high variability in reported
essential amino acid requirements between from practical feedstuffs, it must be taken
studies. This has been attributed to methodol- into account that nutrient bioavailability for
ogy issues, but also to the composition of the the animal will usually be less than from puridiet used, and whether fish were able to reach fied sources.
Another limitation of the currently recomtheir maximum growth potential. In fact, different proteins are not identical in their nutritive mended dietary amino acid values is the fact
value, with amino acid profile and digestibility that a single value is offered to cover the
entire production cycle. We should, therefore,
varying markedly among ingredients.
This raises the question whether the make an extra effort to fulfil information gaps
currently recommended dietary essential about the nutrient requirements of salmonids,
amino acid levels (NRC, 2011) are effec- and to offer recommendations according to
tive in maximising performance of animals specifications of each stage of production.
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January-February 2014 | InternatIonal AquAFeed | 19

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