1

Manipulating Atlantic salmon, Salmo salar, fillet fatty acids:

Can we go from fish to plant and back again?

Malcolm JoblingNFH, University of Tromsø, 9037 Tromsø, Norway

e-mail: malcolmj@nfh.uit.no

The fatty acid compositions of fish often bear a close resemblance of those of their food,

and the fatty acids present in salmon fillets are influenced by the types of oils used in feed

formulation.

General background

Plant (vegetable) and marine fish oils differ in the types of fatty acids they contain (Table 1),

so the fatty acid compositions of salmon fed plant and fish oils also differ (Figure 1).

Table 1. Summary of fatty acids characteristic of some plant (vegetable) and fish oils used in

fish feeds. Fatty acids of the (n-6) and (n-3) series are the essential fatty acids, i.e. animals

must obtain them via their diet. EPA [20:5 (n-3)] and DHA [22:6 (n-3)] are the major (n-3)

highly-unsaturated fatty acids (HUFAs).

Characteristic fatty acids

Plant oils Fish oils

16:0 Palm, Cottonseed 20:1 isomers Herring, Capelin

18:1 (n-9)Rape, Olive, Sunflower,Palm 22:1 isomers Herring, Capelin

18:2 (n-6) Soya, Maize, Cottonseed,Sunflower 20:5 (n-3) Anchovy, Sardine,

Menhaden18:3 (n-3) Linseed 22:6 (n-3) Marine fish oils

2

Plant (vegetable) oils are characterised by 18-carbon (18C) fatty acids of the (n-9), (n-6) and

(n-3) series, whereas marine fish oils contain higher percentages of (n-3) HUFAs, particularly

EPA and DHA. It is the (n-3) HUFAs that have been found to be beneficial for several aspects

of human health. They are required for normal neural and sensory development in babies and

small children, and may also reduce the risk of heart disease and some forms of cancer in

adults. As a result of this there is currently a focus on the influences that plant (vegetable) oils

and fish oils have on the compositions and sensory attributes of fish fillets.

Figure 1. Plant (vegetable) and marine fish oils impart marked differences to the fatty acid

profiles of Atlantic salmon, Salmo salar, fillets. Fish given feeds containing plant oils tend to

have high fillet concentrations of (n-6) fatty acids, whereas salmon fed marine fish oils

deposit large amounts of (n-3) HUFAs (particularly EPA and DHA) in the fillet fat.

0

10

20

30

40

50

Fille

t fat

ty a

cids

(%)

(n-6) Fatty acids (n-3) Fatty acids

Plant oil Fish oil

3

Given this background it would an advantage to be able to predict the time-course of the

change in the fillet fatty acid profile following the introduction of feeds with different types of

oils. There are two inter-related research challenges linked to this problem

� to find a model that describes how fillet fatty acid profiles change following

introduction of a new type of feed

� to be able to provide estimates of the time required for fillet fatty acid profiles to

stabilise following the feed change

The means of change

There are two main ways in which tissue fatty acid profiles may be changed following a

change in feed oil composition; these may be termed wash-out and dilution. The dictionary

definition of wash-out is to remove, eradicate or carry away. Thus, with wash-out a change in

the fatty acid profile of a fillet would arise primarily as a result of metabolism and turnover

(‘burning off’) of fatty acids mobilised from the existing stores. Dilution is defined as a

reduction in strength that results from the addition of more material. As such, with dilution a

change in fatty acid profile would arise because the existing fatty acid stores become diluted

as the fish grow, and accumulate and deposit fatty acids obtained from the diet.

Incorporation of fatty acids into fish tissues is influenced by a number of factors. These

include:

� preferential incorporation of specific fatty acids into the tissue lipids

� preferential metabolism ('burning off') of some fatty acids

� modification of fatty acid structure (i.e. by chain elongation or desaturation).

4

If the factors relating to transformation and metabolism dominate following a change in the

source of dietary fatty acids the prediction of tissue compositional changes will be extremely

difficult. On the other hand, if these factors play a minor role in governing changes in tissue

fatty acid composition following a switch from a feed containing plant (vegetable) oil to one

containing fish oil, and dilution is most important, the change in fatty acid composition

becomes relatively easy to model.

The dilution model

In a dilution model it is assumed that:

� the initial fatty acid content becomes diluted as the fish grow and deposit

increasing amounts of fat

� dietary fatty acids are deposited in the tissues without influencing the metabolism

or turnover of existing fatty acid stores

� incorporation of dietary fatty acids into the fat occurs in the same way independent

of the fatty acid compositions of the existing stores.

A dilution model describes the change in the percentage of a given fatty acid in a tissue when

increasing quantities of fatty acids are added. When there is a change to a new type of feed the

fatty acid profiles of the ‘test’ fish change over time as more fatty acids are deposited in the

tissues. The fatty acid profiles of these ‘test’ fish gradually come to resemble those of fish that

have been fed this feed for prolonged periods (the latter are a ‘reference’ group that provide

the basis for comparison).

The mathematical model

The mathematical description of dilution is:

PT = PR + [(PI - PR)/(QT /QI)]

5

PT is the percentage of a fatty acid in the tissues of the ‘test’ fish at time T, PI is the initial

percentage and PR is the percentage of the fatty acid in the ‘reference’ group. QI is the initial

total amount of fatty acids (or total fat) present, and QT is the amount present at time T. The

equation can be transformed to:

(PT - PR)/(PI - PR) = 1/(QT /QI)

This means that a plot of (PT - PR)/(PI - PR) against QT /QI gives a curve that tends towards an

asymptotic value at infinite QT /QI, giving a 'law of diminishing returns' relationship.

Does this type of curve describe the changes we observe when we change the types of oils in

fish feeds?

The model was first tested using data from a study of feed oils (plant vs. fish) and dietary fat

concentrations (high, ca. 34% vs. low, ca. 22%) on growth and fatty acid profiles of several

tissues (fillet, viscera and carcass) of Atlantic salmon parr and post-smolt. The changes in the

percentages of three fatty acids, typical of plant oils, were examined following the

introduction of a feed containing fish oil. The three fatty acids studied were 18:1, 18:2 (n-6)

and 18:3 (n-3).

The curves fitted to the observed changes were almost identical to the ideal dilution model, so

the results of this first test looked very promising. Figure 2 shows the plot for the pooled data

of the three fatty acids [18:1, 18:2 (n-6) and 18:3 (n-3)] as an example.

6

Figure 2. The ratios of % change [(PT - PR)/(PI - PR)] of the three fatty acids 18:1, 18:2 (n-6)

and 18:3 (n-3) in relation to changes in total fat (QT /QI) in the fillet, viscera and carcass of

Atlantic salmon following a change from feeds containing plant oil to ones containing fish oil.

The circles indicate the observed values and the solid line is the fitted power curve regression.

The calculated regression indicated in the figure is very close to the ideal dilution curve

equation.

The test was then repeated using the changes in the percentages of the three fatty acids in the

fillet (Figure 3). Once again the curve fitted to the observations was very similar to an ideal

dilution curve.

(PT - PR)/(PI - PR) = 0.995(Q T /Q I ) -0.986

(R2 = 0.899; N = 180)

0,0

0,2

0,4

0,6

0,8

1,0

1 3 5 7 9

Q T/Q I

(PT -

PR

)/(P

I - P

R)

7

Figure 3. The ratio of % change [(PT - PR)/(PI - PR)] of three fillet fatty acids [18:1, 18:2 (n-6)

and 18:3 (n-3)] in relation to changes in total fat (QT /QI) in Atlantic salmon fillets following a

change from feeds containing plant oils to ones containing fish oil. The small symbols are the

observed values, the solid line is the fitted power curve regression, and the large grey-tone

circles indicate values calculated using the ideal dilution curve equation.

As a final check the concordance between the predicted percentages of the fatty acids (from

the dilution model) and those found in the fillets 98 days after the feed switch (observed

values) was examined (Figure 4). Values were very similar, giving a regression that was

extremely close to a 'line of equality' (a regression that has a slope of 1 and passes through the

origin).

(P T - P R )/(P I - P R ) = 0.9027(Q T /Q R )-0.9377

(R 2 = 0.923; N = 60)

0,0

0,2

0,4

0,6

0,8

1,0

1 3 5 7 9

Q T /Q I

(PT

- P

R)/(

PI -

PR

)

18:118:2 (n-6)18:3 (n-3)

8

Figure 4. Plot showing the degree of concordance between the fillet percentages of 18:1, 18:2

(n-6) and 18:3 (n-3) predicted by the dilution model and the percentages of the same fatty

acids observed in the fillets 98 days after a change from feeds containing plant oil to ones

containing fish oil. The observed values are the mean�SD for six fish per sample.

Conclusions from the modelling

The results provide evidence that

� a simple dilution model can be used to describe the change in fillet fatty acid

profile of Atlantic salmon following a change in feed oil type

� fatty acid catabolism and turnover ('burning-off') play only a minor role in

governing the compositional changes

� preferential incorporation and structural modification, i.e. elongation and

desaturation, of fatty acids are of limited importance in modulating the fillet fatty

acid profiles of salmon that are feeding and growing well.

Predicted = 0.9654 Observed + 0.2036(R 2 = 0.996; N = 12)

0

5

10

15

20

25

0 5 10 15 20 25

% Observed after 98 days of feeding

% P

redi

cted

from

dilu

tion

mod

el

18:118:2 (n-6)18:3 (n-3)

9

Applications and implications

The dilution model can be used to make predictions about the changes in fatty acid

compositions of salmon fillets that occur following a change from feeds containing one type

of oil to another (e.g. from plant oil to marine fish oil). However, for the predictions to be

accurate quite a lot of information is needed:

� The % of the fatty acids present in the fillet at the time of the feed change, i.e. PI

� The % of the same fatty acids present in the fillet fat of the 'reference' fish, i.e. PR

� An estimate of QT /QI, which requires information about:

� the growth, or size increase, of the fish from the time of the feed change until

the time of harvest

� possible changes in the mass of the fillet relative to body mass, i.e. fillet yield

� possible changes in the fat % in the fillet over the feeding period.

The size increase of the fish from the time of the feed change to harvest will be pre-

determined (e.g. growth from 1 kg � 5 kg), so will not create problems for the estimation.

Both fillet yield and fillet fat % change with increasing fish size, so these factors must be

taken into account when making predictions. There is probably sufficient information about

these changes in farmed Atlantic salmon to enable 'reference' (literature) values to be used for

making predictions.

Fillet yield increases with growth of small salmon, but in farmed salmon above ca. 2 kg in

body weight the yield remains fairly stable at about 55% (54-58%).

10

Atlantic salmon deposit increasing amounts of body fat as they grow, and the fat % in the

fillet and cutlet increase as the fish increase in body size (Figure 5). The rate of increase tends

to be higher in small salmon than in larger fish, so fat % approaches a plateau when salmon

reach large size.

Figure 5. Fat % in the cutlets (NQC = Norwegian Quality Cut) of farmed Atlantic salmon of

different sizes (body weights, kg).

The reliance upon 'reference' values will be adequate for most practical purposes, when a

'rough-and-ready' estimate of the change in fillet fatty acids will suffice. The use of 'reference'

values would also reduce the numbers of analyses needed each time a prognosis (prediction)

is made.

0

5

10

15

20

25

0 3 6 9 12

Fish weight (kg)

Cut

let (

NQ

C) f

at (%

)

11

Is the dilution model also suitable to describe fillet fatty acid changes in other fish species?

Atlantic salmon is a 'medium-fat' fish. The fillets of wild salmon often contain 4-8% lipid, but

the fillets of large, farmed fish may have about 20% fat. The salmon fillet contains a lot of

storage fat, and the fatty acid composition of the fat generally follows that of the feed quite

closely.

On the other hand, some species of fish, termed 'lean' fish (<2% fillet lipid), have very little

storage fat in the fillet. One such species is the Atlantic cod, Gadus morhua. In the 'lean'

species the fillet lipids are dominated by the structural, cell-membrane phospholipids. The

phospholipids contain high proportions of 16:0, 18:1 (n-9), 20:5 (n-3) and 22:6 (n-3), and

their fatty acid profiles are less responsive to change following a change of diet than are those

of the storage fats. One consequence of this is that the dilution model may not provide an

adequate description of the changes in the fillet fatty acid profiles of 'lean' fish following a

switch in dietary oil source.

Can we go from fish to plant and back again?

This was the question posed in the title. When looking at the fatty acids in the salmon fillet it

seemed that the fatty acids derived from plant oils became diluted over time, rather than being

completely eliminated or eradicated. In other words, traces of the fatty acids characteristic of

the plant oils remained even after a prolonged period of feeding with fish oils, so a complete

restoration of fatty acid composition would not seem to be possible. This also means that the

'plant type' fatty acids could probably be used as 'markers' (or 'tracers') to map the dietary

history of the fish.

12

Even though traces of the 'plant type' fatty acids will remain detectable in the fillet for a

prolonged period following a dietary change there is little cause for concern. Over time the

dilution would become so pronounced that the 'plant type' fatty acids would have little

practical significance in terms of either conceptions about the salmon being a 'seafood

product', or with respect to its nutritional value to consumers as a supplier of the 'health-

promoting' (n-3) HUFAs.

Further reading:

Bell, J.G. (1998) Current aspects of lipid nutrition in fish farming. In: Biology of Farmed Fish

(ed. by K.D. Black & A.D. Pickering), pp. 114-145. Sheffield Academic Press, Sheffield.

Higgs, D.A. & Dong, F.M. (2000) Lipids and fatty acids. In: Encyclopedia of Aquaculture

(ed. by R.R. Stickney), pp. 476-496. John Wiley & Sons, New York.

Jobling, M. (2001) Nutrient partitioning and the influence of feed composition on body

composition. In: Food Intake in Fish (ed. by D. Houlihan, T. Boujard & M. Jobling), pp. 354-

375. Blackwell Scientific, Oxford.

Jobling, M. (2003) Do changes in Atlantic salmon, Salmo salar L., fillet fatty acids following

a dietary switch represent wash-out or dilution? Test of a dilution model and its application.

Aquaculture Research 34, 1215-1221.

Jobling, M. (in press) Are modifications in tissue fatty acid profiles following a change in diet

the result of dilution? Test of a simple dilution model. Aquaculture (available on line via

Science Direct)

13

Jobling, M., Larsen, A.V., Andreassen, B., Olsen, R.L. & Sigholt, T. (2002) Influence of a

dietary shift on temporal changes in fat deposition and fatty acid composition of Atlantic

salmon post-smolt during the early phase of seawater rearing. Aquaculture Research 33, 875-

889.

Sargent, J.R., Tocher, D.R. & Bell, J.G. (2002) The Lipids. In: Fish Nutrition, Third edition

(ed. by J.E. Halver & R.W. Hardy), pp.181-257. Academic Press, San Diego.