Farmed
salmon is rich in fat, but fillets of farmed salmon contain less marine omega-3
than previously, because a large fraction of the fish oil in the feed has been
replaced by plant oil. Scientists have discovered a way of stimulating farmed
salmon to convert plant oil in the feed to marine omega-3. This
means that farmed salmon may become a net producer of marine omega-3.
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| (Photo credit: Kris Krug) |
Natural
ability
Salmon
have a natural ability to convert omega-3 fatty acids from plants into the
marine omega-3 fatty acids EPA and DHA. This ability is greatest in the fish
before smoltification. This is the process by which the salmon become
physiologically a
ble to move from freshwater to seawater, which occurs in
nature when the fish follow river water out into the sea. It is this ability
that scientists at the food research institute Nofima have used.
The
scientists wanted to find out whether it is possible to retain the ability of
farmed salmon to convert omega-3 fatty acids by delaying smoltification and
keeping the fish in freshwater longer. It is normal in today’s aquaculture
industry that salmon smoltify and are transferred to seawater before they reach
a weight of 100 gram.
Two
groups of fish
The
timing of salmon smoltification is controlled by light, and this fact was used
to create one group of salmon destined to smoltify early (at weight 85 gram)
and another group destined to smoltify late (at weight 400 gram). After the
salmon that weighed 85 gram had been transferred to seawater, all of the fish
were given feed in which only 10% of the fat came from fish oil, with the
remaining 90% from rapeseed oil.
An investigation
carried out by Nofima of the three largest feed suppliers has shown that the
corresponding figures for commercial feed last year were approximately 35% from
fish oil and 65% from rapeseed oil.
Larger
smolts – richer in omega-3
The
results from the experiment showed that salmon that had remained in freshwater
until they weighed 400 gram had a higher fraction of marine omega-3 fatty acids
than salmon that had remained in freshwater until they weighed 85 gram and then
transferred to seawater, where they remained until they weighed 400 gram. The
fractions of the total fatty acids for marine omega-3 in the two groups were
9.2% and 7.5%, respectively. The experiment has thus shown that it is possible
to retain the ability of the salmon to convert omega-3 fatty acids from plants
to marine omega-3 fatty acids by delaying smoltification and keeping the fish
longer in freshwater. Fish then produce a greater amount of marine omega-3 in
total.
“This
is an important result, since the levels of healthy marine omega-3 fatty acids
in farmed salmon are central to the health of the salmon themselves, and for
the health-promoting qualities of the salmon products that we eat,” says
scientist Tone-Kari Knutsdatter Østbye at Nofima.
A
positive aquaculture industry
The
project has been commissioned by the Norwegian Seafood Research Fund (FHF).
FHF’s R&D director in aquaculture, Kjell Maroni, believes that the
production of larger smolts can be a way forward for more facilities in the
aquaculture industry.
“Many
companies in the aquaculture industry use a strategy in which parts of the
smolt production are kept in freshwater until they are larger than the size at
which smoltification has normally taken place in recent years. This makes the
growth period before slaughter shorter, and contributes to a lower risk of
diseases such as salmon louse. It can only be positive if this strategy
contributes also to an increased use and increased natural production of
omega-3 in the salmon.”
The
scientists wanted to investigate also whether different salmon families differ
in their ability to convert omega-3 from plant oils to marine omega-3 fatty
acids, and how the salmon families react to the delay in smoltification.
A
gene for omega-3 conversion
Salmon
were therefore selected on the basis of a single gene – the gene that controls
the production of an enzyme known as delta-6-desaturase. This enzyme is known
to be a key element in the conversion of plant omega-3 to marine omega-3 fatty
acids. Nofima has carried out its own preliminary studies in selecting for this
particular gene, and the research institute has used information stored in gene
banks, which are freely available databases that store gene sequences.
These
experiments were started after a single generation of salmon families from
breeding company SalmoBreed had been subject to selection. Salmon with a high
or a low expression of delta-6-desaturase were selected as parents, and their
offspring were tested in the experiments.
Offspring
from salmon selected for a high expression of the gene had a higher capacity
for producing omega-3 fatty acids than offspring from salmon with a low
expression of the gene.
“Our
studies of gene expression and fatty acid composition have confirmed that
several factors affect the ability of salmon to convert omega-3 fatty acids.
These factors include light cycling, transfer to seawater, the age of the
salmon, and their genetic background,” says Østbye.
The
project has been carried out as a collaboration between Nofima, SalmoBreed and
Biomar, and Gerd Marit Berge from Nofima has been project manager. The
experiments were carried out at Nofima’s research station at Sunndalsøra.
“This
has been a productive multidisciplinary collaboration in which we have combined
expertise in nutrition, breeding, and biotechnology in Nofima, and where we
have collaborated closely with breeding companies and feed manufacturers. This
has led to exciting results,” concludes Østbye.
Author: Reidun Lilleholt Kraugerud
The Aquaculturists
This blog is maintained by The Aquaculturists staff and is supported by the magazine International Aquafeed which is published by Perendale Publishers Ltd.
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