Live feeds are often essential for larval fish. Live feeds are proven to be essential first-feed for many larval fish, essentially all those that hatch from small eggs with limited yolk reserves and often immature feeding and digestive functions. Live feeds provide larval fish with essential nutrients that are naturally ‘microencapsulated’ in bite-sized packages. They include a high proportion of easily-assimilated free amino acids and free fatty acids, as well as digestive enzymes and beneficial bacterial microfloras in the gut contents of the prey. The swimming activity of live prey also stimulates feeding responses in larval fish, a vital concern because small larvae with very limited metabolic reserves can quickly starve if they do not promptly begin feeding actively.
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3. “Today the most costly and
perhaps least understood live
food are the unicellular algae”
- Dhert & Sorgeloos 1995
L
ive feeds are often essential for
larval fish. Live feeds are proven
to be essential first-feed for many
larval fish, essentially all those that
hatch from small eggs with limited yolk
reserves and often immature feeding and
digestive functions. Live feeds provide larval
fish with essential nutrients that are naturally
‘microencapsulated’ in bite-sized packages.
They include a high proportion of easily-
assimilated free amino acids and free fatty
acids, as well as digestive enzymes and
beneficial bacterial microfloras in the gut
contents of the prey. The swimming activity
of live prey also stimulates feeding responses
in larval fish, a vital concern because small
larvae with very limited metabolic reserves
can quickly starve if they do not promptly
begin feeding actively.
The natural live foods of such larvae are of
course microplankton, both zooplankton and
(although often not appreciated) phytoplank-
ton. Natural zooplankton assemblages are
often highly diverse and may include protozoa,
rotifers, arroworms, microcrustaceans such as
copepods, and eggs and larvae of nearly every
group of marine animals including sponges,
coelenterates, polychaetes, various crusta-
ceans, molluscs, echinoderms, and even fish.
This diverse array of prey organisms supplies
multiple sources of essential nutrients. But it
can be very difficult to obtain sufficient natural
plankton to supply the needs of a hatchery,
and natural plankton can introduce predators,
parasites and pathogens. Hatchery-cultured
live feeds are therefore the only practical and
safe feed for many larval fish.
Use of live feeds in aquaculture
By far the most commonly-used live feeds
in hatcheries are rotifers (Brachionus spp.)
and brine shrimp (Artemia) (Conceição et
al. 2010), with some use of copepods such
as species of Acartia, Calanus, Tisbe, and
Parvocalanus. Although copepods gener-
ally provide better nutritional value, their
culture presents so many difficulties that
they are not commonly used in hatcheries
(Drillet et al. 2006, 2011). Rotifers can read-
ily be mass-cultured at high densities and
can double their numbers in a day. Rotifers
are smaller than newly-hatched Artemia,
which can be too large for some larvae.
Artemia are most convenient because their
resting eggs (cysts) can be purchased and
hatched when needed, but newly-hatched
Artemia nauplii do not begin to feed until
after the first molt, so their nutritional value
depends entirely on the nutritional environ-
ment of the previous wild generation that
produced the eggs. One study found that
the content of the important omega-3
Poly-Unsaturated Fatty Acid (PUFA) EPA
in Artemia cysts from the same source can
vary as much as 44-fold (Dhert & Sorgeloos
1995). Such variations mean that the nutri-
tional content of newly hatched Artemia
may be largely unknown, and only after
the first molt can their nutritional value be
improved by feeding.
It is important to understand that nei-
ther Brachionus rotifers nor Artemia are
truly marine organisms. Rather they are
found in ‘saline’ habitats, which are mostly
inland environments with often extreme
seasonal variations in temperature, salinity,
and even availability of water. Adaptation
to such extreme conditions has endowed
these species with characteristics that are
very useful in aquaculture, such as tolerance
of a wide range of culture conditions, rapid
asexual reproduction by parthenogenesis
(Brachionus), and formation of resistant rest-
ing cysts (Brachionus and Artemia). They are
also relatively omnivorous and do not have
stringent nutritional requirements, and so
can be fed on low-cost feeds such as yeast,
starch, rice bran, and dried Spirulina (cyano-
bacteria).
It may be no surprise that feeding larvae
only one or two species of hatchery-pro-
duced live feeds might not provide adequate
nutrition. But the underlying cause of such
nutritional inadequacy is often the low quality
of the low-cost food sources used to pro-
duce the live feeds. It is therefore necessary
to choose carefully the food sources used
for hatchery-produced live feeds if they are
to provide adequate nutritional support for
larval fish.
They are what they eat
Enhancing the nutritional value of
live feeds with microalgae
by Eric C Henry PhD, research scientist, Reed Mariculture Inc., USA
12 | InternatIonal AquAFeed | May-June 2013
FEATURE
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5. Limitations of formulated feeds
for live feed production
Formulated feeds offer low cost and con-
venience, but they have fundamental short-
comings. Zooplankton, including rotifers and
Artemia, can feed only on micro particles of
appropriate size (from bacteria to 10 µm for
Brachionus [Baer et al. 2008, Vadstein et al.
1993], and from bacteria to 28 µm, with the
optimum about 8-16 µm for Artemia [Makridis
and Vadstein 1999, Fernández 2001]). It is
difficult to produce dry feeds that provide
uniform particle sizes, and even when uniform
dry particles can be produced they can be
subject to clumping when dispersed into
water for feeding. But probably the most criti-
cal shortcoming of dry feeds is rapid leaching
of water-soluble nutrients; the smaller the par-
ticle, the faster nutrients are leached out. Not
only are leached nutrients unavailable to the
live feeds, they can cause fouling of the water.
Rotifer Brachionus
plicatilis
Algae concentrate (Reed
Mariculture Tetraselmis 3600)
May-June 2013 | InternatIonal AquAFeed | 13
FEATURE
6. Lipid emulsions of high-PUFA oils may be
used to improve the fatty acid profile of live
feeds. Although their contents are not subject
to leaching, lipid droplets are prone to stick
to surfaces, including the walls of the culture
tank and the live feed organisms themselves.
Lipid enrichment protocols therefore often
must include a rinsing step to clean the
rotifers or Artemia of adhering lipid droplets,
which would otherwise foul the larval tank.
Short-term feeding of oil emulsions results
in lipid-enriched rotifers with high EPA and
DHA levels, but, they are prone to rapid
loss of their gut contents and acquire an
extreme lipid:protein ratio (Dhert et al. 2001).
Moreover, it has been shown that when the
rotifers are collected on screens, as they are
for rinsing, this mechanical stress can cause
ejection of the nutritious gut contents that
were ingested during enrichment feeding
(Romero-Romero & Yúfera 2012), defeating
the purpose of the enrichment.
Advantages of microalgae
Microalgae are the base of the plankton
food web, and their great biochemical diver-
sity is the source of the high nutritional value
of natural zooplankton. As the natural food
of zooplankton, microalgae offer a number of
advantages over formulated feeds. They are
natural ‘microencapsulation’ particles bounded
by a cell membrane that retains the nutri-
tious contents. They naturally contain a wide
spectrum of nutritional components, such as
essential amino acids, PUFAs, sterols, vitamins,
and phytopigments. Different species provide
a wide range of cell sizes and nutritional
factors, as well as components that enhance
digestion and immune functions (Guedes
& Malcata 2012). Some strains have been
found to have antibacterial effects (Austin &
Day 1990, Kokou et al. 2012, Regunathan &
Wesley 2004).
Selecting the right microalgae
Although hundreds of microalgae strains
have been tested as feeds for aquaculture,
fewer than 20 are in widespread use (Guedes
& Malcata 2012). Because these strains
vary so greatly in their nutritional profiles,
careful consideration is necessary in order
to select the most nutritionally appropri-
ate strains. Such algae as Spirulina, Chlorella,
Haematococcus, and Dunaliella are easily mass-
produced because they can be cultivated
in open ponds at low cost, but they all lack
the omega-3 PUFAs EPA and DHA that are
essential for production of live feeds that
provide adequate nutrition to marine fish.
High-PUFA algae in wide use include strains
of Nannochloropsis (Eustigmatophyceae),
favoured for rotifer production and green-
water; Tetraselmis (Prasinophyceae); Isochrysis
and Pavlova (Prymnesiophyceae); Thalassiosira,
Chaetoceros, and
Skeletonema (diatoms);
and Rhodomonas
(Cryptophyceae).
Although the PUFA
content of many strains
has by now been well-
documented, sterol
profiles have been
more challenging to
characterise because
there is far more strain-
to-strain variation, even
among strains suppos-
edly of the same spe-
cies, as revealed in a
recent investigation of
over 100 diatom strains
(Rampen et al. 2010).
Protein content is less
variable, with a study of
40 strains of microalgae
in seven algal classes
finding consistently high
contents of essential
amino acids (Brown
et al. 1997). Vitamin
contents of microalgae
also appear to be con-
sistently high (Brown
& Miller 1992, Brown
et al. 1999, De Roeck-
Holtzhauer et al. 1991).
Although various nutritional components
have been well-documented in many strains,
it remains difficult to assemble complete
nutritional profiles of many strains so that the
optimal combination of strains can be selected
for a particular application. It is unfortunate
that so many studies of the nutritional per-
formance of microalgae have tested single
strains as the only feed, when it should be
obvious that no single strain is likely to provide
an optimal nutritional profile comparable to
that provided by a natural phytoplankton
assemblage.
In practice, microalgae have repeatedly
been shown to dramatically improve the PUFA
content of rotifers and Artemia (Chakraborty
et al. 2007, Ferreira et al. 2008, Kjell et al.
1993, Lie et al. 1997, Øie et al. 1994, Reitan et
al. 1997), which frequently results in improved
larval performance. But it is important to
recognise that the high nutritional quality of
enriched live feeds can be maintained after
delivery to the larval tank only by application
of ‘greenwater’ techniques. Unless microalgae
are added to the larval tank water, the live
feed organisms quickly begin to starve, and
can metabolize a significant fraction of their
biomass before they are eaten by the larvae.
The algal cells themselves can also function
as live feeds, since they have been shown to
be eaten and digested by larvae (Reitan et al.
1997, Van Der Meeren et al. 2007), and may
Nauplius stage of copepod
Parvocalanus crassirostris
14 | InternatIonal AquAFeed | May-June 2013
FEATURE
7.
8. also stimulate digestive enzyme production
(Cahu et al. 1998).
Production of microalgae
Despite the many advantages of microalgae,
their wider use is hampered by difficulties in
culturing, storage, and high costs. Microalgae
culture can consume a significant fraction of the
resources of a hatchery, and requires special
equipment, skilled labour, and a large alloca-
tion of space that is unproductive during the
seasons when live feeds are not needed.
Low-cost open-pond culture methods
carry high risks of contamination and culture
failure due to the impossiblity of tightly con-
trolling culture conditions, and the most highly
prized high-PUFA strains such as Isochrysis and
Pavlova require indoor culture.
It is very difficult to synchronize microalgal
production with live feed requirements to
prevent feed shortages or wasteful overpro-
duction, and it is difficult to accurately dose
algae cultures directly into live feed cultures.
If the algae are harvested and concentrated,
the tightly-packed cells can deteriorate rapidly
in refrigerated storage. Some microalgae have
been freeze- or spray-dried, but dried cells
are subject to protein denaturation, and when
they are rehydrated the leaching of water-
soluble substances can rapidly deplete their
nutritional value, as with other dry feeds.
Microalgae concentrates
The best solution to these problems
can be the use of commercially-available
refrigerated or frozen algae concentrates
or ‘pastes’ (Guedes & Malcata 2012,
Shields & Lupatsch 2012). These products,
which are actually viscous liquids, have
proven to be effective feeds for rotifers,
Artemia, shellfish and other filter-feeders,
as well as for greenwater applications.
In products formulated to provide a
long shelf-life, the concentrated microalgae
are suspended in buffer media that pre-
serve cellular integrity and nutritional value,
although the cells are non-viable. When
concentrates with well-defined biomass
densities are employed, the algae can be
accurately dosed into live feed cultures
with a metering pump, and non-viability
confers the advantage that the products
pose no risk of introducing exotic algal
strains. The best refrigerated products typi-
cally have a shelf-life of 3-6 months, and
frozen products several years. This means
that a reliable supply of algae can be kept
on hand, available for use in any season or
if an unexpected need arises. Algae costs
become predictable, and often prove to
be less than on-site production when total
production costs and inefficiencies are
accounted for.
Although costs of liquid algae concen-
trates are higher than for dried algae or
formulated feeds, they offer all the nutritional
advantages of live cultures. The nutritional
quality of live feeds can be no better than
the food sources used to produce them.
Success of early larvae is so critical to the
success of a hatchery that even a relatively
small improvement in survival or growth rate
can yield great benefits.
Outlook
Live feeds remain indispensable for
larviculture of many fish. Although micro-
algae are among the costliest food sources
used to produce live feeds, their many
advantages justify the cost for hatcheries
producing high-value fish. Research contin-
ues to better characterise the nutritional
properties of various algae strains and to
optimise algae production technologies.
We can anticipate that introduction of
novel algae strains and nutritionally-opti-
mised combinations of strains, along with
improved feeding protocols, will ensure
that microalgae remain the food of choice
for production of the highest-quality live
feeds.
References
www.aquafeed.co.uk/referencesIAF1303
May-June 2013 | InternatIonal AquAFeed | 15
FEATURE
Naturally ahead
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Myco
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Mycotoxins decrease performance and interfere
with the health status of your animals.
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is the solution for mycotoxin risk management.
mycofix.biomin.net
9. www.aquafeed.co.uk
LINKS
• See the full issue
• Visit the International Aquafeed website
• Contact the International Aquafeed Team
• Subscribe to International Aquafeed
They are what they eat
Enhancing the nutritional value of live feeds
with microalgae
Controlling mycotoxins with
binders
Ultraviolet
water disinfection for fish
farms and hatcheries
Niacin
– one of the key B vitamins for sustaining
healthy fish growth and production
Volume 16 Issue 3 2013 - mAY | Ju Ne
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