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November | December 2010 Feature title: They are what they eat Enhancing the nutritional value of live feeds with microalgae

Deadline: 30/04/2013

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FEATURE

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

“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 easilyassimilated 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) phytoplankton. 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, mollusks, 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 generally 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 readily 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 environment 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 10 | International AquaFeed | March-April 2013

vary as much as 44-fold (Dhert & Sorgeloos 1995). Such variations mean that the nutritional 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 neither 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 resting 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 (cyanobacteria).

It may be no surprise that feeding larvae only one or two species of hatchery-produced 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 produce 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.

FEATURE

Limitations of formulated feeds for live feed production Formulated feeds offer low cost and convenience, but they have fundamental shortcomings. 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

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subject to clumping when dispersed into water for feeding. But probably the most critical shortcoming of dry feeds is rapid leaching of water-soluble nutrients; the smaller the particle, the faster nutrients are leached out. Not only are leached nutrients unavailable to the live feeds, they can cause fouling of the water.

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Contact the Marketing team on - Tel: +44 1242 267706

Contact the Marketing team on - Tel: +44 1242 267706

Lee Bastin - leeb@aquafeed.co.uk

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FEATURE

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 diversity 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 appropriate 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 greenwater; Tetraselmis (Prasinophyceae); Isochrysis and Pavlova 12 | International AquaFeed | March-April 2013

(Prymnesiophyceae); Thalassiosira, Chaetoceros, and Skeletonema (diatoms); and Rhodomonas (Cryptophyceae).

Although the PUFA content of many strains has by now been welldocumented, sterol profiles have been more challenging to characterise because there is far more strainto-strain variation, even among strains supposedly of the same species, 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 consistently 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 performance 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

FEATURE

be eaten and digested by larvae (Reitan et al.

1997, Van Der Meeren et al. 2007), and may also stimulate digestive enzyme production (Cahu et al. 1998).

they are rehydrated the leaching of watersoluble substances can rapidly deplete their nutritional value, as with other dry feeds.

Production of microalgae

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 filterfeeders, as well as for greenwater applications.

In products formulated to provide a long shelf-life, the concentrated microalgae are suspended in buffer media that preserve 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 typically 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

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 allocation 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 controlling 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 overproduction, 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

Microalgae concentrates

total production costs and inefficiencies are accounted for.

Although costs of liquid algae concentrates 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 microalgae are among the costliest food sources used to produce live feeds, their many advantages justify the cost for hatcheries producing highvalue fish. Research continues 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-optimised combinations of strains, along with improved feeding protocols, will ensure that microalgae remain the food of choice for production of the highestquality live feeds.

References Available on request

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