May | June 2013

They are what they eat - Enhancing the nutritional value of live feeds with

microalgae

The International magazine for the aquaculture feed industry

International Aquafeed is published six times a year by Perendale Publishers Ltd of the United Kingdom.All data is published in good faith, based on information received, and while every care is taken to prevent inaccuracies, the publishers accept no liability for any errors or omissions or for the consequences of action taken on the basis of information published. ©Copyright 2013 Perendale Publishers Ltd. All rights reserved. No part of this publication may be reproduced in any form or by any means without prior permission of the copyright owner. Printed by Perendale Publishers Ltd. ISSN: 1464-0058

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the services we provide.

“Today the most costly and

perhaps least understood live

food are the unicellular algae”

- Dhert & Sorgeloos 1995

Live feeds are often essential forlarval fish. Live feeds are provento be essential first-feed for manylarval fish, essentially all those that

hatch from small eggs with limited yolkreserves and often immature feeding anddigestivefunctions.Livefeedsprovide larvalfishwithessentialnutrientsthatarenaturally‘microencapsulated’ in bite-sized packages.They include a high proportion of easily-assimilated free amino acids and free fattyacids, as well as digestive enzymes andbeneficial bacterial microfloras in the gutcontentsof theprey.Theswimmingactivityoflivepreyalsostimulatesfeedingresponsesin larval fish, a vital concern because smalllarvaewithvery limitedmetabolic reservescan quickly starve if they do not promptlybeginfeedingactively.

Thenaturallivefoodsofsuchlarvaeareofcoursemicroplankton,bothzooplanktonand(althoughoftennotappreciated)phytoplank-ton. Natural zooplankton assemblages areoftenhighlydiverseandmayincludeprotozoa,rotifers,arroworms,microcrustaceanssuchascopepods,andeggsandlarvaeofnearlyeverygroup of marine animals including sponges,coelenterates, polychaetes, various crusta-

ceans, molluscs, echinoderms, and even fish.Thisdiversearrayofpreyorganismssuppliesmultiple sourcesof essential nutrients. But itcanbeverydifficulttoobtainsufficientnaturalplankton to supply the needs of a hatchery,andnaturalplanktoncanintroducepredators,parasites and pathogens. Hatchery-culturedlivefeedsarethereforetheonlypracticalandsafefeedformanylarvalfish.

Use of live feeds in aquacultureByfarthemostcommonly-usedlivefeeds

in hatcheries are rotifers (Brachionus spp.)and brine shrimp (Artemia) (Conceição etal.2010),withsomeuseofcopepodssuchas species of Acartia, Calanus, Tisbe, andParvocalanus. Although copepods gener-ally provide better nutritional value, theirculture presents so many difficulties thatthey are not commonly used in hatcheries(Drilletetal.2006,2011).Rotiferscanread-ily be mass-cultured at high densities andcandoubletheirnumbersinaday.Rotifersare smaller than newly-hatched Artemia,which can be too large for some larvae.Artemia aremostconvenientbecause theirresting eggs (cysts) can be purchased andhatched when needed, but newly-hatchedArtemia nauplii do not begin to feed untilafterthefirstmolt,sotheirnutritionalvaluedependsentirelyonthenutritionalenviron-ment of the previous wild generation thatproduced the eggs. One study found thatthe content of the important omega-3Poly-Unsaturated Fatty Acid (PUFA) EPAinArtemia cysts from the same sourcecan

varyasmuchas44-fold(Dhert&Sorgeloos1995).Suchvariationsmeanthatthenutri-tional content of newly hatched Artemia may be largely unknown, and only afterthe firstmolt can theirnutritional valuebeimprovedbyfeeding.

It is important to understand that nei-ther Brachionus rotifers nor Artemia aretruly marine organisms. Rather they arefound in ‘saline’ habitats, which are mostlyinland environments with often extremeseasonal variations in temperature, salinity,and even availability of water. Adaptationto such extreme conditions has endowedthese species with characteristics that areveryuseful inaquaculture,suchastoleranceof awide rangeof cultureconditions, rapidasexual reproduction by parthenogenesis(Brachionus),andformationofresistantrest-ingcysts (BrachionusandArtemia).Theyarealso relativelyomnivorous anddonothavestringent nutritional requirements, and socanbe fedon low-cost feedssuchasyeast,starch, ricebran,anddriedSpirulina (cyano-bacteria).

It may be no surprise that feeding larvaeonly one or two species of hatchery-pro-ducedlivefeedsmightnotprovideadequatenutrition. But the underlying cause of suchnutritionalinadequacyisoftenthelowqualityof the low-cost food sources used to pro-duce the live feeds. It is therefore necessaryto choose carefully the food sources usedfor hatchery-produced live feeds if they areto provide adequate nutritional support forlarvalfish.

They are what they eat Enhancing the nutritional value of live feeds with microalgaeby Eric C Henry PhD, research scientist, Reed Mariculture Inc., USA

12 | InternatIonal AquAFeed | May-June 2013

FEATURE

May-June 2013 | InternatIonal AquAFeed | 13

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Limitations of formulated feeds for live feed production

Formulatedfeedsoffer lowcostandcon-venience, but they have fundamental short-comings. Zooplankton, including rotifers andArtemia, can feedonlyonmicroparticlesofappropriatesize(frombacteriato10µmfor

Brachionus [Baer et al. 2008, Vadstein et al.1993],andfrombacteriato28µm,withtheoptimumabout8-16µmforArtemia [Makridisand Vadstein 1999, Fernández 2001]). It isdifficult to produce dry feeds that provideuniformparticlesizes,andevenwhenuniformdry particles can be produced they can be

subject to clumping when dispersed intowaterforfeeding.Butprobablythemostcriti-calshortcomingofdryfeedsisrapidleachingofwater-solublenutrients;thesmallerthepar-ticle,thefasternutrientsareleachedout.Notonly are leachednutrientsunavailable to thelivefeeds,theycancausefoulingofthewater.

Rotifer Brachionus plicatilis

Algae concentrate (Reed Mariculture Tetraselmis 3600)

12 | InternatIonal AquAFeed | May-June 2013 May-June 2013 | InternatIonal AquAFeed | 13

FEATURE

Lipidemulsionsofhigh-PUFAoilsmaybeused to improve the fattyacidprofileof livefeeds.Althoughtheircontentsarenotsubjectto leaching, lipid droplets are prone to sticktosurfaces, includingthewallsof theculturetankand the live feedorganisms themselves.Lipid enrichment protocols therefore oftenmust include a rinsing step to clean therotifersorArtemia ofadhering lipiddroplets,which would otherwise foul the larval tank.Short-term feeding of oil emulsions resultsin lipid-enriched rotifers with high EPA andDHA levels, but, they are prone to rapidloss of their gut contents and acquire anextremelipid:proteinratio(Dhertetal.2001).Moreover, ithasbeenshownthatwhentherotifersarecollectedonscreens,as theyarefor rinsing, this mechanical stress can causeejection of the nutritious gut contents thatwere ingested during enrichment feeding(Romero-Romero&Yúfera 2012), defeatingthepurposeoftheenrichment.

Advantages of microalgaeMicroalgae are the base of the plankton

foodweb,andtheirgreatbiochemicaldiver-sityisthesourceofthehighnutritionalvalueof natural zooplankton. As the natural foodofzooplankton,microalgaeofferanumberofadvantages over formulated feeds. They arenatural‘microencapsulation’particlesboundedby a cell membrane that retains the nutri-

tiouscontents.Theynaturallycontainawidespectrumof nutritional components, such asessentialaminoacids,PUFAs,sterols,vitamins,andphytopigments.Differentspeciesprovidea wide range of cell sizes and nutritionalfactors, aswell as components that enhancedigestion and immune functions (Guedes& Malcata 2012). Some strains have beenfound to have antibacterial effects (Austin&Day1990,Kokouet al. 2012,Regunathan&Wesley2004).

Selecting the right microalgaeAlthough hundreds of microalgae strains

have been tested as feeds for aquaculture,fewerthan20areinwidespreaduse(Guedes& Malcata 2012). Because these strainsvary so greatly in their nutritional profiles,careful consideration is necessary in orderto select the most nutritionally appropri-ate strains. Such algae as Spirulina, Chlorella,Haematococcus,andDunaliellaareeasilymass-produced because they can be cultivatedin openponds at low cost, but they all lacktheomega-3PUFAsEPAandDHAthatareessential for production of live feeds thatprovide adequate nutrition to marine fish.High-PUFA algae in wide use include strainsof Nannochloropsis (Eustigmatophyceae),favoured for rotifer production and green-water;Tetraselmis (Prasinophyceae); IsochrysisandPavlova(Prymnesiophyceae);Thalassiosira,

Chaetoceros , andSkeletonema (diatoms);and Rhodomonas(Cryptophyceae).

Although the PUFAcontentofmanystrainshasbynowbeenwell-documented, sterolprofiles have beenmore challenging tocharacterise becausethereisfarmorestrain-to-strainvariation,evenamong strains suppos-edly of the same spe-cies, as revealed in arecent investigation ofover100diatomstrains(Rampen et al. 2010).Protein content is lessvariable,withastudyof40strainsofmicroalgaein seven algal classesfindingconsistentlyhighcontents of essentialamino acids (Brownet al. 1997). Vitamincontents of microalgaealsoappear tobecon-sistently high (Brown& Miller 1992, Brownet al. 1999,DeRoeck-Holtzhaueretal.1991).

Although various nutritional componentshavebeenwell-documented inmany strains,it remains difficult to assemble completenutritionalprofilesofmanystrainssothattheoptimalcombinationofstrainscanbeselectedfor a particular application. It is unfortunatethat so many studies of the nutritional per-formance of microalgae have tested singlestrains as the only feed, when it should beobviousthatnosinglestrainislikelytoprovidean optimal nutritional profile comparable tothat provided by a natural phytoplanktonassemblage.

In practice, microalgae have repeatedlybeenshowntodramaticallyimprovethePUFAcontentofrotifersandArtemia (Chakrabortyet al. 2007, Ferreira et al. 2008, Kjell et al.1993,Lieetal.1997,Øieetal.1994,Reitanetal.1997),whichfrequentlyresultsinimprovedlarval performance. But it is important torecognise that the high nutritional quality ofenriched live feeds can be maintained afterdeliverytothelarvaltankonlybyapplicationof‘greenwater’techniques.Unlessmicroalgaeare added to the larval tank water, the livefeed organisms quickly begin to starve, andcan metabolize a significant fraction of theirbiomassbeforetheyareeatenbythelarvae.The algal cells themselves can also functionas livefeeds,sincetheyhavebeenshowntobeeatenanddigestedbylarvae(Reitanetal.1997,VanDerMeerenetal.2007),andmay

Nauplius stage of copepod Parvocalanus crassirostris

14 | InternatIonal AquAFeed | May-June 2013

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May-June 2013 | InternatIonal AquAFeed | 15

Volume 16 / Issue 3 / May-June 2013 / © Copyright Perendale Publishers Ltd 2013 / All rights reserved

also stimulate digestive enzyme production(Cahuetal.1998).

Production of microalgaeDespitethemanyadvantagesofmicroalgae,

their wider use is hampered by difficulties inculturing, storage, and high costs. Microalgaeculturecanconsumeasignificantfractionoftheresources of a hatchery, and requires specialequipment, skilled labour, and a large alloca-tion of space that is unproductive during theseasonswhenlivefeedsarenotneeded.

Low-cost open-pond culture methodscarryhigh risksof contaminationandculturefailuredue to the impossiblityof tightlycon-trollingcultureconditions,andthemosthighlyprizedhigh-PUFAstrainssuchasIsochrysisandPavlovarequireindoorculture.

Itisverydifficulttosynchronizemicroalgalproduction with live feed requirements toprevent feed shortagesorwasteful overpro-duction, and it is difficult to accurately dosealgae cultures directly into live feed cultures.If the algae are harvested and concentrated,thetightly-packedcellscandeterioraterapidlyinrefrigeratedstorage.Somemicroalgaehavebeen freeze- or spray-dried, but dried cellsaresubjecttoproteindenaturation,andwhenthey are rehydrated the leaching of water-soluble substances can rapidly deplete theirnutritionalvalue,aswithotherdryfeeds.

Microalgae concentratesThe best solution to these problems

can be the use of commercially-availablerefrigerated or frozen algae concentratesor ‘pastes’ (Guedes & Malcata 2012,Shields&Lupatsch2012).Theseproducts,which are actually viscous liquids, haveproven to be effective feeds for rotifers,Artemia, shellfish and other filter-feeders,aswellasforgreenwaterapplications.

In products formulated to provide alongshelf-life,theconcentratedmicroalgaeare suspended in buffer media that pre-servecellularintegrityandnutritionalvalue,although the cells are non-viable. Whenconcentrates with well-defined biomassdensities are employed, the algae can beaccurately dosed into live feed cultureswith a metering pump, and non-viabilityconfers the advantage that the productspose no risk of introducing exotic algalstrains.Thebestrefrigeratedproductstypi-cally have a shelf-life of 3-6 months, andfrozenproducts several years.Thismeansthatareliablesupplyofalgaecanbekeptonhand,availableforuseinanyseasonorif an unexpected need arises. Algae costsbecome predictable, and often prove tobelessthanon-siteproductionwhentotalproduction costs and inefficiencies areaccountedfor.

Although costs of liquid algae concen-trates are higher than for dried algae orformulatedfeeds,theyofferallthenutritionaladvantages of live cultures. The nutritionalquality of live feeds can be no better thanthe food sources used to produce them.Success of early larvae is so critical to thesuccess of a hatchery that even a relativelysmallimprovementinsurvivalorgrowthratecanyieldgreatbenefits.

OutlookLive feeds remain indispensable for

larviculture of many fish. Although micro-algaeareamongthecostliestfoodsourcesused to produce live feeds, their manyadvantages justify the cost for hatcheriesproducinghigh-valuefish.Researchcontin-ues to better characterise the nutritionalproperties of various algae strains and tooptimise algae production technologies.We can anticipate that introduction ofnovel algae strains and nutritionally-opti-mised combinations of strains, along withimproved feeding protocols, will ensurethatmicroalgaeremainthefoodofchoicefor production of the highest-quality livefeeds.

Referenceswww.aquafeed.co.uk/referencesIAF1303

14 | InternatIonal AquAFeed | May-June 2013 May-June 2013 | InternatIonal AquAFeed | 15

FEATURE

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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 I s sue 3 2 013 - mAY | J uNe

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