March | April 2013

Bioenergetics - application in aquaculture nutrition

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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36 | InternAtIonAl AquAFeed | March-April 2013

Copyright,  ©,  2013,  Alltech.  All  rights  reserved

B ioenergetics describe the flowofenergyandnutrientswithinabiologicalsysteminourexamplea fish or shrimp. It describes

the biological process of utilisation andtransformation of absorbed nutrients forenergy, for own body synthesis.The feed,that is consumed, is transformed in thebody, complex chemical compounds arebroken down into simpler components -protein into amino acids, carbohydratesintoglucose,lipidsintofattyacidsandwiththisprocessenergy is released -which isused formaintenance, for renewingwornout tissue and building new tissue - forgrowth. The major organic compoundsin feeds such as lipid, protein and carbo-hydrates are the sources of energy butthey also supply the building material forgrowth.

There are different types of energy,chemicalenergy,electricalenergy,mechani-cal energy and heat. These different formsof energy can be transformed into eachotherbutonlyatacost,thetransformationis not 100 percent efficient. What is lostis mostly in the form of heat. Heat is alsotheonly formofenergy, intowhichall theothers can be transformed and measured.The chemical energy stored in feed andanimal tissue is measured using a bombcalorimeter.Theamountofheatproducedby complete oxidation of feed or tissueis known as the heat of combustion orgross energy (GE). Heat energy is usuallyexpressed in kilocalories (kcal) or kilojoule(kJ).Onekcalequalstheenergyneededtoraisethetemperatureofonekgofwaterbyone degree Celsius (°C). One kcal equals4.184kJ.

Forthebio-energeticmodel,thetwolawsofthermodynamicscanbeapplied

1.Energy cannot be created ordestroyedwithinasystembutmaybe changed into different forms(whatgoesinmustgoout)

2.Inasystemwhereenergyistrans-formed(fromfeedto flesh) thereisadegradationandlossofenergyintheformofheat(nothingis100percentefficient)

Theflowofenergy fromfeedtogrowthinananimalisillustratedinFigure1.Notallthe energy from the feed is digested, sub-stancessuchasfibreandcellulosefromplantingredientspassthroughthedigestivesystemwithoutbeingavailabletothefish.Thecon-sumed GE minus faecal energy losses (FE)iscalled thedigestibleenergy(DE)which isthenavailableforthemetabolicprocessesofananimal.

The next majorlosses occur, whenenergy containing com-pounds (on DE basis)are transformed by thefish, broken down tosmaller units and thenused to build its ownenergy reserves or todeposit protein asgrowth. As mentionedabove, this process oftransformation is never100 percent, there arealways losses and theyare mostly in the formofheat.Inpoikilotherms

such as fish this heat is lost to the sur-roundingwater,inhomeothermsitispartlyused to keep the body temperature con-stant. Only the net energy (NE) is nowavailable for maintenance and for growth.Maintenance requirement represents ener-gyneededformovements,osmo-regulation,blood circulation, first this energy has tobe supplied before the remainder can bechanneled into growth - themain productinfishculture.

Quantification of energy demand in fish

By quantifying the energy budget - theenergy input on one hand and the variousenergy losses on the other hand, valuableinformation can be gained in order to opti-misefeedsandguaranteeoptimalfishgrowth.By defining demands for maintenance andgrowth (Figure 1) and anticipating certainlosses beforehand, feeds can be formulatedandfeedingtablesestablished.

Bioenergetics - application in aquaculture nutrition

by Ingrid Lupatsch, Centre for Sustainable Aquaculture, Swansea University, United Kingdom

Figure 1: Schematic presentation of the energy flow through a fish

18 | InternAtIonAl AquAFeed | March-April 2013

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Maintenance requirementFish require energy for maintaining basic

processes of life such as blood circulation,osmo-regulation, excretion and movement,regardless of whether or not feed is con-sumed.Ananimaldeprivedoffeedcontinuesto require energy for those processes andwill obtain it from the catabolism of ownbody reserves. Depending on the activity,severalmetaboliclevelscanbedistinguished:basal, standard, routine and active metabo-lism.

Metabolicrate(Q)atall levelsofactivity,depends largely on the size of the fish andthe water temperature, and is (at constanttemperature) proportional to the metabolicbodyweightintheformof

Q = a BW(kg)b

Where(kg)b:Metabolicbodyweighta is the constant for given conditions

(species,activity,temperature)b isthescalingexponentofthemetabolic

bodyweightMostmetabolicstudiesonfisharecarried

outvia indirectcalorimetry.This isbasedonthe assumption, that energy production inananimal isanaerobicprocessandrequiresoxygen for oxidising nutrients either fromthefoodorfromthetissue. Inthiscase it isassumed that the amount of oxygen takenup by respiration will release an equivalentamount of energy which can be calculatedfromtheoxy-caloricvalue.Anothermethodisthecomparativeslaughtertechniquewhichmeasures the caloric value of the tissuesutilisedduringfasting.

Figure2illustratestherelationshipbetweenmetabolic rate of a fasting fish (gilthead seabream)andweight.

The relationship between fasting metabo-lism and fish weight is not linear and results(Figure 2) were fitted to ln - ln functions ashave traditionally been used by animal nutri-tionists to express metabolic body weight.The antilog of these functions describes theallometric relationship common in biologicalmeasurements.

Metabolic rate (kJ /fish /day) = 41.5 BW(kg)0.80

(1)

With an exponent of b= 0.80 for the metabolicbody weight, the implica-tion is that metabolic rateis increasingwith increasingfishweightinabsoluteterms(kJ/fish/day),butsmallerfishspendmoreenergyperunitsize than bigger fish. Thisconcept of metabolic bodyweight will be clarified fur-theron.

It should be noted thatthe fasting metabolism isonly an approximation ofthemaintenancerequirement;allowancemustbe made for the efficiency of utilisation ofthe dietary energy. This can be achieved byfeeding fish graded levels fromzero feeduptomaximumintake.Energygainorlossinfishis thendeterminedbycomparative slaughtertechnique. The following Figures 3 and 4describetherelationshipbetweenenergyfed(DE) and energy retained for sea bream oftwodifferentsizes.(at210C).

It is obvious from Figure 3 that as moreenergy is consumed the more energy isgained,untilthefishrefusetoeatmore.Figure3 also demonstrates that the relationshipbetweendailyDEconsumed (x) andenergyretained (y) is linear and can be describedby the followingequations foreach the twofishsizes:

Sea bream of 30 g y = - 2.2 + 0.66 × (2)

Sea bream of 100 g y = - 4.6 + 0.67 × (3)

During fasting the fishwould lose energyasexpected-2.2kJper fishof30gand4.6kJperfishof100gperday.TheDErequire-ment for maintenance (no energy gain orloss) can be found where energy gain (y)is set at zero. According to the equationsabove,themaintenancerequirementperday

would amount to 2.2 / 0.66 = 3.33 kJ forthe 30 g fish and 6.86 kJ for the 100 g fish.As mentioned before, absolute maintenancerequirement is increasingwith increasing fishweights,butregardedperunitofweightgainit is decreasing. Energy requirement of thesmallerfishis110kJ/kgandforthelargerfishonly69kJ/kg.

Theslopesofthelinesarenearlyidenticalat0.67;theycanberegardedastheefficiencyof utilisation of energy. Per unit of DE con-sumed67percent is retainedasgrowth, theremainderislostasheattothewater.

InFigure4 thesamedataset isusedbutdaily energy retention in fish is presentedreferring to the metabolic weight of kg0.80.ByexpressingDE intakeandthesubsequentretention of energy per metabolic weight(kg0.80) the resulting regressions of the rela-tionshipsforbothfishsizescanbecombined.

ThustherelationshipbetweenDEfed(x)andenergygained(y)bothexpressed inkJ /kg0.80/dayisasfollows:

at 21ºC y = - 33.7 + 0.67 × (4)

Accordingtotheequation(4),themain-tenancerequirementperdaywouldamountto 33.7/0.67 = DEmaint = 50.3 kJ x kg0.80(at 21ºC). Again the slope of the line, the

Figure 2: Metabolic rate (kJ/fish/day) of gilthead sea bream at increasing sizes

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efficiency of energy utiliza-tionforgrowthremainsthesame at 0.67. The recipro-cal of 0.67 is 1.49 (1/0.67),which means that 1.49 kJof DE have to be investedto produce 1 kJ of energyas growth, in other words,the energy cost to depositoneunitofenergyasgainisclosetooneandahalfunitsof energy from the feed(basedinDE).

Besidesfishweight,watertemperature is one of themajor factors to determinemaintenance requirement.Addingdataofanadditionaltrial with sea bream per-formed at 27ºC providesthe following equation fortherelationshipbetweenDEfed and energy gained per(kg)0.80(Figure5):

at 27ºC y = - 51.5 + 0.66 × (5)

According to equation(5), themaintenanceenergyrequirement would amountto DEmaint = 78 kJ kg0.80at a temperature of 27ºC,while at 21ºC the mainte-nance requirement was cal-culated as 50.3 kJ kg0.80 asshown before. However inboth instances the slope ofthe line (efficiency) remainsthesameevenatthehighertemperature.

Requirements for growth

To be able to estimatefeedrequirementsitisessen-tial to predict the growthpotential of the target spe-cies. In contrast to terres-trialanimalsfishseemtogrowcontinuously, growth doesnot cease and reaches anasymptote, which in aquacul-turehowevermightneverbeattained.Asgrowthisaffectedby temperature, it increaseswith increasing temperaturesup to an optimum abovewhichgrowthdecreases,untilthe upper lethal temperatureisreached.

Togetherwiththeantici-pated increase in weight,the energy content of thisgainisanotherfactordeter-

miningthesubsequenttotalenergydemandoffish.

Thefollowingequationsdescribethedailyweight gain of gilthead sea bream for watertemperatures ranging between 20 and 28ºCand the energy content per unit of weightgain.

Weight gain (g / fish / day) = 0.024 × body weight (g) 0.514 × exp 0.060 × Temp (6)

Energy content of fish (kJ / g wet weight) = 4.66 × BW(g) 0.139 (7)

Modelling requirements Thecalculationofdailyenergyandconse-

quentlythefeeddemand(basedondigestibleenergyDE,i.e.theamountabsorbedthroughthe gut) for fish can then be described asfollows:

DE intake (kJ/day) = a x BW (kg)b + c x energy gain (kJ/day)

where DE = digestible energy intake

BW = body weight (kg)

The expected live weight gain, which isdependent upon fish size and water tem-perature,canbepredictedwiththefollowingcommonequation,whereagaina,b,andcareconstantstypicalforafishspecies:

Weight gain (g/day) = a x BW (g)b x expc x Temp

Theaverageenergycontentoftheweightgainforafishisdependentonthefishsizeandcanbedescribedas:

Energy content (kJ/g fish) = a x BW (g)b (i.e. it is body weight dependent)

The expected daily energy gain is there-fore:

Weight gain (g) x energy content of fish (kJ/g)

Forthequantificationofdailymaintenancerequirementwhichistheenergyrequirementatzerogrowth:

DEmaint (kJ) = a x BW (kg)b

The cost of production as DE intake(in units of kJ for energy) for one unitof energy deposited as fish energy (asgrowth) is for many fish species around1.50 or 1 / 1.50 = 0.67 = efficiency forgrowth

Combining those equations suggests thatthe feed allowance based on energy intakecanbecalculatedasfollows:

Feed (g) = [(Maintenance + (weight gain) x (composition) x (1.50)]

Figure 3: Relationship between DE consumed and energy gained (in kJ / fish /

day) for two sizes of gilthead sea bream

Figure 4: Relationship between DE consumed and energy gained (in kJ /

kg0.80 / day) for two sizes of gilthead sea bream (at 210C)

Figure 5: Relationship between DE consumed and energy gained (in kJ / kg0.80 / day) for gilthead sea

bream at increasing temperatures.

20 | InternAtIonAl AquAFeed | March-April 2013

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Bioenergetics – application in aquaculture nutrition

Volume 16 I s sue 2 2 013 - mARCH | APR I l

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