Response to selection for harvest body weight of Oreochromis shiranus

Response to selection for harvest body weight

of Oreochromis shiranus

Alfred Ochanza Maluwa a ,Bjarne Gjerde b,?

National Aquaculture Centre,P .O.Box 44,Domasi,Malawi

AKVAFORSK,Institute of Aquaculture Research,AS,P .O.Box 5010,N-1432,?s,Norway

Received 8June 2007;accepted 16June 2007

Abstract

Realised and predicted responses to selection were obtained for harvest body weight of Oreochromis shiranus from two generations of selection at the National Aquaculture Centre,Domasi,Malawi.The realised response was estimated from the difference in the least squares means of selected and unselected control populations,while the predicted response was obtained from the difference of the mean breeding values between generations.The realised (13.2%)and predicted (14.4%)responses for harvest body weight over the two generations of selection were not significantly different (P N 0.05).The intensity of selection from F 1to F 2(0.66)and from F 2to F 3(0.59)was low due to availability of few selection candidates per family at harvest caused by mortality,tag loss and the need to form broad heterogeneous multi-strain F 2and F 3populations.Within generation,the heritability estimates for the F 1(0.31±0.09),F 2(0.31±0.10)and F 3(0.35±0.11)generations were moderate.Across generation,the heritability estimate was low (0.14±0.27).Within generation,the magnitude of the common full-sib effect was moderate (0.08–0.09)and not significantly different from zero (P N 0.05).However,across generations,the common full-sib effect was high (0.27±0.05)and significantly different from zero (P b 0.05).A substantial correlated selection response due to selection for increased harvest body weight was observed for both tagging body weight and for survival from tagging to harvest;for tagging body weight 22.8%from F 1to F 2and 15.9%from F 2to F 3;for survival 7.2%from F 1to F 2and 13.8%from F 2to F 3.The results are discussed in relation to the maximisation of the genetic gain in the future generations while constraining the rate of inbreeding.?2007Elsevier B.V .All rights reserved.

Keywords:Oreochromis shiranus ;Harvest body weight;Heritability;Realised and predicted genetic gain;Breeding values;Selection intensity

1.Introduction

Breeding programs are designed to improve traits that respond to selection and which have a market or a non-market economic value (Olesen et al.,2000).The primary factors that affect genetic gain and thus response to selection per generation are the selection intensity,

accuracy of selection and the magnitude of the additive genetic variance in the population under study (Muir,2000).To monitor genetic gain in a breeding program,both the least squares (Becker,1984)and mixed model (Henderson,1973)methods can be used.For the least squares method,the environmental effects in different generations of selection are accounted for by using an unselected control or divergent selection (Falconer and Mackay,1996).The realised genetic gain is estimated from the difference in the least squares means of the selected and control populations (Becker,1984).The

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?Corresponding author.Tel.:+4764949514;fax:+4764949502.E-mail address:bjarne.gjerde@akvaforsk.no (B.Gjerde).

limitation with this method is that environmental effects may bias the estimated gains because it becomes practically difficult to keep and maintain sufficient numbers of contemporary groups of animals that are stable over generations especially in large animal breeding programs(Hill,1972a,b).However,high fecundity in fish species makes it possible to produce sufficiently large numbers of both the selected and control populations in a short period of time,provided that the control population is reproduced using sufficiently large numbers of contributing parents to prevent random drift and inbreeding.A major constraint with mass spawning in fish,especially in tilapias,is the inability to control the number of contributing sires to the progeny because only a few sires may dominate(Fessehaye et al.,2006).In view of the constraints with the production of representative control populations,the best option is to reproduce a new control population in each generation using sufficiently large number of broodstock whose breeding values are around the population mean(Rutten,2005).

Henderson(1973)demonstrated the use of mixed model equations as an alternative technique that enables the separation of genetic and environmental effects when predicting breeding values using the best linear unbiased prediction(BLUP).Henderson(1984)demonstrated that (BLUP)increases the accuracy of selection and thus genetic gain provided that a complete additive genetic relationship matrix is used.With BLUP procedure, breeding values are obtained that are comparable across generations from which the predicted genetic gain can be obtained as the difference in the mean breeding values of individuals between generations.Fish are prolific bree-ders and a large number of progeny may be produced and recorded for each breeding pair.When sib information is used,the predicted breeding values within families become highly correlated.Consequently,when applying truncation selection,the selected animals originate from only a few families,leading to high levels of inbreeding (Wray and Thompson,1990).The inbreeding levels in a breeding program can be minimised by applying the optimum contribution selection,which maximises genetic gain at a predefined rate of inbreeding(Meuwissen, 1997).Sorensen and Kennedy(1984)showed that both the least squares and mixed model estimators are unbiased when the assumptions underlying each model are satisfied and the recorded data has been properly adjusted for the fixed effects.

In aquaculture,selection experiments that yielded positive response to selection were first reported as early as the1920s.Embody and Hayford(1925)reported improved resistance against furunculosis in brook trout. Lewis(1944)reported increased early spawning,egg number and yearling weight in rainbow trout.More recently,successful breeding programs have been imple-mented,with varying results on the response to selection. The response to selection for growth in the samonids and trout has ranged from10.1to20%per generation (Gjedrem,2000).In Oreochromis niloticus the responses to selection that have been reported by several authors are: 12–17%per generation(Eknath et al.,1998),40%per generation(Gall and Bakar,2002),12%per generation (Bolivar and Newkirk,2002),23%per generation (Bentsen et al.,2003),8.4to11.4%per generation (Ponzoni et al.,2005).

In Malawi,small-scale aquaculture has boomed in all potential areas of the country.The potential areas are those with shallow perennial streams that are easy to divert into farms by gravity.The total annual fish production in Malawi is currently estimated at850tonnes and O.shiranus comprise90%of the total yield (National Aquaculture Centre,2005).A tilapia-breeding program was initiated in1996at the National Aquacul-ture Centre,Domasi.Four strains of O.shiranus were collected from their wild sources in the country.These strains were;Shire,which was collected from Shire river, Nkhotakota,from Lake Malawi at Nkhotakota,Chilwa from L.Chilwa and Chiuta from L.Chiuta.The species has undergone three generations of breeding and data on growth performance from two generations of selection is available.For the trait harvest body weight,estimates of the strain additive genetic and strain heterosis effects were obtained from a diallel cross of the F1generation, (Maluwa and Gjerde,2006a).Estimates of the strain additive and maternal genetic effects,and strain individual,maternal and paternal heterosis effects were obtained from the F2generation(Maluwa and Gjerde, 2006b).Estimates of heritability and magnitude of genotype by test environment interaction for the harvest body weight trait were obtained from F1,F2and F3 (Maluwa et al.,2006).However,estimates of genetic gain for harvest body weight or any other traits in O. shiranus are hitherto not available.

The objective of this study was to obtain estimates of realised and predicted genetic gain(response to selection)for harvest body weight of O.shiranus across the two generations of selection.

2.Materials and methods

2.1.Data on fish harvest body weight

Harvest body weight data were obtained from the tilapia-breeding program at the National Aquaculture Centre,Domasi, Malawi.The F1was formed using a diallel cross experiment involving four wild parental strains(Maluwa and Gjerde,

34 A.O.Maluwa,B.Gjerde/Aquaculture273(2007)33–41

2006a).The F2and F3generations were produced from the mating of F1and F2individuals,respectively,that had breeding values for harvest body weight above the population mean in the actual population.The F1consisted of full-sib families produced through single pair matings in1m3hapas installed in one common pond.In the F2and F3generations,full-and half-sib families were produced by a nested mating design,in which three dams were mated to the same sire.Half-sibs were produced by moving the sire to the second and third dams placed in different hapas after mating with the first dam.In each generation,after selection,the mating of full-and half-sibs was avoided during mate allocation in the breeding hapas. The analysed harvest body weight records were obtained from the full-sib families in the F1,and from the full-and half-sib families in the F2and F3.

2.2.Control populations

Control populations were established for the F2and F3 generations by mass spawning of broodstock from F1and F2 respectively.These control broodstock had breeding values around the population mean in each generation.For the F2,a total of37males and46females that were0.8g above the F1mean breeding value were mass spawned in a1m3concrete tank. Similarly,for the F3,the control population comprised F2 individuals(83males and94females)with a mean breeding value of?0.1g below the F2generation mean.In each generation,the fry that were spawned on the same day with the families in the1m3breeding hapas were used as a control for that group of families.As a result,each age group of families from the breeding hapas had a corresponding control age group.In the F2 there were51control groups corresponding to the99full-sib families produced,while in the F3there were43control groups corresponding to the84full-sib families produced.

2.3.Fry tagging and growth testing

The fry were transferred from the breeding hapas and tanks 14days after spawning to1m3concrete rearing tanks.Each family and control group was reared separately for a period ranging from76to131days.The fry were then individually tagged with Floy tags while the control fish were tagged with a blank plastic tag.About3000and4000fish from the selected populations in the F2and F3,respectively and about1500and 1400control fish in the F2and F3,respectively were individually tagged.The selected and control populations were reared communally in one500m2pond at each of three test environments that roughly represented the topography and climatic conditions of the country.The first test farm environment was Chisitu,with an altitude of1200m above the sea level and a mean annual water temperature of20°C.The second test farm environment was Domasi with an altitude of 800m above the sea level and a mean annual water temperature of24°C.The third was Kasinthula in the lower shire valley with an altitude of150m above the sea level and a mean annual water temperature of27°C.The fish were harvested after a growth-testing period of180days.2.4.Data analyses

2.4.1.Estimates of realised genetic gain from the control populations

Estimates of the realised genetic gain for harvest body weights from F1to F2(and from F2to F3)were obtained from the following linear model;

Y ijk?ltES

eTitP jte ijke1T

where;Y ijk is the observed harvest body weight of the k th individual of the selected or the control population in F2or F3;μis the overall mean;(ES)i is the fixed effect of the i th level of test environment(three levels)by sex(two levels)subclass (i=1,2,…,6);P j is the fixed effect of the j th population(j=1 for the control population and2for the selected population); and e ijk is the random residual error of the k th individual.The covariate age at harvest,nested within the ES effect,on harvest body weight was not significantly different from zero (P N0.05)and was therefore excluded from the model.

An estimate of the realised correlated genetic gain for tagging body weight from F1to F2and from F2to F3was obtained as the difference in the mean tagging body weight of the selected and control populations in the F2and F3, respectively.

The differences in fish growth rates yielded heterogeneous variances of harvest body weight among the eighteen subclasses of the fixed ES effect within generation.Conse-quently,to obtain constant error variances over the level of this fixed effect,estimates of the ES effect in Model(1)were obtained by weighted least squares(Chatterjee and Price, 1977)in which each harvest body weight record was weighted by the reciprocal of the variance of the harvest body weight of the actual test environment by sex subclass.

The realised genetic gain for harvest body weight from F1to F2,and from F2to F3generations was estimated as the difference in the least squares means of the selected and the control populations in the F2and F3,respectively.The genetic gain(g)was expressed as a percentage of the least squares mean of the selected population in the previous generation.The cumulative genetic gain was calculated as the sum of the within generation genetic gain and was expressed as a percentage of the least squares mean of the F1base population.

A similar model including the two fixed effects of test environment and population and with the binary trait survival from tagging to harvest as the dependent variable(1=live,0= dead)was used to obtain an estimate of the difference in survival between the selected and control populations,separately for the F2and F3generations.

2.4.2.Variance components and heritability estimates

Maluwa et al.(2006)reported that the genetic correlation of harvest body weight between pairs of test environments was not significantly different from unity,implying a low individual genotype by environment interaction.Therefore in this study,harvest body weights from the three test farm environments were treated as the same trait.The harvest body

A.O.Maluwa,

B.Gjerde/Aquaculture273(2007)33–41

weight data were analysed using the following mixed model (Model(2)),which in matrix notation can be written as;

y?XbtZ1atZ2ctee2Twhere,y is a vector of observed body weights at harvest;b is a vector of a fixed effect of the generation(three levels)by test environment(three levels)by sex(two levels)subclass(with a total of eighteen levels),and including a covariate of age at harvest for each level of the fixed effect;a is a vector of the animal additive genetic effect,a～(0,Aσa2),where A is the additive genetic relationship matrix among all fish,including the parent of the F1,which had no harvest body weight record;

c is a vector of random common full-sib effects,c～(0,Iσc2), which comprises the hapa/tank effect due to separate rearing of the full-sib families before tagging an

d on

e quarter o

f the non-additive(dominance)genetic effect common to full-sibs;e is a vector of random residual errors,e～(0,Iσe2).X,Z1,Z2are design or incidence matrices assignin

g harvest body weight observations to the levels of b,a and c,respectively.

The random effects in the model were estimated using ASReml software package(Gilmour et al.,2002).The additive (σa2),common full-sib(σc2)and residual(σe2)variances were estimated within and across generations.Phenotypic variance was calculated asσp2=σa2+σc2+σe2.Heritability(h2)was calculated as h2=σa2/σp2,while the common full-sib effect (c2)was calculated as c2=σc2/σp2.As for Model(1),to account for the heterogeneous variances of harvest body weight among the eighteen subclasses of the combined generation by test environment by sex effect in Model(2),the harvest body weights were weighted by the reciprocal of the variances for each of the actual level of the fixed effect.

2.4.

3.Predicted genetic gain and intensity of selection

BLUP breeding values for harvest body weight within and across generations were obtained using two different sets of parameters.The within generation BLUP breeding values were obtained using the mean of the estimated within generation heritability and common full-sib effect(Set1);the across generation BLUP breeding values were obtained using the estimates of heritability and common full-sib effect across generation(Set2).The predicted genetic gain for harvest body weight from F1to F2was obtained as the difference in the mean predicted breeding values for harvest body weight of individuals in the F1and F2.Similarly,the predicted genetic gain from F2to F3was obtained.The genetic gain per generation was expressed as a percentage of the mean body weight of the selected population in the previous generation.The cumulative genetic gain from F1to F3was obtained as the difference between the mean breeding values of the F3and F1and was expressed as a percentage of the phenotypic mean body weight of the F1base population.The intensity of selection(i)for each generation was estimated as,i=ΔG?/σB?V(Falconer and Mackay,1996)where ΔG?is the predicted genetic gain(g)per generation andσB?V is the standard deviation of the predicted breeding values in the actual generation.3.Results

3.1.Descriptive statistics

In each of the three generations the number of families harvested was substantially lower than the number of families tagged:In the F1generation the number of full-sib families tagged was79,while harvest body weight records were obtained from only59families.In the F2generation,99full-sib and52 half-sib families were produced,while harvest body weight records were obtained from52full-sib and33half-sib families. In the F3generation,84full-sib and55half-sib families were tagged,while harvest body weight records were obtained from 55full-sib and25half-sib families respectively.The reason for this reduced number of families was low survival rates(including tag losses),particularly in the F2and F3generations(Table1).

Table1gives for each generation,population and test environment,the number of fish stocked,the mean and coefficient of variation for harvest body weight and survival from tagging to harvest.At each test environment the mean harvest body weight of the F2selected population was higher than that of the F1base population.Similarly,the mean body weight of the F3selected population was higher than that of the F2population.At each test environment in F2and F3,both the mean harvest body weight and survival of the selected fish were higher than those of the control fish.

3.2.Realised genetic gain

For the harvest body weight,the realised genetic gains are shown in Table2.The realised genetic gain from both generation Table1

The number of fish stocked(N),the mean observed harvest body weight and survival(including tag losses)from tagging to harvest for each generation,population and test environment of O.shiranus Generation Population Test

environment

stocked

Harvest

body

weight(g)

Survival

(%)a

Mean CV

F1Base Chisitu14267.233.484.1

Domasi13868.323.079.5

Kasinthula16889.823.587.2

F2Selected Chisitu100573.421.442.8

Domasi100971.519.441.9

Kasinthula100089.419.239.4 Control Chisitu50168.323.815.2

Domasi48170.421.518.0

Kasinthula51083.020.317.0

F3Selected Chisitu101280.518.033.9

Domasi100085.919.938.4

Kasinthula102296.124.432.9 Control Chisitu49465.825.216.2

Domasi52172.731.319.6

Kasinthula51790.129.117.8

a Survival figures exclude fish that lost tags.

36 A.O.Maluwa,B.Gjerde/Aquaculture273(2007)33–41

F 1to F 2and from F 2to F 3was significantly different from zero (P b 0.05).The two gain estimates were not significantly different (P N 0.05).The accumulated genetic gain over the two generations of selection was 13.2%or on average,6.6%per generation.The mean breeding values for the parents of the control populations as deviations from the population mean in each generation was (0.8g)for the F 2and ?0.1g for the F 3.In the F 1generation,the mean tagging body weight was 8.8±0.36g.In F 2the mean tagging body weight of the selected population (7.9±0.17g)was significantly higher than that of the control population (6.1±0.30g).Similarly,in F 3the mean tagging body weight of the selected population (6.9±0.17g)was significantly higher than that of the control population (5.8±0.30g).For tagging body weight this corresponds to a realised correlated genetic gain of 22.8%from F 1to F 2and of 15.9%from F 2to F 3.For survival from tagging to harvest,the selected fish had a much higher survival than the control fish;25.0±1.5%-units in F 2and 17.2±1.4%-units in F 3.

3.3.Variance components,heritability and common full-sib effect

Estimates of variance components,heritability and common full-sib effect for harvest body weight within and across generations are shown in Table 3.Within generation,heritability estimates (0.31to 0.35)were of similar magnitude and significantly different from zero (P b 0.05).Across generation,the heritability estimate was low (0.14±0.08)and not signifi-cantly different from zero (P N 0.05).The common full-sib effects within each generation were moderate (0.08to 0.09)and not significantly different from zero (P N 0.05).Across generations,the common full-sib effect was very high (0.27±0.05)and significantly different from zero (P b 0.05).3.4.Predicted genetic gain and selection intensity

The predicted genetic gains for harvest body weight obtained from the two selected generations (F 2and F 3)are shown in Table 4.The predicted genetic gains from both sets of parameters were significantly different from zero (P b 0.05).

7As expected from the magnitude of the Set 1and Set 2parameter estimates,the predicted genetic gains using Set 1were significantly high from those predicted using Set 2(P b 0.05).The predicted genetic gains in each generation of selection using Set 1parameters were not significantly different (P N 0.05)from the realised genetic gain in the same generation (Table 3).The predicted accumulated gain over the two generations of selection using Set 1of the parameters was 14.4%or on average,7.2%per generation.The selection intensity in each generation was fairly low and of similar magnitudes when using the two sets of the parameters (Table 4).

3.5.The breeding values of the selected broodstock A plot of the breeding values of the F 1selected broodstock on the mean breeding value of their F 1strain combination is shown in Fig.1.A similar plot of the F 2selected broodstock on the mean breeding value of their F 2full-sib families is shown in Fig.2.The mean breeding values of the F 1and F 2populations were 1.6g and 6.8g,respectively.The breeding values for the F 1selected broodstock were above the mean of their population and comprised individuals from both high and low ranking strain combinations (Fig.1).Similarly,almost all the breeding values of the F 2selected broodstock were above their population mean and the broodstock comprised indivi-duals from both high and low ranking families (Fig.2).

The number of breeding candidates per strain combination in the F 1population and the number of male and female broodstock selected from each strain combination (and contributed progenies with records in F 2)are given in Appendix A.Similarly,the number of breeding candidates per full-sib family in the F 2population and the number of male and female broodstock selected from each family (and contributed progeny with records in the F 3)are given in Appendix B.

4.Discussion

The higher mean harvest body weight of the selected populations in two subsequent generations (F 2vs.F 1and F 3vs.F 2)indicates a positive realized selection response for harvest body weight given the equal

Table 2

Estimates of realised genetic gain for harvest body weight of Oreochromis shiranus within and across generation Generation

Population

Least squares mean (g)

Realised genetic gain ?Mean±se (g)

(%)F 1Base 78.5–

–F 2Selected 80.8 4.5±1.0a 5.7Control 76.3F 3

Selected 88.3 5.9±1.5a 7.3Control 82.4Cumulative

–

10.4±1.6

13.2

?Figures with the same letter in the same column are not significantly different (P N 0.05).

Table 3

Estimates of the additive,common full-sib,residual and phenotypic variances,heritability and common full-sib effect for harvest body weight of Oreochromis shiranus within and across generation Generation Variance components

Heritability Common

full-sib effect σa

2σc 2σe 2σp 2h 2±se c 2±se F 1

203.5–456.2659.70.31±0.09–

F 2223.758.3429.5711.50.31±0.100.08±0.08F 3

295.677.7481.3854.70.35±0.110.09±0.07Across

122.8

237.5

534.2

894.5

0.14±0.08

0.27±0.05

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B.Gjerde /Aquaculture 273(2007)33–41

growth-testing period of 180days and same test environments and management in each generation.This is confirmed by the higher mean harvest body weight of the selected than the control population in both the F 2and F 3generations.

The differences in tagging body weight and sur-vival between the selected and control populations in both F 2and F 3,indicate a favourable correlated selection response for these traits due to the selec-tion performed for increased harvest body weight.A favourable correlated selection response for tagging body weight is expected due to the positive genetic correlation (r =0.38±0.13)found between harvest and tagging body weights (Maluwa et al.,2006).The large

difference in survival between the selected and control population could be due the higher body weight of the selected as compared to the control fish at tagging and stocking.A correlated selection response would as-sume that survival from tagging to harvest is herita-ble and that there is a favourable genetic correlation between survival and harvest body weight.This phenomenon could have been reported by Maluwa et al.(2006)when estimating the genetic parame-ters for the species,however,the genetic parameters for survival for O.shiranus were not reported in the study.

The mean breeding value for harvest body weight of the control population parents in each generation was around the population mean.Hence,assuming an equal contribution of progeny at harvest from all control parents,the control population in each generation was appropriate for the estimation of the realised genetic gain for harvest body weight without bias.The as-sumption of the equal contribution from all parents may not be realistic in this study.Fessehaye et al.(2006)reported for Nile tilapia (Oreochromis niloticus )that when the species were mass spawned as the control population in this study,only one third of the males dominated and contributed more than 70%of the off-spring.However,the control population in this study was still appropriate because all control population parents had similar breeding values for harvest body weight.

The similarity of the realised genetic gain and the predicted genetic gain (using Set 1parameters)sug-gests that the within generation parameter estimates (Set 1)were more accurate than the across generation parameters (Set 2).The within generation heritability estimates for harvest body weight were within the range reported for body weight of Nile tilapia (Bentsen et al.,

Table 4

Predicted mean breeding values (BV

ˉ)and their standard deviation (σB ?V ),genetic gain (Δ?G )and selection intensity (i )using two sets of

parameters ?for harvest body weight of Oreochromis shiranus Generation

Breeding value (g)Genetic gain ??i

ˉσB ?V g±se %Set 1:h 2=0.33;c 2=0.085

F 1Base population 1.010.2–

––F 27.19.2 6.1±0.53b 8.10.66F 3

11.88.0 4.7±0.47b 6.00.59Cumulative

–

10.8±0.60

14.4

–

Set 2:h 2=0.14;c 2=0.27F 1Base population 0.7 5.0–

––F 2 3.6 4.8 2.9±0.42a 3.90.60F 3

6.4 3.8 2.8±0.37a 3.60.74Cumulative

–– 5.7±0.487.6–

?Set 1are the means of the within generation estimates;set 2are the across generation estimates.

??Figures with the same letter in the same column are not significantly different (P N

0.05).

Fig.1.A plot of the breeding values of the F 1selected broodstock (Set 1parameters)on the mean breeding values of their F 1strain

combinations.

Fig.2.A plot of the breeding values of the F 2selected broodstock (Set 1parameters)on the mean breeding values of their F 2full-sib families.

38 A.O.Maluwa,B.Gjerde /Aquaculture 273(2007)33–41

2003;Ponzoni et al.,2005;Rutten et al.,2005a,b)and higher than the across generation estimates.Higher heritability estimates for harvest body weight with-in than across generations were also obtained in Nile tilapia(Bentsen et al.,2003).These results may be attributed to the discrete nature of the generations in this study.The generations were discrete because parental fish were not re-used in the subsequent gen-erations,thus only parent–offspring genetic ties connected the generations.The low heritability and high magnitude of the common full-sib effect across generations suggest that the parent–offspring genetic ties were not strong enough to provide unbiased es-timates of heritability across generations.Stronger genetic ties between generations could be obtained by repeating the use of some sires and/or dams in the subsequent generations.

The realised and predicted genetic gains in this study are lower than the range(8.4to20%per generation) reported for most fish species in aquaculture(Eknath et al.,1998;Gjedrem,2000;Bolivar and Newkirk, 2002;Ponzoni et al.,2005).These results are likely caused by the low numbers of breeding candidates per family at harvest(Appendix A and B),thus resulting in low selection intensity and accuracy.Part of the low selection intensity in these two initial generations of the breeding program was intended in order to form broad heterogeneous composite multi-strain populations(Mal-uwa and Gjerde,2006a,b).The F2was a four-way multi-strain population that was formed from the mating of F1 selected broodstock with breeding values for harvest body weight above the mean breeding value of their populations and comprised individuals from both high and low ranking strain combinations(Fig.1,Appendix A).Similarly,the F3was formed by mating of F2se-lected broodstock that had breeding values above the mean population breeding values and comprised in-dividuals from both high and low ranking families (Fig.2,Appendix B).

The low number of breeding candidates at harvest was caused by tag losses and high fish mortality in grow-out ponds.Most of the mortality occurred after tagging(Maluwa and Gjerde,2006a,b)and some occurred when fingerlings entangled among them-selves with the thread from the Floy tags.Therefore, there is a need to explore alternative tagging or iden-tification methods to reduce tag losses and improve survival.The low number of families at harvest was also caused by the low success rate of reproduction in the breeding hapas.In the F1and F2,only99and84 full-sib families,respectively were produced out of the planned150.

The non-significant effect of age at harvest on harvest body weight is in agreement with earlier re-ported results on the species(Maluwa and Gjerde, 2006a,b;Maluwa et al.,2006).Hence,to produce more families and thus allow for increased selection intensity with the same number of breeding hapas available,the breeding period may be further extend-ed.However,to minimise age effects on harvest body weight,the production period for all families should be as short as possible.Therefore,rather than in-creasing the breeding period,the number of available breeding hapas should be increased to accommodate more matings within the same or shorter period of time.

For the two generations of selection in this study, the genetic gain is still substantial and significantly different from zero.In the future generations,higher genetic gain can be obtained by increasing the num-ber of selection candidates per family and the number of families.However,with increasing number of fish per family,the number of broodstock selected per family will have to be restricted to avoid high rate of inbreeding that may result in inbreeding depression for important fitness and production traits(Gjerde et al.,1983).Inbreeding reduces genetic variance (Gjerde et al.,1996)leading to reduction in genetic gain.Therefore,in the future generations,optimum contribution selection should be applied which maxi-mises genetic gain at a predefined rate of inbreeding (Meuwissen,1997).

Acknowledgements

This study was funded by the United Nations Development Program using the Japan Human Resources Development Fund through the World Fish Centre in Penang,Malaysia,under the project;‘Transfer of GIFT Technology from South East Asia to Sub-Saharan Africa and Egypt’.European Commission support is also acknowledged.Data analysis and write-up was carried out at AKVAFORSK,in Norway as part of the first author's PhD studies at the Norwegian University of Life Sciences,(Universitetet for milj?-og biovitenskap,(UMB))on a Kvote L?nekassen funded scholarship.Our sincere gratitude is due to Dr.Raul, Ponzoni of the World Fish Centre,Penang,Malaysia for the managerial guidance during the research project and Dr.Hans Bentsen of AKVAFORSK for reviewing the manuscript.We would also like to thank the staff, especially the breeding group at the National Aquacul-ture Centre,Domasi,Malawi,for their assistance during experimentation.

A.O.Maluwa,

B.Gjerde/Aquaculture273(2007)33–41

Appendix A.The ranking of the F1strain combina-tions on the mean predicted breeding values of their offspring(BVˉ)(Set1parameters),the number of full-sib families per strain combination,the number of selection candidate per strain and the number of broodstock selected from each strain combination

F1strain combinations Number of

selection

candidates

per strain

combination

Number

selected

per strain

combination a

Rank BVˉ

(g)Number

of families

per strain

combination

♂♀♂♀

114.63221674

2 6.548733

3 6.23141451

4 5.6513945

5 4.949854

6 4.43161016

7 3.4151004 80.71111803 9?1.94211814 10?2.1451224 11?2.6491714 12?2.94121322 13?3.228722 14?5.5281603 15?6.23121101 16?9.2281002 Total––––3352 a Number of broodstock that contributed progeny with harvest body weight record in F2.

Appendix B.The ranking of the F2full-sib families on the mean predicted breeding values of their offspring(BV)(Set1parameters),the number of selection candidates per family,and the number of broodstock selected from each family

F2full-sib family breeding values Number of

selection

candidates

per family

Number of

broodstock

selected

per family a

Rank BVˉ(g)♂♀♂♀124.915614 221.5191733 320.14303 418.8262223 518.3251723 616.1301923 714.8132302 814.78524 913.213621

(continued)

F2full-sib family

breeding values

Number of

selection

candidates

per family

Number of

broodstock

selected

per family a Rank BVˉ(g)♂♀♂♀1012.3121201 1111.7313412 1211.0131112 1310.412613 149.891513 159.071312 168.611702 178.4282222 188.4241711 198.1302724 207.3364710

21 6.8152203

22 5.1322504 N22b5.027925800 Total–6926342555 a Number of broodstock that contributed progeny with harvest body weight records in F3.The range in the mean breeding values for the rank N22group was4.7to?15.8g.

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