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Definition of a breeding objective for commercial production of the freshwater crayfish, marron (Cherax tenuimanus)
Aquaculture 173 Ž1999. 179–195
Definition of a breeding objective for commercial production of the freshwater crayfish, marron žCherax tenuimanus / Mark Henryon a
a,)
, Ian W. Purvis b, Peer Berg
a
Danish Institute of Agricultural Sciences, Department of Breeding and Genetics, Research Centre Foulum, P.O. Box 50, 8830 Tjele, Denmark b CSIRO DiÕision of Animal Production, Pastoral Research Laboratory, Armidale, NSW 2350, Australia Accepted 14 October 1998
Abstract This study tested the hypothesis that profitability of commercial marron production would be increased by the development of a rapidly growing strain that has a large tail, a proportional claw size, and a high survival, food conversion efficiency, reproductive rate, and fecundity. A profit equation was developed for commercial marron production, and expressed as function of the production characteristics associated with maintenance of the broodstock Žgrowth rate, survival, food conversion efficiency, reproductive rate, and fecundity., incubation of the eggs and hatchlings Žsurvival., rearing of the juveniles Žgrowth rate, survival, and food conversion efficiency., and grow out of the marron Žgrowth rates of the carapace, tail, and claws, survival, and food conversion efficiency.. Economic values were estimated for these characteristics when profit was set to zero, and the sensitivity of selection response to potential errors in these estimates was analysed. The results showed that profit was increased by a genetic increase in the growth rate of the juveniles, growth rates of the carapace and tail of the marron, survivals of the broodstock, eggs and hatchlings, juveniles, and marron, food conversion efficiencies of the broodstock, juveniles, and marron, and the reproductive rate and fecundity of the broodstock. By contrast, profit was decreased by a genetic increase in the growth rate of the broodstock, and a genetic increase in the growth rate of the claws of the marron, given that there was a smaller claw weight to carapace weight ratio that was ideal. Growth rate of the tail of the marron was the most economically important characteristic. Its economic value Žper genetic standard deviation improvement. was between 4.7 and 11.6 times larger than an improvement in the growth rate of the carapace, survival of the marron, and growth rate of the claws. In turn, growth rate of the tail was between
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0044-8486r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 9 8 . 0 0 4 8 6 - 4
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30 and 7900 times more important than the food conversion efficiency of the marron, and the characteristics associated with the broodstock, eggs and hatchlings, and juveniles. The sensitivity analysis indicated that response to selection was not sensitive to potential errors in the magnitude of the economic values. These results demonstrate that breeding programs for commercial production should concentrate on the improvement of those characteristics associated with the marron, with emphasis on the growth rate of their tail. They also suggest that the economic values are suitable for implementation in breeding programs, even when future production systems and market conditions are uncertain. The hypothesis tested in this study was supported. Profitability of commercial marron production would be increased by the development of a rapidly growing strain that has a large tail, a proportional claw size, and a high survival, food conversion efficiency, reproductive rate, and fecundity. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Cherax tenuimanus; Marron; Crayfish; Breeding objective; Selection; Animal breeding
1. Introduction The first step in the development of a breeding program for any commercial animal species is to define the breeding objective ŽDickerson, 1970; James, 1986.. The breeding objective of a species identifies the biological traits Ži.e., production characteristics. which are likely to be commercially important under future conditions of production, and estimates their relative economic values. Producers of the freshwater crayfish, marron Ž Cherax tenuimanus., are currently paid by the live weight of the marron they sell for human consumption. Hence, profitability is dependent upon marron with a rapid growth rate, and a high survival and food conversion efficiency, throughout their production cycle. Profitability is likely to be further increased by utilising marron with a high reproductive rate and fecundity, as these characteristics would reduce the number of broodstock that are maintained. In future, quality characteristics of the marron may also be commercially important. For example, marron with a large tail size may be desirable because the tail is the edible component. Similarly, marron with a proportional claw size Žrelative to carapace size. may be desirable because the claws are important for the visual presentation of marron as a gourmet item. In order to maximise the efficiency of breeding programs for commercial marron production, the relative economic importance of these production characteristics needs to be estimated. In this study, a breeding objective was defined for breeding programs which aim to genetically improve the marron used for commercial production. A profit equation was developed for commercial marron production, and expressed as function of the production characteristics associated with maintenance of the broodstock Žgrowth rate, survival, food conversion efficiency, reproductive rate, and fecundity., incubation of the eggs and hatchlings Žsurvival., rearing of the juveniles Žgrowth rate, survival, and food conversion efficiency., and grow out of the marron Žgrowth rates of the carapace, tail, and claws, survival, and food conversion efficiency.. Economic values were estimated for these characteristics when profit was set to zero, and the sensitivity of selection response to uncertainties in these estimates was analysed. The objective was to test the hypothesis that profitability of commercial marron production would be increased by the develop-
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ment of a rapidly growing strain that has a large tail, a proportional claw size, and a high survival, food conversion efficiency, reproductive rate, and fecundity.
2. Materials and methods 2.1. Production system The production system assumed was a commercial marron farm where the stock are semi-intensively managed. In such a system, broodstock marron, maintained in broodstock ponds, mate during the annual spring breeding season ŽOctober.. After mating, the females spawn and incubate their eggs for three months, during which time the eggs hatch, develop into juveniles, and are released. Following release, the broodstock are discarded, while the juveniles are transferred to juvenile ponds for a three months rearing period ŽJanuary–March.. At the end of this rearing period, the juveniles, now referred to as marron, are transferred to growout ponds, from which they are harvested after 12 months ŽApril.. Some of the harvested marron are maintained as broodstock to be bred during the upcoming breeding season in six months time. However, most are sold to a local wholesaler who markets them as a gourmet item. The growout ponds are 900 m2 , and the marron are initially stocked in these ponds at 3 marronrm2 . The juveniles are initially stocked in the juvenile ponds at 25rm2 , and the broodstock in the broodstock ponds at 2rm2 . The broodstock, juveniles, and marron are all fed detrital material plus a supplement. The detritus is established with organic material prior to the introduction of the broodstock, juveniles, and marron to their respective ponds. The supplement is a commercial pellet feed. The farm operates at maximum efficiency. No more marron can be produced by each growout pond, given the current size and management of the ponds, and the genetic makeup of the marron. In turn, only the broodstock required to produce the eggs and hatchlings, juveniles, and ultimately, the marron needed to initially stock the growout ponds, are maintained. Consequently, the size andror number of broodstock and juvenile ponds is determined by the number of broodstock and juveniles that are required. For more marron to be produced would require a proportional increase in the size andror number of the broodstock, juvenile, and growout ponds. This results in a proportional increase in the total costs of production because the size of the farm is assumed to be within a range of farm sizes where long-term average costs for each unit of production are constant and minimised Ži.e., cost economies and diseconomies of scale exist only for much smaller and larger farms.. 2.2. Sources of returns and costs Returns to producers come from the sale of harvested marron for human consumption. To produce these marron, variable and fixed costs are incurred during the maintenance, rearing, and grow out of the broodstock, juveniles, and marron.
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2.3. Production characteristics The production characteristics associated with the broodstock, eggs and hatchlings, juveniles, and marron, and assumed to have an effect on future returns and costs, were: Ø Broodstock
Ø Egg and hatchling Ø Juvenile
Ø Marron
Growth rate ŽW b . Survival Ž S b . Food conversion efficiency of the supplement feed ŽFCE b . Reproductive rate Ž R f . Fecundity Ž Ne . Survival Ž Se . Growth rate ŽWj . Survival Ž S j . Food conversion efficiency of the supplement feed ŽFCE j . Growth rate of the carapace ŽWcp . Growth rate of the tail ŽWt . Growth rate of the claws ŽWc . Survival Ž Sm . Food conversion efficiency of the supplement feed ŽFCE m .
The definitions and current commercial values of these production characteristics are presented in Table 1. The current commercial values for most of the production characteristics were either derived from Morrissy et al. Ž1995., assessed on a commercial population ŽT.B.S. Pastoral Marron farmers, Australia., or obtained from marron producers. However, no information was available for the food conversion efficiencies of the broodstock and juveniles, and survival of the eggs and hatchlings. The commercial values for the food conversion efficiencies of the broodstock and juveniles were estimated to be lower and higher than that of the marron, as this is the case for fish Že.g., Kinghorn, 1983b.. By contrast, the survival of the eggs and hatchlings was derived from the survival of eggs and hatchlings in other crayfish species ŽKurtis, 1991.. The growth rates of the carapace, tail, and claws represent a partition of marron growth rate into the growth rates of their three major body components Ži.e., summation of these individual growth rates is the growth rate of the marron.. By partitioning marron growth rate in this way, the growth rates of the carapace, tail, and claws not only determined marron growth rate and, in turn, liveweight production, but also quality characteristics of the marron. The quality characteristics considered were the ratios of tail weight to live weight, and claw weight to carapace weight. The ratio of tail weight to live weight was a measure of the edible tail component. The ratio of claw weight to carapace weight was a measure of the visual presentation of marron as a gourmet item. 2.4. Profit equation Annual profit per growout pond ŽP., expressed as a function of the production characteristics, is shown in Scheme 1. Line Ž1. is the returns from the sale of harvested
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Table 1 Definitions and current values of the production characteristics associated with commercial marron production Definition Broodstock Wb Live weight gain of the broodstock during their maintenance period Žgrbroodstock. Sb Proportion of broodstock that survive their maintenance period FCE b Ratio of liveweight gain to weight of supplement feed fed to the broodstock Rf Proportion of female broodstock that mate and spawn during the annual breeding season Ne Number of eggs spawned by each female broodstock
Value 66.1 0.90 0.27 0.80 300
Eggs and hatchlings Se Proportion of eggs and hatchlings that survive their incubation period
0.70
JuÕeniles Wj Live weight gain of the juveniles during their rearing period Žgrjuvenile. Sj Proportion of juveniles that survival their rearing period FCE j Ratio of liveweight gain to weight of supplement feed fed to the juveniles
3.0 0.70 0.36
Marron Wcp Wt Wc Sm FCE m
Live weight gain of the carapace of the marron during their growout period Žgrmarron. Live weight gain of the tail of the marron during their growout period Žgrmarron. Live weight gain of both claws of the marron during their growout period Žgrmarron. Proportion of marron that survival their growout period Ratio of liveweight gain to weight of supplement feed fed to the marron
30.7 22.2 11.4 0.87 0.29
The production characteristics are the growth rates of the broodstock ŽW b . and juveniles ŽWj ., growth rates of the carapace ŽWcp ., tail ŽWt ., and claws ŽWc . of the marron, survivals of the broodstock Ž S b ., eggs and hatchlings Ž Se ., juveniles Ž S j ., and marron Ž Sm ., food conversion efficiencies of the broodstock ŽFCE b ., juveniles ŽFCE j ., and marron ŽFCE m ., and the reproductive rate Ž R f . and fecundity Ž Ne . of the broodstock.
marron, line Ž2. is the cost to maintain the broodstock, line Ž3. is the cost to rear the juveniles, line Ž4. is the cost to grow the marron, and line Ž5. is the opportunity cost of maintaining some of the harvested marron as broodstock. Upper case letters refer to the production characteristics, and are as previously defined ŽTable 1.. Lower case letters are the prices, costs, constants, and functions associated with production. These are defined and presented in Table 2, while the derivation of the prices, costs, and constants are described in the following sections. 2.4.1. Prices The sale price of the marron is the current market price. Producers are paid by the live weight of marron they sell, with higher prices paid for larger individuals. Specifically, marron less than 40 g cannot be sold, whereas individuals between 40 and 120 g are sold for $ŽAus.25rkg, and those larger than 120 g for $ŽAus.30rkg ŽR. Greenhalgh, Marron Marketing Co-operative, Australia.. Price premiums for the ratios of tail weight to live weight, and claw weight to carapace weight, were estimated after consultation with a major consumer of marron ŽLamont Winery and Restaurant, Millendon, Australia.. This consumer was prepared to pay a price premium for marron that have a larger tail weight to live weight ratio than those currently used for commercial production. They also preferred marron with a claw weight to carapace weight ratio of approximately 0.28, as this was considered optimal for the visual presentation of marron.
M. Henryon et al.r Aquaculture 173 (1999) 179–195
Scheme 1.
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Table 2 Description and values of the prices, costs, constants, and functions associated with commercial marron production
Prices pw1 pw2 pt pc
Costs c food_b c food_ j c food_m cb cj cm c fixed_b c fixed_ j c fixed_m
Description
Value
Sale price of marron Ž$Ausrg. with a live weight between 40 and 120 g Sale price of marron Ž$Ausrg. with a live weight larger than 120 g Price premium for a unit increase in the tail weight to live weight ratio of marron Ž$Ausrunitrg marron live weight. Price premium for a unit increase in the claw weight to carapace weight ratio of marron Ž$Ausrunitrg marron live weight.
0.025 0.030 0.0725
Cost of the supplement feed fed to the broodstock Ž$Ausrg. Cost of the supplement feed fed to the juveniles Ž$Ausrg. Cost of the supplement feed fed to the marron Ž$Ausrg. Variable costs incurred during the maintenance of the broodstock Ž$Ausrbroodstock. Variable costs incurred during the rearing of the juveniles Ž$Ausrjuvenile. Variable costs incurred during the grow out of the marron Ž$Ausrmarron. Fixed costs of production allocated to the broodstock Ž$Ausrbroodstock. Fixed costs of production allocated to the juveniles Ž$Ausrjuvenile. Fixed costs of production allocated to the marron Ž$Ausrmarron.
Constants nf Number of female marron harvested from each growout pond and maintained as broodstock to produce the eggs and hatchlings, juveniles, and ultimately, the marron required to initially stock each growout pond m Factor used to represent the number of male broodstock maintained for breeding. The product of m and n f is the total number of broodstock sw2 Live weight variance of the harvested marron mt Ratio of tail weight to live weight of the marron where no price premium is paid mc Ratio of claw weight to carapace weight of marron where no price premium is paid Functions k Ž a, Wj qWcp qWt qWc , sw . Proportion of harvested marron which have a live weight less than a grams, i.e., k Ž a,Wj qWcp qWt qWc , sw . s p Ž x - a < Wj qWcp qWt qWc , sw . s
F
ž
ay Ž Wj qWcp qWt qWc .
sw
/
where F Ž.. is the standard cumulative normal distribution, with mean Wj qWcp qWt qWc , variance sw2 , and a is 40 or 120 hŽ a, b, Wj qWcp qWt qWc , sw . Mean live weight Žg. of harvested marron which have a live weight between a and b grams, i.e., h Ž a,b,Wj qWcp qWt qWc , sw . s E Ž x < aF= - b . s
Hab x.E HabE
Ž x .d x Ž x .d x
where EŽ x . is the normal density ; N ŽWj qWcp qWt qWc , sw2 . with mean Wj qWcp qW1 qWc , variance sw2 , and Ž a, b . is Ž40, 120. or Ž120, `.
y0.0111
0.0005 0.0007 0.0006 0.6731 0.0405 0.6834 0.4766 0.0323 0.1702
25.4
1.5 407.2 0.33 0.37
0.088 or 0.996
70.5 or 126.3
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The estimate of a price premium for the ratio of tail weight to live weight was based on the change that an increase in this ratio had on the weight of tail meat produced by a unit live weight of marron. Considering the current live weight, tail weight, and sale price of harvested marron presented in Tables 1 and 2, and assuming that the marron are sold for the consumption of their tail meat, tail meat was calculated to be worth $ŽAus.0.072rg. By contrast, the price premium for the ratio of claw weight to carapace weight was estimated after the consumer was prepared to pay an extra $ŽAus.1.00rkg of marron live weight for individuals with a claw weight to carapace ratio of 0.28, instead of the current ratio of 0.37. 2.4.2. Costs The costs were those currently incurred by commercial producers. The supplement feed of the juveniles was assumed to be more expensive than the supplement feed of the marron and, in turn, the broodstock, because younger marron have a higher protein requirement Žc.f. Goddard, 1988.. The variable costs of the broodstock, juveniles, and marron were the costs to prepare their respective ponds prior to stocking, and the daily costs incurred during their maintenance, rearing, and growout periods. The broodstock also incurred the cost to transfer the juveniles from the broodstock ponds to the juvenile ponds, while the juveniles incurred a cost to be transferred from the juvenile ponds to the growout ponds. Similarly, the marron incurred a cost to be harvested, processed, transported to the local wholesaler, and marketed by the wholesaler. Other variable costs, incurred during the operation of the whole farm Ži.e., general maintenance, environmental taxes, consultation fees, accounting, and telephone., were allocated to the broodstock, juveniles, and marron by a weighting factor. The weighting factors for the broodstock, juveniles, and marron were the product of their pond sizes and the proportion of the annual production cycle during which they are maintained, reared, or grown. Broodstock, juveniles, and marron that died were assumed to incur half the variable costs of surviving broodstock, juveniles, and marron. The fixed costs of the broodstock, juveniles, and marron were the capital costs of production and the farm’s normal profit. Capital costs were assumed to be variable in the long-term. Those allocated to the broodstock, juveniles, and marron were the capital costs incurred during their respective maintenance, rearing, and growout periods Že.g., cost of the broodstock, juvenile, and marron ponds.. Other capital costs incurred during the operation of the whole farm Ži.e., land, buildings, machinery, water recirculation system, vehicles, alarm, and farming license., as well as the farm’s normal profit, were allocated to the broodstock, juveniles, and marron by the same weighting factor used to allocate the variable costs incurred during the operation of the whole farm. Fixed costs were allocated to all broodstock, juveniles, and marron, regardless of whether they survived the maintenance, rearing, and growout periods. The farm’s normal profit was the imputed returns to capital and risk taking required for the producer to stay in business in the long-term ŽLipsey and Steiner, 1981.. Including the farm’s normal profit as a cost of production set the profit function equal to zero, which corresponds to economic theory where the average long-term profit of an industry at equilibrium is zero. Under a situation of zero profit, the relative economic values derived from the profit equation are the same for all perspectives of the industry
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Že.g., producer, wholesaler, or consumer perspective. and bases of production Že.g., per marron, pond, or farm. ŽBrascamp et al., 1985; Smith et al., 1986.. These economic values are also rescaled against changes in the size of the production system Že.g., pond size andror number. and, as such, represent an increase in the efficiency of production from a genetic improvement in each of the production characteristics. The relative economic values derived from the profit equation also apply when production is adapted to quota. In future, quotas could be imposed to protect natural waterways from production effluent by restricting either the live weight or number of marron produced, number andror size of the growout ponds, amount of water usage, or amount of supplement feed fed. Such quotas are a constraint on total outputs or inputs of production, and result in the same relative economic values as those derived when the profit equation is set to zero ŽSmith et al., 1986.. 2.4.3. Constants The constants used in the profit equation emulate current commercial conditions. The number of female marron maintained as broodstock was derived using the reproductive rate and fecundity of the broodstock, and the survivals of the broodstock, eggs and hatchlings, juveniles, and marron. The factor used to represent the number of male broodstock maintained was obtained from marron producers. The live weight variance of the harvested marron was derived from Morrissy et al. Ž1995.. The ratios of tail weight to live weight, and claw weight to carapace weight, where no price premiums are paid, were set at the ratios of those marron currently used for commercial production. This equates to the current commercial situation where returns are dependent only on the live weight of the marron. 2.5. DeriÕation of economic Õalues The economic value of a production characteristic was calculated as the partial derivative of the profit equation with respect to that characteristic, evaluated with all other characteristics maintained at their current commercial values. However, survivals of the broodstock, eggs and hatchlings, juveniles, and marron, and reproductive rate of the broodstock, were treated as threshold characteristics. Each was assumed to be a discrete characteristic with an underlying liability, such that, Si s F Ž l i . where Si is the survival of the broodstock, eggs and hatchlings, juveniles, or marron, or the reproductive rate of the broodstock, and l i is the underlying liability of Si , normally distributed with mean zero and variance one. The economic value of a threshold characteristic was calculated as the partial derivative of the profit equation with respect to its underlying liability. The economic values for the production characteristics were presented as the increase in profit resulting from an increase in each characteristic of one genetic standard deviation Žor one genetic standard deviation in the underlying liability of the threshold characteristics.. The genetic variance of each characteristic was calculated from their estimates of heritability and phenotypic variance presented in Table 4 Žreference sources for these estimates are described in Section 2.6..
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2.6. SensitiÕity analysis The sensitivity of selection response to potential errors in the estimated economic values was analysed by assuming that the estimates were in error of their true values. It was assumed that the estimated economic value for each production characteristic was in error of it’s true value by a factor of 0, 0.5, 1.1, 1.5, 2 and 5, while the estimated values for the other characteristics were considered to be their true values. Sensitivity was then the relative response in the true aggregate breeding value from selection on indices derived from the estimated and true economic values. Specifically, the aggregate breeding values derived from the estimated economic values Ž Hu ., and those assumed to be the true values Ž Ht ., were defined: Hu s ÕuX g and Ht s ÕtX g where Õu is the vector of absolute economic values estimated in this study for each of the production characteristics defined previously, Õt is a vector of absolute economic values, assumed to be the true values, for each of the production characteristics, and g is the vector of additive genetic values for each of the production characteristics Žor the additive genetic values for the underlying liability of those characteristics treated as threshold characteristics.. The selection indices derived from Hu and Ht Ž Iu and It . were: Iu s buX x and It s btX x where the vector of selection criteria, x, are phenotypic measures of the production characteristics obtained from the selection candidates or their relatives, and are defined in Table 3. The vectors of the index coefficients for each of these criteria, bu and bt , are those which maximise the correlation between their respective selection indices and aggregate breeding values, and were calculated as: b k s Py1 G Õk where k is u or t, P is the matrix of phenotypic Žco. variances among the selection criteria in the selection index, G is the matrix of genetic Žco. variances among the selection criteria in the selection index and the production characteristics in the aggregate breeding value, and Õ k , is the vector of economic values for the production characteristics in the aggregate breeding value. Predicted gain in Ht , resulting from one round of selection on the index Ik Ž k s u,t., was calculated as: R k s irH t Ik sH t where R k is the predicted gain in Ht , i is the standardised selection differential, rH t Ik is the correlation between Ik and Ht , and sH2t is the variance of Ht . The sensitivity Ž S . of response in Ht resulting from selection on Iu , relative to selection on It , was expressed by: S s R urR t and calculated as Žafter Smith, 1983.: Ss
ÕuX GX Py1 G Õt X u
X
(Õ G P
y1
G Õu ÕtX GX Py1 G Õt
(
.
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Table 3 Definitions of the selection criteria used to assess the sensitivity of selection response to potential errors in economic values estimated for the production characteristics associated with commercial marron production Selection criteria
Definition
xWb
Mean of broodstock growth rate obtained from five full-siblings of the selection candidate’s dam Žincludes the selection candidate’s dam.. These individuals were also assessed for x FC E b and x l R f . Mean of juvenile growth rate obtained from 30 full-siblings of the selection candidate Žincludes the selection candidate.. These individuals were also assessed for x FC E . j Mean of marron growth rate obtained from five full-siblings of the selection candidate Žexcludes the selection candidate.. These individuals were also assessed for x W , x W , j t x W c, x FCE j , and x FCE m. Mean tail weight of the marron obtained from five full-siblings of the selection candidate Žexcludes the selection candidate.. These individuals were also assessed for x W , x W , j cp x W c, x F C E j , and x F C E m. Mean claw weight of the marron obtained from five full-siblings of the selection candidate Žexcludes the selection candidate.. These individuals were also assessed for x W , x W , j cp x W t, x FCE j , and x FCE m. Mean liability of broodstock survival obtained from five full-sibling equivalents of the selection candidate’s dam Žexcludes the selection candidate’s dam.. Mean liability of egg and hatchling survival obtained from 50 full-sibling equivalents of the selection candidate Žexcludes the selection candidate.. Mean liability of juvenile survival obtained from 20 full-sibling equivalents of the selection candidate Žexcludes the selection candidate.. Mean liability of marron survival obtained from ten full-sibling equivalents of the selection candidate Žexcludes the selection candidate.. These individuals were also assessed for x W j and x FC E j . Mean of broodstock food conversion efficiency obtained from five full-siblings of the selection candidate’s dam Žincludes the selection candidate’s dam.. These individuals were also assessed for x W b and x l R f . Mean of juvenile food conversion efficiency obtained from 30 full-siblings of the selection candidate Žincludes the selection candidate.. These individuals were also assessed for x W j . Mean of marron food conversion efficiency obtained from 20 full-siblings of the selection candidate Žincludes the selection candidate.. These individuals were also assessed for x W j and x F C E j . Mean liability of reproduction rate obtained from five full-sibling equivalents of the selection candidate’s dam Žincludes the selection candidate’s dam.. These individuals were also assessed for x W b and x FCE b. Fecundity of the selection candidate’s dam. The selection candidate’s dam was also assessed for x W b, x FCE b, and x l R f .
xWj x W cp
xWt
xWc
x lS b x lSe x lS j x lS m
x FC E b
x FC E j x FC E m
xlRf
x Ne
The phenotypic and genetic parameters assumed are presented in Table 4. Estimates involving the growth rates and survivals were obtained from Henryon Ž1995.. For the remaining characteristics, there was no information available for marron, or other crayfish species. Hence, these parameters were derived from estimates for commercial fish species Že.g., Gjedrem, 1983; Gjedrem, 1992; Gjerde, 1986; Gjerde et al., 1994; Huang and Gall, 1990; Kanis et al., 1976; Kinghorn, 1983; Kinghorn, 1983b; Rye et al., 1990; Su et al., 1997.. However, there were many genetic correlations, particularly those involving food conversion efficiency, where no estimates were available for fish species.
190 Table 4 Estimates of heritability, phenotypic variance, and phenotypic and genetic correlations, for the production characteristics associated with commercial marron production Wb 0.35 393.2
Wj
Wcp
Wt
Wc
0.05 0.81
0.38 129.2
0.42 67.3
0.15 21.7
0.05 1
0.02 1
0.02 1
0.03 1
0.30
0.50 0.38
0.46 0.36 0.95
0.30 0.15 0.72 0.59
y0.07 y0.02 y0.10 y0.08 y0.04
0 y0.04 y0.01 y0.01 0 0.03
y0.01 y0.10 y0.05 y0.04 y0.01 0.05 0.15
y0.04 y0.05 y0.12 y0.10 y0.05 0.18 0.08 0.20
0.38 0.36 0.15
0.95 0.80
0.74
0.15 0.30
0.14 0.28
Sb
Se
Sj
Sm
y0.05
FCE b 0.05 0.0010 0.25 0.05 0.18 0.17 0.10 y0.03 0 0 y0.01
0.25 0.30 0.15 0.15 0.25
0.08 0.15
FCE j 0.03 0.0008 0.05 0.30 0.15 0.14 0.08 0 y0.01 y0.02 y0.01 0.25
y0.01 0.45 0.03 0.08
FCE m 0.05 0.0012 0.18 0.15 0.30 0.28 0.15 y0.01 0 y0.01 y0.03 0.45 0.45
Rf
Ne 0.10 1
0.20 5638
0.15 0.02 0.10 0.10 0.05 y0.05 0 y0.01 y0.03 0.03 0 0.01
0.25 0.05 0.10 0.10 0.05 y0.10 0 y0.02 y0.05 0.08 0.01 0.05 0.10
0.10
The production characteristics are the growth rates of the broodstock ŽW b . and juveniles ŽWj ., growth rates of the carapace ŽWcp ., tail ŽWt ., and claws ŽWc . of the marron, survivals of the broodstock Ž S b ., eggs and hatchlings Ž Se ., juveniles Ž S j ., and marron Ž Sm ., food conversion efficiencies of the broodstock ŽFCE b ., juveniles ŽFCE j ., and marron ŽFCE m ., and the reproductive rate Ž R f . and fecundity Ž Ne . of the broodstock. Phenotypic correlations are below, and genetic correlations above, the diagonal. a S b , Se , S j , Sm , and R f are treated as threshold characteristics which have an underlying liability, normally distributed with mean zero and variance one. W b , Wj , Wcp , Wt , and Wc are measured in grams, FCE b , FCE j , and FCE m are ratios, and Ne is a count.
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Heritability Phenoytpic Variancea Wb Wj Wcp Wt Wc Sb Se Sj Sm FCE b FCE j FCE m Rf Ne
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191
These parameters were estimated by assuming genetic correlations between characteristics were of the same sign and similar magnitude as their phenotypic correlations Žafter Koots and Gibson, 1996., and that correlations between characteristics expressed at the same chronological age were larger than correlations between characteristics expressed at different chronological ages. The phenotypic and genetic Žco. variance matrices resulting from these parameter estimates were positive-definite. 3. Results Profit was increased by a genetic increase in the growth rate of the juveniles, growth rates of the carapace and tail of the marron, survivals of the broodstock, eggs and hatchlings, juveniles, and marron, food conversion efficiencies of the broodstock, juveniles, and marron, and the reproductive rate and fecundity of the broodstock ŽTable 5.. By contrast, profit was decreased by a genetic increase in the growth rate of the broodstock, and a genetic increase in the growth rate of the claws of the marron, given that there was a smaller claw weight to carapace weight ratio which was ideal. The production characteristics associated with the marron, with the exception of food conversion efficiency, were the most economically important. Of these, growth rate of the tail had the largest economic value. A genetic improvement Žone genetic standard
Table 5 Economic value estimates for the production characteristics associated with commercial marron production Production characteristic
Economic value
Wt Wcp Sm Wc Sj Ne Rf Se Wj FCE m Sb Wb FCE j FCE b
941.50 Ž177.06rgrmarron. 200.53 Ž28.62rgrmarron. 105.10 Ž606.81runderlying liability unit. y81.43 Žy45.08rgrmarron. 31.37 Ž221.83runderlying liability unit. 21.61 Ž0.64reggrfemale broodstock. 21.39 Ž67.65runderlying liability unit. 13.58 Ž96.02runderlying liability unit. 13.18 Ž65.51rgrjuvenile. 8.88 Ž1125.42runit. 6.02 Ž26.92runderlying liability unit. y0.79 Žy0.07rgrbroodstock. 0.27 Ž53.98runit. 0.12 Ž16.45runit.
The production characteristics are the growth rates of the broodstock ŽW b . and juveniles ŽWj ., growth rates of the carapace ŽWcp ., tail ŽWt ., and claws ŽWc . of the marron, survivals of the broodstock Ž S b ., eggs and hatchlings Ž Se ., juveniles Ž S j ., and marron Ž Sm ., food conversion efficiencies of the broodstock ŽFCE b ., juveniles ŽFCE j ., and marron ŽFCE m ., and the reproductive rate Ž R f . and fecundity Ž Ne . of the broodstock. Economic values are presented as the increase in annual profit ŽA$rper marron growout pond. resulting from an increase in each characteristic of one genetic standard deviation. However, S b , Se , S j , Sm , and R f were treated as threshold characteristics, and their economic values are the increase in profit resulting from a genetic standard deviation increase in their underlying liability. Values in parenthesis are absolute values, and represent the increase in profit from a unit increase in each of the characteristics.
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Table 6 Sensitivity Ž%. of selection response when selection was on an index derived from economic values estimated for the production characteristics associated with commercial marron production, but the estimates for each characteristic were assumed to be in error of their true values by a factor of 0, 0.5, 1.1, 1.5, 2, and 5
0 0.5 1.1 1.5 2 5
Wb
Wj
Wcp
100 100 100 100 100 100
100 100 100 100 100 99.9
99.9 93.9 99.9 100 99.9 100 100 100 100 100 100 100 100 100 99.9 99.7 99.9 98.2
Wt
Wc
Sb
Se
Sj
100 100 100 100 100 100
100 100 100 100 100 99.9
100 99.9 100 100 100 100 100 100 100 99.9 99.8 98.3
Sm
FCE b FCE j FCE m
Rf
Ne
100 100 100 100 100 100
100 100 100 100 100 100
100 100 100 100 100 99.9
100 100 100 100 100 100
100 100 100 100 100 100
The production characteristics are the growth rates of the broodstock ŽW b . and juveniles ŽWj ., growth rates of the carapace ŽWcp ., tail ŽWt ., and claws ŽWc . of the marron, survivals of the broodstock Ž S b ., eggs and hatchlings Ž Se ., juveniles Ž S j ., and marron Ž Sm ., food conversion efficiencies of the broodstock ŽFCE b ., juveniles ŽFCE j ., and marron ŽFCE m ., and the reproductive rate Ž R f . and fecundity Ž Ne . of the broodstock.
deviation. in the growth rate of the tail was worth between 4.7 and 11.6 times more than an improvement in the growth rate of the carapace, survival of the marron, and growth rate of the claws ŽTable 5.. By comparison, food conversion efficiency of the marron, and the production characteristics associated with the broodstock, eggs and hatchlings, and juveniles were of minor importance. A genetic improvement in the growth rate of the tail was worth 30 times more than an improvement in juvenile survival, 44 times more than an improvement in the reproductive rate and fecundity of the broodstock, and between 69 and 7900 times more than an improvement in the growth rates of the broodstock and juveniles, an improvement in the survivals of the broodstock and eggs and hatchlings, and an improvement in the food conversion efficiencies of the broodstock, juveniles, and marron. Response to selection was not sensitive to potential errors in the magnitude of the estimated economic values. When the estimated value for each production characteristic was assumed to be in error by between zero and five times its true value, selection response was at least 98% as high as the response obtained using an index derived from the true values ŽTable 6.. The only exception occurred when growth rate of the tail of the marron was assumed to have a true economic value of zero. Including growth rate of the tail in the breeding objective with its estimated economic value, should it have a true value of zero, was predicted to decrease selection response by 6%. 4. Discussion It was hypothesised that the profitability of commercial marron production would be increased by the development of a rapidly growing strain that has a large tail, a proportional claw size, and a high survival, food conversion efficiency, reproductive rate, and fecundity. This was shown to be the case, the only exception being that a genetic increase in the growth rate of the broodstock caused a small decrease in profit. It was also shown that the production characteristics associated with the marron, with the exception of food conversion efficiency, were the most economically important. Of these, growth rate of the tail was by far the most important, having an economic value
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between 4.7 and 11.6 times larger than the growth rate of the carapace, survival of the marron, and growth rate of the claws. The importance of the characteristics associated with the marron was of little surprise considering that these characteristics largely determined the returns to production, most of the production costs were incurred during the grow out of the marron, andror these characteristics tended to exhibit a high level of genetic variation. By comparison, food conversion efficiency of the marron, and those characteristics associated with the broodstock, eggs and hatchlings, and juveniles were of minor importance as it was inexpensive to feed the marron, maintain the broodstock, and rear the juveniles, and many of these characteristics tended to exhibit a low level of genetic variation. These results demonstrate that breeding programs for commercial marron production should concentrate on the genetic improvement of those characteristics associated with the marron. In particular, emphasis should be to increase the growth rate of the marron through increases in the growth rate of their tail. The breeding objective was, strictly speaking, peculiar to the production system defined, the current commercial values of the production characteristics, and the prices and costs specified. Both the production characteristics of commercial importance, and their economic values, could vary between individual farms. Of particular interest, was the growth rate of the broodstock, and the growth rates of the carapace, tail, and claws of the marron. A genetic increase in the growth rate of the broodstock was shown to decrease profit by a small amount, presumably because faster growing broodstock incur higher feeding costs. However, it was assumed that the broodstock were discarded following the breeding season. Certainly, the economic value of this characteristic could change, in both direction and magnitude, should the broodstock be sold for human consumption. The relative economic importance of the growth rates of the carapace, tail, and claws remains uncertain. It is clear that each of these characteristics is economically important, as they are the components of marron growth rate and, in turn, liveweight production, upon which the value of marron is based. However, differences between the economic values derived for the growth rates of the carapace, tail, and claws were due to additional price premiums paid for the ratios of tail weight to live weight, and claw weight to carapace weight. These price premiums were obtained after consultation with a particular consumer who had a preference for marron with a large tail weight to live weight ratio, and specific claw weight to carapace ratio. Yet, it is conceivable that some consumers may not be prepared to pay additional price premiums for these characteristics, while others may differ in their perception of ideal ratios and the price premiums they are prepared to pay. Despite this uncertainty, the differences between the economic values derived for the growth rates of the carapace, tail, and claws do serve notice that any quality characteristic included in the breeding objective will most likely be economically important, as they provide returns over and above liveweight production. For marron, other quality characteristics may include palatability, and shell and meat colour. Response to selection was insensitive to errors in the magnitude of the economic values because most of the production characteristics were favourably correlated, both genetically and phenotypically. Consequently, most of the production characteristics were simultaneously improved by selection, and errors in the magnitude of the economic
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values only caused minimal reductions in economic returns. Similar results were also found in other sensitivity studies ŽPease et al., 1967; Rønningen, 1971; Vandepitte and Hazel, 1977; Smith, 1983.. The minor exception, of course, was predicted when growth rate of the tail was included in the breeding objective with its estimated economic value, but its true value was assumed to be zero. Because growth rate of the tail was estimated to be the most economically important characteristic, selection emphasis was placed on its improvement when it was actually of no economic value. Smith Ž1983. also highlighted the sensitivity of selection response when a characteristic is considered to be of major importance, but actually has no true value. However, in practice, it is unlikely that growth rate of the tail will be of no economic value. The insensitivity of selection response has two applications for marron breeding programs. First, the economic values estimated in this study should be suitable for implementation in these programs, even when future production systems and market conditions are uncertain. Second, the production characteristics estimated to be of minor importance could be assumed to have zero economic value and removed from the breeding objective. Decreasing the number of characteristics in the breeding objective decreases the number of genetic parameters which need to be estimated, increases the probability of obtaining estimates within the parameter space, and increases the accuracy of predicted response ŽHill and Thompson, 1978.. However, it must be remembered that the sensitivity analysis was carried out conditional on a selection index where phenotypic measures of each production characteristic were obtained from the selection candidates or their relatives, and the magnitude of only one economic value was assumed to be in error of its true value. Selection response may be sensitive should indices with different selection criteria be established, andror more than one economic value be in error. The accuracy of the sensitivity analysis was largely dependent upon the phenotypic and genetic parameters used in this study. How well these parameters apply to marron populations is an obvious proviso to the sensitivity of selection response to errors in the economic values. Although the parameters used were derived from the best available sources, there is little information available for marron. Consequently, many of the parameter estimates were unreliable. This study defined a breeding objective for commercial marron production by identifying the production characteristics which influence returns and costs to production, and estimating their relative economic values. Before these estimates are chosen as those implemented in breeding programs for marron, a more thorough analysis should be conducted on the sensitivity of selection response to potential errors in the estimates. However, the evidence to date indicates that selection response is largely insensitive to errors, justifying the use of these economic values in the development of breeding programs for marron. Acknowledgements This study was funded by the Faculty of Agriculture at the University of Western Australia, Australia, and the Danish Agricultural and Veterinary Research Council, Denmark.
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References Brascamp, E.W., Smith, C., Guy, D.R., 1985. Derivation of economic weights from profit equations. Anim. Prod. 40, 175–180. Dickerson, G.E., 1970. Efficiency of animal production—molding the biological components. J. Anim. Sci. 30, 849–859. Gjedrem, T., 1983. Genetic variation in quantitative traits and selective breeding in fish and shellfish. Aquaculture 33, 51–72. Gjedrem, T., 1992. Breeding plans for rainbow trout. Aquaculture 100, 73–83. Gjerde, B., 1986. Growth and reproduction in fish and shellfish. Aquaculture 57, 37–55. Gjerde, B., Simianer, H., Refstie, T., 1994. Estimates of genetic and phenotypic parameters for body weight, growth rate and sexual maturity in Atlantic salmon. Livest. Prod. Sci. 38, 133–143. Goddard, J.S., 1988. Food and Feeding. In: Holdich, D.M., Lowery, R.S. ŽEds.., Freshwater Crayfish. Biology, Management and Exploitation. Croom Helm, London, pp. 145–167. Henryon, M., 1995. Genetic variation in wild marron can be used to develop an improved commercial strain. Ph.D Thesis, University of Western Australia, Perth, Australia, 128 pp. James, J.W., 1986. Economic evaluation of breeding objectives in sheep and goats—general considerations. Proc. 3rd Wld. Congr. Genet. Appl. Livest. Prod. IX, pp. 470–478. Hill, W.G., Thompson, R., 1978. Probabilities of non-positive definite between-group or genetic covariance matrices. Biometrics 34, 429–439. Huang, N., Gall, G.A.E., 1990. Correlation of body weight and reproductive characteristics in rainbow trout. Aquaculture 86, 191–200. Kanis, E., Refstie, T., Gjedrem, T., 1976. A genetic analysis of egg, alevin and fry mortality in salmon Ž Salmo salar ., sea trout Ž Salmo trutta. and rainbow trout Ž Salmo gairdneri .. Aquaculture 8, 259–268. Kinghorn, B.P., 1983. A review of quantitative genetics in fish breeding. Aquaculture 31, 283–304. Kinghorn, B., 1983b. Genetic variation in food conversion efficiency and growth in rainbow trout. Aquaculture 32, 141–155. Kurtis, A.M., 1991. A review of patterns and causes of crustacean brood mortality. In: Wenner, A., Kurtis, A. ŽEds.., Crustacean Egg Production. A.A. Balkema, Rotterdam, pp. 117–141. Lipsey, R.G., Steiner, P.O., 1981. Economics, 6th edn. Harper and Row, New York, 958 pp. Koots, K.R., Gibson, J.P., 1996. Realized sampling variances of estimates of genetic parameters and the difference between genetic and phenotypic correlations. Genetics 143, 1409–1416. Morrissy, N.M., Walker, P., Moore, W., 1995. Predictive equations for managing semi-intensive grow-out of a freshwater crayfish Žmarron., Cherax tenuimanus ŽSmith 1912. ŽDecapoda: Parastacidae., on a commercial farm. Aquacult. Res. 26, 71–80. Pease, A.H.R., Cook, G.L., Greig, M., Cuthbertson, A., 1967. Combined testing. Report DA 188. Pig Industry Development Authority, Hitchin, Herts., England, 41 pp. Rye, M., Lillevik, K.M., Gjerde, B., 1990. Survival in early life of Atlantic salmon and rainbow trout: estimates of heritabilities and genetic correlations. Aquaculture 89, 209–216. Rønningen, K., 1971. Tables for estimating the loss in efficiency when selecting according to an index based on a false economic ratio between two traits. Acta Agr. Scand. 21, 33–49. Smith, C., 1983. Effects of changes in economic weights on the efficiency of index selection. J. Anim. Sci. 56, 1057–1064. Smith, C., James, J.W., Brascamp, E.W., 1986. On the derivation of economic weights in livestock improvement. Anim. Prod. 43, 545–551. Su, G.-S., Liljedahl, L.-E., Gall, G.A.E., 1997. Genetic and environmental variation of female reproductive traits in rainbow trout Ž Oncorhynchus mykiss .. Aquaculture 154, 115–124. Vandepitte, W.M., Hazel, L.N., 1977. The effect of errors in the economic weights on the accuracy of selection indexes. Ann. Genet. Select. Anim. 9, 87–103.
Definition of a breeding objective for commercial production of the freshwater crayfish, marron žCherax tenuimanus / Mark Henryon a
a,)
, Ian W. Purvis b, Peer Berg
a
Danish Institute of Agricultural Sciences, Department of Breeding and Genetics, Research Centre Foulum, P.O. Box 50, 8830 Tjele, Denmark b CSIRO DiÕision of Animal Production, Pastoral Research Laboratory, Armidale, NSW 2350, Australia Accepted 14 October 1998
Abstract This study tested the hypothesis that profitability of commercial marron production would be increased by the development of a rapidly growing strain that has a large tail, a proportional claw size, and a high survival, food conversion efficiency, reproductive rate, and fecundity. A profit equation was developed for commercial marron production, and expressed as function of the production characteristics associated with maintenance of the broodstock Žgrowth rate, survival, food conversion efficiency, reproductive rate, and fecundity., incubation of the eggs and hatchlings Žsurvival., rearing of the juveniles Žgrowth rate, survival, and food conversion efficiency., and grow out of the marron Žgrowth rates of the carapace, tail, and claws, survival, and food conversion efficiency.. Economic values were estimated for these characteristics when profit was set to zero, and the sensitivity of selection response to potential errors in these estimates was analysed. The results showed that profit was increased by a genetic increase in the growth rate of the juveniles, growth rates of the carapace and tail of the marron, survivals of the broodstock, eggs and hatchlings, juveniles, and marron, food conversion efficiencies of the broodstock, juveniles, and marron, and the reproductive rate and fecundity of the broodstock. By contrast, profit was decreased by a genetic increase in the growth rate of the broodstock, and a genetic increase in the growth rate of the claws of the marron, given that there was a smaller claw weight to carapace weight ratio that was ideal. Growth rate of the tail of the marron was the most economically important characteristic. Its economic value Žper genetic standard deviation improvement. was between 4.7 and 11.6 times larger than an improvement in the growth rate of the carapace, survival of the marron, and growth rate of the claws. In turn, growth rate of the tail was between
)
Corresponding author. Tel.: q45-89-99-1220; Fax: q45-89-99-1300; E-mail: [email protected]
0044-8486r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 9 8 . 0 0 4 8 6 - 4
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30 and 7900 times more important than the food conversion efficiency of the marron, and the characteristics associated with the broodstock, eggs and hatchlings, and juveniles. The sensitivity analysis indicated that response to selection was not sensitive to potential errors in the magnitude of the economic values. These results demonstrate that breeding programs for commercial production should concentrate on the improvement of those characteristics associated with the marron, with emphasis on the growth rate of their tail. They also suggest that the economic values are suitable for implementation in breeding programs, even when future production systems and market conditions are uncertain. The hypothesis tested in this study was supported. Profitability of commercial marron production would be increased by the development of a rapidly growing strain that has a large tail, a proportional claw size, and a high survival, food conversion efficiency, reproductive rate, and fecundity. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Cherax tenuimanus; Marron; Crayfish; Breeding objective; Selection; Animal breeding
1. Introduction The first step in the development of a breeding program for any commercial animal species is to define the breeding objective ŽDickerson, 1970; James, 1986.. The breeding objective of a species identifies the biological traits Ži.e., production characteristics. which are likely to be commercially important under future conditions of production, and estimates their relative economic values. Producers of the freshwater crayfish, marron Ž Cherax tenuimanus., are currently paid by the live weight of the marron they sell for human consumption. Hence, profitability is dependent upon marron with a rapid growth rate, and a high survival and food conversion efficiency, throughout their production cycle. Profitability is likely to be further increased by utilising marron with a high reproductive rate and fecundity, as these characteristics would reduce the number of broodstock that are maintained. In future, quality characteristics of the marron may also be commercially important. For example, marron with a large tail size may be desirable because the tail is the edible component. Similarly, marron with a proportional claw size Žrelative to carapace size. may be desirable because the claws are important for the visual presentation of marron as a gourmet item. In order to maximise the efficiency of breeding programs for commercial marron production, the relative economic importance of these production characteristics needs to be estimated. In this study, a breeding objective was defined for breeding programs which aim to genetically improve the marron used for commercial production. A profit equation was developed for commercial marron production, and expressed as function of the production characteristics associated with maintenance of the broodstock Žgrowth rate, survival, food conversion efficiency, reproductive rate, and fecundity., incubation of the eggs and hatchlings Žsurvival., rearing of the juveniles Žgrowth rate, survival, and food conversion efficiency., and grow out of the marron Žgrowth rates of the carapace, tail, and claws, survival, and food conversion efficiency.. Economic values were estimated for these characteristics when profit was set to zero, and the sensitivity of selection response to uncertainties in these estimates was analysed. The objective was to test the hypothesis that profitability of commercial marron production would be increased by the develop-
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ment of a rapidly growing strain that has a large tail, a proportional claw size, and a high survival, food conversion efficiency, reproductive rate, and fecundity.
2. Materials and methods 2.1. Production system The production system assumed was a commercial marron farm where the stock are semi-intensively managed. In such a system, broodstock marron, maintained in broodstock ponds, mate during the annual spring breeding season ŽOctober.. After mating, the females spawn and incubate their eggs for three months, during which time the eggs hatch, develop into juveniles, and are released. Following release, the broodstock are discarded, while the juveniles are transferred to juvenile ponds for a three months rearing period ŽJanuary–March.. At the end of this rearing period, the juveniles, now referred to as marron, are transferred to growout ponds, from which they are harvested after 12 months ŽApril.. Some of the harvested marron are maintained as broodstock to be bred during the upcoming breeding season in six months time. However, most are sold to a local wholesaler who markets them as a gourmet item. The growout ponds are 900 m2 , and the marron are initially stocked in these ponds at 3 marronrm2 . The juveniles are initially stocked in the juvenile ponds at 25rm2 , and the broodstock in the broodstock ponds at 2rm2 . The broodstock, juveniles, and marron are all fed detrital material plus a supplement. The detritus is established with organic material prior to the introduction of the broodstock, juveniles, and marron to their respective ponds. The supplement is a commercial pellet feed. The farm operates at maximum efficiency. No more marron can be produced by each growout pond, given the current size and management of the ponds, and the genetic makeup of the marron. In turn, only the broodstock required to produce the eggs and hatchlings, juveniles, and ultimately, the marron needed to initially stock the growout ponds, are maintained. Consequently, the size andror number of broodstock and juvenile ponds is determined by the number of broodstock and juveniles that are required. For more marron to be produced would require a proportional increase in the size andror number of the broodstock, juvenile, and growout ponds. This results in a proportional increase in the total costs of production because the size of the farm is assumed to be within a range of farm sizes where long-term average costs for each unit of production are constant and minimised Ži.e., cost economies and diseconomies of scale exist only for much smaller and larger farms.. 2.2. Sources of returns and costs Returns to producers come from the sale of harvested marron for human consumption. To produce these marron, variable and fixed costs are incurred during the maintenance, rearing, and grow out of the broodstock, juveniles, and marron.
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2.3. Production characteristics The production characteristics associated with the broodstock, eggs and hatchlings, juveniles, and marron, and assumed to have an effect on future returns and costs, were: Ø Broodstock
Ø Egg and hatchling Ø Juvenile
Ø Marron
Growth rate ŽW b . Survival Ž S b . Food conversion efficiency of the supplement feed ŽFCE b . Reproductive rate Ž R f . Fecundity Ž Ne . Survival Ž Se . Growth rate ŽWj . Survival Ž S j . Food conversion efficiency of the supplement feed ŽFCE j . Growth rate of the carapace ŽWcp . Growth rate of the tail ŽWt . Growth rate of the claws ŽWc . Survival Ž Sm . Food conversion efficiency of the supplement feed ŽFCE m .
The definitions and current commercial values of these production characteristics are presented in Table 1. The current commercial values for most of the production characteristics were either derived from Morrissy et al. Ž1995., assessed on a commercial population ŽT.B.S. Pastoral Marron farmers, Australia., or obtained from marron producers. However, no information was available for the food conversion efficiencies of the broodstock and juveniles, and survival of the eggs and hatchlings. The commercial values for the food conversion efficiencies of the broodstock and juveniles were estimated to be lower and higher than that of the marron, as this is the case for fish Že.g., Kinghorn, 1983b.. By contrast, the survival of the eggs and hatchlings was derived from the survival of eggs and hatchlings in other crayfish species ŽKurtis, 1991.. The growth rates of the carapace, tail, and claws represent a partition of marron growth rate into the growth rates of their three major body components Ži.e., summation of these individual growth rates is the growth rate of the marron.. By partitioning marron growth rate in this way, the growth rates of the carapace, tail, and claws not only determined marron growth rate and, in turn, liveweight production, but also quality characteristics of the marron. The quality characteristics considered were the ratios of tail weight to live weight, and claw weight to carapace weight. The ratio of tail weight to live weight was a measure of the edible tail component. The ratio of claw weight to carapace weight was a measure of the visual presentation of marron as a gourmet item. 2.4. Profit equation Annual profit per growout pond ŽP., expressed as a function of the production characteristics, is shown in Scheme 1. Line Ž1. is the returns from the sale of harvested
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Table 1 Definitions and current values of the production characteristics associated with commercial marron production Definition Broodstock Wb Live weight gain of the broodstock during their maintenance period Žgrbroodstock. Sb Proportion of broodstock that survive their maintenance period FCE b Ratio of liveweight gain to weight of supplement feed fed to the broodstock Rf Proportion of female broodstock that mate and spawn during the annual breeding season Ne Number of eggs spawned by each female broodstock
Value 66.1 0.90 0.27 0.80 300
Eggs and hatchlings Se Proportion of eggs and hatchlings that survive their incubation period
0.70
JuÕeniles Wj Live weight gain of the juveniles during their rearing period Žgrjuvenile. Sj Proportion of juveniles that survival their rearing period FCE j Ratio of liveweight gain to weight of supplement feed fed to the juveniles
3.0 0.70 0.36
Marron Wcp Wt Wc Sm FCE m
Live weight gain of the carapace of the marron during their growout period Žgrmarron. Live weight gain of the tail of the marron during their growout period Žgrmarron. Live weight gain of both claws of the marron during their growout period Žgrmarron. Proportion of marron that survival their growout period Ratio of liveweight gain to weight of supplement feed fed to the marron
30.7 22.2 11.4 0.87 0.29
The production characteristics are the growth rates of the broodstock ŽW b . and juveniles ŽWj ., growth rates of the carapace ŽWcp ., tail ŽWt ., and claws ŽWc . of the marron, survivals of the broodstock Ž S b ., eggs and hatchlings Ž Se ., juveniles Ž S j ., and marron Ž Sm ., food conversion efficiencies of the broodstock ŽFCE b ., juveniles ŽFCE j ., and marron ŽFCE m ., and the reproductive rate Ž R f . and fecundity Ž Ne . of the broodstock.
marron, line Ž2. is the cost to maintain the broodstock, line Ž3. is the cost to rear the juveniles, line Ž4. is the cost to grow the marron, and line Ž5. is the opportunity cost of maintaining some of the harvested marron as broodstock. Upper case letters refer to the production characteristics, and are as previously defined ŽTable 1.. Lower case letters are the prices, costs, constants, and functions associated with production. These are defined and presented in Table 2, while the derivation of the prices, costs, and constants are described in the following sections. 2.4.1. Prices The sale price of the marron is the current market price. Producers are paid by the live weight of marron they sell, with higher prices paid for larger individuals. Specifically, marron less than 40 g cannot be sold, whereas individuals between 40 and 120 g are sold for $ŽAus.25rkg, and those larger than 120 g for $ŽAus.30rkg ŽR. Greenhalgh, Marron Marketing Co-operative, Australia.. Price premiums for the ratios of tail weight to live weight, and claw weight to carapace weight, were estimated after consultation with a major consumer of marron ŽLamont Winery and Restaurant, Millendon, Australia.. This consumer was prepared to pay a price premium for marron that have a larger tail weight to live weight ratio than those currently used for commercial production. They also preferred marron with a claw weight to carapace weight ratio of approximately 0.28, as this was considered optimal for the visual presentation of marron.
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Scheme 1.
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Table 2 Description and values of the prices, costs, constants, and functions associated with commercial marron production
Prices pw1 pw2 pt pc
Costs c food_b c food_ j c food_m cb cj cm c fixed_b c fixed_ j c fixed_m
Description
Value
Sale price of marron Ž$Ausrg. with a live weight between 40 and 120 g Sale price of marron Ž$Ausrg. with a live weight larger than 120 g Price premium for a unit increase in the tail weight to live weight ratio of marron Ž$Ausrunitrg marron live weight. Price premium for a unit increase in the claw weight to carapace weight ratio of marron Ž$Ausrunitrg marron live weight.
0.025 0.030 0.0725
Cost of the supplement feed fed to the broodstock Ž$Ausrg. Cost of the supplement feed fed to the juveniles Ž$Ausrg. Cost of the supplement feed fed to the marron Ž$Ausrg. Variable costs incurred during the maintenance of the broodstock Ž$Ausrbroodstock. Variable costs incurred during the rearing of the juveniles Ž$Ausrjuvenile. Variable costs incurred during the grow out of the marron Ž$Ausrmarron. Fixed costs of production allocated to the broodstock Ž$Ausrbroodstock. Fixed costs of production allocated to the juveniles Ž$Ausrjuvenile. Fixed costs of production allocated to the marron Ž$Ausrmarron.
Constants nf Number of female marron harvested from each growout pond and maintained as broodstock to produce the eggs and hatchlings, juveniles, and ultimately, the marron required to initially stock each growout pond m Factor used to represent the number of male broodstock maintained for breeding. The product of m and n f is the total number of broodstock sw2 Live weight variance of the harvested marron mt Ratio of tail weight to live weight of the marron where no price premium is paid mc Ratio of claw weight to carapace weight of marron where no price premium is paid Functions k Ž a, Wj qWcp qWt qWc , sw . Proportion of harvested marron which have a live weight less than a grams, i.e., k Ž a,Wj qWcp qWt qWc , sw . s p Ž x - a < Wj qWcp qWt qWc , sw . s
F
ž
ay Ž Wj qWcp qWt qWc .
sw
/
where F Ž.. is the standard cumulative normal distribution, with mean Wj qWcp qWt qWc , variance sw2 , and a is 40 or 120 hŽ a, b, Wj qWcp qWt qWc , sw . Mean live weight Žg. of harvested marron which have a live weight between a and b grams, i.e., h Ž a,b,Wj qWcp qWt qWc , sw . s E Ž x < aF= - b . s
Hab x.E HabE
Ž x .d x Ž x .d x
where EŽ x . is the normal density ; N ŽWj qWcp qWt qWc , sw2 . with mean Wj qWcp qW1 qWc , variance sw2 , and Ž a, b . is Ž40, 120. or Ž120, `.
y0.0111
0.0005 0.0007 0.0006 0.6731 0.0405 0.6834 0.4766 0.0323 0.1702
25.4
1.5 407.2 0.33 0.37
0.088 or 0.996
70.5 or 126.3
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The estimate of a price premium for the ratio of tail weight to live weight was based on the change that an increase in this ratio had on the weight of tail meat produced by a unit live weight of marron. Considering the current live weight, tail weight, and sale price of harvested marron presented in Tables 1 and 2, and assuming that the marron are sold for the consumption of their tail meat, tail meat was calculated to be worth $ŽAus.0.072rg. By contrast, the price premium for the ratio of claw weight to carapace weight was estimated after the consumer was prepared to pay an extra $ŽAus.1.00rkg of marron live weight for individuals with a claw weight to carapace ratio of 0.28, instead of the current ratio of 0.37. 2.4.2. Costs The costs were those currently incurred by commercial producers. The supplement feed of the juveniles was assumed to be more expensive than the supplement feed of the marron and, in turn, the broodstock, because younger marron have a higher protein requirement Žc.f. Goddard, 1988.. The variable costs of the broodstock, juveniles, and marron were the costs to prepare their respective ponds prior to stocking, and the daily costs incurred during their maintenance, rearing, and growout periods. The broodstock also incurred the cost to transfer the juveniles from the broodstock ponds to the juvenile ponds, while the juveniles incurred a cost to be transferred from the juvenile ponds to the growout ponds. Similarly, the marron incurred a cost to be harvested, processed, transported to the local wholesaler, and marketed by the wholesaler. Other variable costs, incurred during the operation of the whole farm Ži.e., general maintenance, environmental taxes, consultation fees, accounting, and telephone., were allocated to the broodstock, juveniles, and marron by a weighting factor. The weighting factors for the broodstock, juveniles, and marron were the product of their pond sizes and the proportion of the annual production cycle during which they are maintained, reared, or grown. Broodstock, juveniles, and marron that died were assumed to incur half the variable costs of surviving broodstock, juveniles, and marron. The fixed costs of the broodstock, juveniles, and marron were the capital costs of production and the farm’s normal profit. Capital costs were assumed to be variable in the long-term. Those allocated to the broodstock, juveniles, and marron were the capital costs incurred during their respective maintenance, rearing, and growout periods Že.g., cost of the broodstock, juvenile, and marron ponds.. Other capital costs incurred during the operation of the whole farm Ži.e., land, buildings, machinery, water recirculation system, vehicles, alarm, and farming license., as well as the farm’s normal profit, were allocated to the broodstock, juveniles, and marron by the same weighting factor used to allocate the variable costs incurred during the operation of the whole farm. Fixed costs were allocated to all broodstock, juveniles, and marron, regardless of whether they survived the maintenance, rearing, and growout periods. The farm’s normal profit was the imputed returns to capital and risk taking required for the producer to stay in business in the long-term ŽLipsey and Steiner, 1981.. Including the farm’s normal profit as a cost of production set the profit function equal to zero, which corresponds to economic theory where the average long-term profit of an industry at equilibrium is zero. Under a situation of zero profit, the relative economic values derived from the profit equation are the same for all perspectives of the industry
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187
Že.g., producer, wholesaler, or consumer perspective. and bases of production Že.g., per marron, pond, or farm. ŽBrascamp et al., 1985; Smith et al., 1986.. These economic values are also rescaled against changes in the size of the production system Že.g., pond size andror number. and, as such, represent an increase in the efficiency of production from a genetic improvement in each of the production characteristics. The relative economic values derived from the profit equation also apply when production is adapted to quota. In future, quotas could be imposed to protect natural waterways from production effluent by restricting either the live weight or number of marron produced, number andror size of the growout ponds, amount of water usage, or amount of supplement feed fed. Such quotas are a constraint on total outputs or inputs of production, and result in the same relative economic values as those derived when the profit equation is set to zero ŽSmith et al., 1986.. 2.4.3. Constants The constants used in the profit equation emulate current commercial conditions. The number of female marron maintained as broodstock was derived using the reproductive rate and fecundity of the broodstock, and the survivals of the broodstock, eggs and hatchlings, juveniles, and marron. The factor used to represent the number of male broodstock maintained was obtained from marron producers. The live weight variance of the harvested marron was derived from Morrissy et al. Ž1995.. The ratios of tail weight to live weight, and claw weight to carapace weight, where no price premiums are paid, were set at the ratios of those marron currently used for commercial production. This equates to the current commercial situation where returns are dependent only on the live weight of the marron. 2.5. DeriÕation of economic Õalues The economic value of a production characteristic was calculated as the partial derivative of the profit equation with respect to that characteristic, evaluated with all other characteristics maintained at their current commercial values. However, survivals of the broodstock, eggs and hatchlings, juveniles, and marron, and reproductive rate of the broodstock, were treated as threshold characteristics. Each was assumed to be a discrete characteristic with an underlying liability, such that, Si s F Ž l i . where Si is the survival of the broodstock, eggs and hatchlings, juveniles, or marron, or the reproductive rate of the broodstock, and l i is the underlying liability of Si , normally distributed with mean zero and variance one. The economic value of a threshold characteristic was calculated as the partial derivative of the profit equation with respect to its underlying liability. The economic values for the production characteristics were presented as the increase in profit resulting from an increase in each characteristic of one genetic standard deviation Žor one genetic standard deviation in the underlying liability of the threshold characteristics.. The genetic variance of each characteristic was calculated from their estimates of heritability and phenotypic variance presented in Table 4 Žreference sources for these estimates are described in Section 2.6..
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2.6. SensitiÕity analysis The sensitivity of selection response to potential errors in the estimated economic values was analysed by assuming that the estimates were in error of their true values. It was assumed that the estimated economic value for each production characteristic was in error of it’s true value by a factor of 0, 0.5, 1.1, 1.5, 2 and 5, while the estimated values for the other characteristics were considered to be their true values. Sensitivity was then the relative response in the true aggregate breeding value from selection on indices derived from the estimated and true economic values. Specifically, the aggregate breeding values derived from the estimated economic values Ž Hu ., and those assumed to be the true values Ž Ht ., were defined: Hu s ÕuX g and Ht s ÕtX g where Õu is the vector of absolute economic values estimated in this study for each of the production characteristics defined previously, Õt is a vector of absolute economic values, assumed to be the true values, for each of the production characteristics, and g is the vector of additive genetic values for each of the production characteristics Žor the additive genetic values for the underlying liability of those characteristics treated as threshold characteristics.. The selection indices derived from Hu and Ht Ž Iu and It . were: Iu s buX x and It s btX x where the vector of selection criteria, x, are phenotypic measures of the production characteristics obtained from the selection candidates or their relatives, and are defined in Table 3. The vectors of the index coefficients for each of these criteria, bu and bt , are those which maximise the correlation between their respective selection indices and aggregate breeding values, and were calculated as: b k s Py1 G Õk where k is u or t, P is the matrix of phenotypic Žco. variances among the selection criteria in the selection index, G is the matrix of genetic Žco. variances among the selection criteria in the selection index and the production characteristics in the aggregate breeding value, and Õ k , is the vector of economic values for the production characteristics in the aggregate breeding value. Predicted gain in Ht , resulting from one round of selection on the index Ik Ž k s u,t., was calculated as: R k s irH t Ik sH t where R k is the predicted gain in Ht , i is the standardised selection differential, rH t Ik is the correlation between Ik and Ht , and sH2t is the variance of Ht . The sensitivity Ž S . of response in Ht resulting from selection on Iu , relative to selection on It , was expressed by: S s R urR t and calculated as Žafter Smith, 1983.: Ss
ÕuX GX Py1 G Õt X u
X
(Õ G P
y1
G Õu ÕtX GX Py1 G Õt
(
.
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Table 3 Definitions of the selection criteria used to assess the sensitivity of selection response to potential errors in economic values estimated for the production characteristics associated with commercial marron production Selection criteria
Definition
xWb
Mean of broodstock growth rate obtained from five full-siblings of the selection candidate’s dam Žincludes the selection candidate’s dam.. These individuals were also assessed for x FC E b and x l R f . Mean of juvenile growth rate obtained from 30 full-siblings of the selection candidate Žincludes the selection candidate.. These individuals were also assessed for x FC E . j Mean of marron growth rate obtained from five full-siblings of the selection candidate Žexcludes the selection candidate.. These individuals were also assessed for x W , x W , j t x W c, x FCE j , and x FCE m. Mean tail weight of the marron obtained from five full-siblings of the selection candidate Žexcludes the selection candidate.. These individuals were also assessed for x W , x W , j cp x W c, x F C E j , and x F C E m. Mean claw weight of the marron obtained from five full-siblings of the selection candidate Žexcludes the selection candidate.. These individuals were also assessed for x W , x W , j cp x W t, x FCE j , and x FCE m. Mean liability of broodstock survival obtained from five full-sibling equivalents of the selection candidate’s dam Žexcludes the selection candidate’s dam.. Mean liability of egg and hatchling survival obtained from 50 full-sibling equivalents of the selection candidate Žexcludes the selection candidate.. Mean liability of juvenile survival obtained from 20 full-sibling equivalents of the selection candidate Žexcludes the selection candidate.. Mean liability of marron survival obtained from ten full-sibling equivalents of the selection candidate Žexcludes the selection candidate.. These individuals were also assessed for x W j and x FC E j . Mean of broodstock food conversion efficiency obtained from five full-siblings of the selection candidate’s dam Žincludes the selection candidate’s dam.. These individuals were also assessed for x W b and x l R f . Mean of juvenile food conversion efficiency obtained from 30 full-siblings of the selection candidate Žincludes the selection candidate.. These individuals were also assessed for x W j . Mean of marron food conversion efficiency obtained from 20 full-siblings of the selection candidate Žincludes the selection candidate.. These individuals were also assessed for x W j and x F C E j . Mean liability of reproduction rate obtained from five full-sibling equivalents of the selection candidate’s dam Žincludes the selection candidate’s dam.. These individuals were also assessed for x W b and x FCE b. Fecundity of the selection candidate’s dam. The selection candidate’s dam was also assessed for x W b, x FCE b, and x l R f .
xWj x W cp
xWt
xWc
x lS b x lSe x lS j x lS m
x FC E b
x FC E j x FC E m
xlRf
x Ne
The phenotypic and genetic parameters assumed are presented in Table 4. Estimates involving the growth rates and survivals were obtained from Henryon Ž1995.. For the remaining characteristics, there was no information available for marron, or other crayfish species. Hence, these parameters were derived from estimates for commercial fish species Že.g., Gjedrem, 1983; Gjedrem, 1992; Gjerde, 1986; Gjerde et al., 1994; Huang and Gall, 1990; Kanis et al., 1976; Kinghorn, 1983; Kinghorn, 1983b; Rye et al., 1990; Su et al., 1997.. However, there were many genetic correlations, particularly those involving food conversion efficiency, where no estimates were available for fish species.
190 Table 4 Estimates of heritability, phenotypic variance, and phenotypic and genetic correlations, for the production characteristics associated with commercial marron production Wb 0.35 393.2
Wj
Wcp
Wt
Wc
0.05 0.81
0.38 129.2
0.42 67.3
0.15 21.7
0.05 1
0.02 1
0.02 1
0.03 1
0.30
0.50 0.38
0.46 0.36 0.95
0.30 0.15 0.72 0.59
y0.07 y0.02 y0.10 y0.08 y0.04
0 y0.04 y0.01 y0.01 0 0.03
y0.01 y0.10 y0.05 y0.04 y0.01 0.05 0.15
y0.04 y0.05 y0.12 y0.10 y0.05 0.18 0.08 0.20
0.38 0.36 0.15
0.95 0.80
0.74
0.15 0.30
0.14 0.28
Sb
Se
Sj
Sm
y0.05
FCE b 0.05 0.0010 0.25 0.05 0.18 0.17 0.10 y0.03 0 0 y0.01
0.25 0.30 0.15 0.15 0.25
0.08 0.15
FCE j 0.03 0.0008 0.05 0.30 0.15 0.14 0.08 0 y0.01 y0.02 y0.01 0.25
y0.01 0.45 0.03 0.08
FCE m 0.05 0.0012 0.18 0.15 0.30 0.28 0.15 y0.01 0 y0.01 y0.03 0.45 0.45
Rf
Ne 0.10 1
0.20 5638
0.15 0.02 0.10 0.10 0.05 y0.05 0 y0.01 y0.03 0.03 0 0.01
0.25 0.05 0.10 0.10 0.05 y0.10 0 y0.02 y0.05 0.08 0.01 0.05 0.10
0.10
The production characteristics are the growth rates of the broodstock ŽW b . and juveniles ŽWj ., growth rates of the carapace ŽWcp ., tail ŽWt ., and claws ŽWc . of the marron, survivals of the broodstock Ž S b ., eggs and hatchlings Ž Se ., juveniles Ž S j ., and marron Ž Sm ., food conversion efficiencies of the broodstock ŽFCE b ., juveniles ŽFCE j ., and marron ŽFCE m ., and the reproductive rate Ž R f . and fecundity Ž Ne . of the broodstock. Phenotypic correlations are below, and genetic correlations above, the diagonal. a S b , Se , S j , Sm , and R f are treated as threshold characteristics which have an underlying liability, normally distributed with mean zero and variance one. W b , Wj , Wcp , Wt , and Wc are measured in grams, FCE b , FCE j , and FCE m are ratios, and Ne is a count.
M. Henryon et al.r Aquaculture 173 (1999) 179–195
Heritability Phenoytpic Variancea Wb Wj Wcp Wt Wc Sb Se Sj Sm FCE b FCE j FCE m Rf Ne
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These parameters were estimated by assuming genetic correlations between characteristics were of the same sign and similar magnitude as their phenotypic correlations Žafter Koots and Gibson, 1996., and that correlations between characteristics expressed at the same chronological age were larger than correlations between characteristics expressed at different chronological ages. The phenotypic and genetic Žco. variance matrices resulting from these parameter estimates were positive-definite. 3. Results Profit was increased by a genetic increase in the growth rate of the juveniles, growth rates of the carapace and tail of the marron, survivals of the broodstock, eggs and hatchlings, juveniles, and marron, food conversion efficiencies of the broodstock, juveniles, and marron, and the reproductive rate and fecundity of the broodstock ŽTable 5.. By contrast, profit was decreased by a genetic increase in the growth rate of the broodstock, and a genetic increase in the growth rate of the claws of the marron, given that there was a smaller claw weight to carapace weight ratio which was ideal. The production characteristics associated with the marron, with the exception of food conversion efficiency, were the most economically important. Of these, growth rate of the tail had the largest economic value. A genetic improvement Žone genetic standard
Table 5 Economic value estimates for the production characteristics associated with commercial marron production Production characteristic
Economic value
Wt Wcp Sm Wc Sj Ne Rf Se Wj FCE m Sb Wb FCE j FCE b
941.50 Ž177.06rgrmarron. 200.53 Ž28.62rgrmarron. 105.10 Ž606.81runderlying liability unit. y81.43 Žy45.08rgrmarron. 31.37 Ž221.83runderlying liability unit. 21.61 Ž0.64reggrfemale broodstock. 21.39 Ž67.65runderlying liability unit. 13.58 Ž96.02runderlying liability unit. 13.18 Ž65.51rgrjuvenile. 8.88 Ž1125.42runit. 6.02 Ž26.92runderlying liability unit. y0.79 Žy0.07rgrbroodstock. 0.27 Ž53.98runit. 0.12 Ž16.45runit.
The production characteristics are the growth rates of the broodstock ŽW b . and juveniles ŽWj ., growth rates of the carapace ŽWcp ., tail ŽWt ., and claws ŽWc . of the marron, survivals of the broodstock Ž S b ., eggs and hatchlings Ž Se ., juveniles Ž S j ., and marron Ž Sm ., food conversion efficiencies of the broodstock ŽFCE b ., juveniles ŽFCE j ., and marron ŽFCE m ., and the reproductive rate Ž R f . and fecundity Ž Ne . of the broodstock. Economic values are presented as the increase in annual profit ŽA$rper marron growout pond. resulting from an increase in each characteristic of one genetic standard deviation. However, S b , Se , S j , Sm , and R f were treated as threshold characteristics, and their economic values are the increase in profit resulting from a genetic standard deviation increase in their underlying liability. Values in parenthesis are absolute values, and represent the increase in profit from a unit increase in each of the characteristics.
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Table 6 Sensitivity Ž%. of selection response when selection was on an index derived from economic values estimated for the production characteristics associated with commercial marron production, but the estimates for each characteristic were assumed to be in error of their true values by a factor of 0, 0.5, 1.1, 1.5, 2, and 5
0 0.5 1.1 1.5 2 5
Wb
Wj
Wcp
100 100 100 100 100 100
100 100 100 100 100 99.9
99.9 93.9 99.9 100 99.9 100 100 100 100 100 100 100 100 100 99.9 99.7 99.9 98.2
Wt
Wc
Sb
Se
Sj
100 100 100 100 100 100
100 100 100 100 100 99.9
100 99.9 100 100 100 100 100 100 100 99.9 99.8 98.3
Sm
FCE b FCE j FCE m
Rf
Ne
100 100 100 100 100 100
100 100 100 100 100 100
100 100 100 100 100 99.9
100 100 100 100 100 100
100 100 100 100 100 100
The production characteristics are the growth rates of the broodstock ŽW b . and juveniles ŽWj ., growth rates of the carapace ŽWcp ., tail ŽWt ., and claws ŽWc . of the marron, survivals of the broodstock Ž S b ., eggs and hatchlings Ž Se ., juveniles Ž S j ., and marron Ž Sm ., food conversion efficiencies of the broodstock ŽFCE b ., juveniles ŽFCE j ., and marron ŽFCE m ., and the reproductive rate Ž R f . and fecundity Ž Ne . of the broodstock.
deviation. in the growth rate of the tail was worth between 4.7 and 11.6 times more than an improvement in the growth rate of the carapace, survival of the marron, and growth rate of the claws ŽTable 5.. By comparison, food conversion efficiency of the marron, and the production characteristics associated with the broodstock, eggs and hatchlings, and juveniles were of minor importance. A genetic improvement in the growth rate of the tail was worth 30 times more than an improvement in juvenile survival, 44 times more than an improvement in the reproductive rate and fecundity of the broodstock, and between 69 and 7900 times more than an improvement in the growth rates of the broodstock and juveniles, an improvement in the survivals of the broodstock and eggs and hatchlings, and an improvement in the food conversion efficiencies of the broodstock, juveniles, and marron. Response to selection was not sensitive to potential errors in the magnitude of the estimated economic values. When the estimated value for each production characteristic was assumed to be in error by between zero and five times its true value, selection response was at least 98% as high as the response obtained using an index derived from the true values ŽTable 6.. The only exception occurred when growth rate of the tail of the marron was assumed to have a true economic value of zero. Including growth rate of the tail in the breeding objective with its estimated economic value, should it have a true value of zero, was predicted to decrease selection response by 6%. 4. Discussion It was hypothesised that the profitability of commercial marron production would be increased by the development of a rapidly growing strain that has a large tail, a proportional claw size, and a high survival, food conversion efficiency, reproductive rate, and fecundity. This was shown to be the case, the only exception being that a genetic increase in the growth rate of the broodstock caused a small decrease in profit. It was also shown that the production characteristics associated with the marron, with the exception of food conversion efficiency, were the most economically important. Of these, growth rate of the tail was by far the most important, having an economic value
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between 4.7 and 11.6 times larger than the growth rate of the carapace, survival of the marron, and growth rate of the claws. The importance of the characteristics associated with the marron was of little surprise considering that these characteristics largely determined the returns to production, most of the production costs were incurred during the grow out of the marron, andror these characteristics tended to exhibit a high level of genetic variation. By comparison, food conversion efficiency of the marron, and those characteristics associated with the broodstock, eggs and hatchlings, and juveniles were of minor importance as it was inexpensive to feed the marron, maintain the broodstock, and rear the juveniles, and many of these characteristics tended to exhibit a low level of genetic variation. These results demonstrate that breeding programs for commercial marron production should concentrate on the genetic improvement of those characteristics associated with the marron. In particular, emphasis should be to increase the growth rate of the marron through increases in the growth rate of their tail. The breeding objective was, strictly speaking, peculiar to the production system defined, the current commercial values of the production characteristics, and the prices and costs specified. Both the production characteristics of commercial importance, and their economic values, could vary between individual farms. Of particular interest, was the growth rate of the broodstock, and the growth rates of the carapace, tail, and claws of the marron. A genetic increase in the growth rate of the broodstock was shown to decrease profit by a small amount, presumably because faster growing broodstock incur higher feeding costs. However, it was assumed that the broodstock were discarded following the breeding season. Certainly, the economic value of this characteristic could change, in both direction and magnitude, should the broodstock be sold for human consumption. The relative economic importance of the growth rates of the carapace, tail, and claws remains uncertain. It is clear that each of these characteristics is economically important, as they are the components of marron growth rate and, in turn, liveweight production, upon which the value of marron is based. However, differences between the economic values derived for the growth rates of the carapace, tail, and claws were due to additional price premiums paid for the ratios of tail weight to live weight, and claw weight to carapace weight. These price premiums were obtained after consultation with a particular consumer who had a preference for marron with a large tail weight to live weight ratio, and specific claw weight to carapace ratio. Yet, it is conceivable that some consumers may not be prepared to pay additional price premiums for these characteristics, while others may differ in their perception of ideal ratios and the price premiums they are prepared to pay. Despite this uncertainty, the differences between the economic values derived for the growth rates of the carapace, tail, and claws do serve notice that any quality characteristic included in the breeding objective will most likely be economically important, as they provide returns over and above liveweight production. For marron, other quality characteristics may include palatability, and shell and meat colour. Response to selection was insensitive to errors in the magnitude of the economic values because most of the production characteristics were favourably correlated, both genetically and phenotypically. Consequently, most of the production characteristics were simultaneously improved by selection, and errors in the magnitude of the economic
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values only caused minimal reductions in economic returns. Similar results were also found in other sensitivity studies ŽPease et al., 1967; Rønningen, 1971; Vandepitte and Hazel, 1977; Smith, 1983.. The minor exception, of course, was predicted when growth rate of the tail was included in the breeding objective with its estimated economic value, but its true value was assumed to be zero. Because growth rate of the tail was estimated to be the most economically important characteristic, selection emphasis was placed on its improvement when it was actually of no economic value. Smith Ž1983. also highlighted the sensitivity of selection response when a characteristic is considered to be of major importance, but actually has no true value. However, in practice, it is unlikely that growth rate of the tail will be of no economic value. The insensitivity of selection response has two applications for marron breeding programs. First, the economic values estimated in this study should be suitable for implementation in these programs, even when future production systems and market conditions are uncertain. Second, the production characteristics estimated to be of minor importance could be assumed to have zero economic value and removed from the breeding objective. Decreasing the number of characteristics in the breeding objective decreases the number of genetic parameters which need to be estimated, increases the probability of obtaining estimates within the parameter space, and increases the accuracy of predicted response ŽHill and Thompson, 1978.. However, it must be remembered that the sensitivity analysis was carried out conditional on a selection index where phenotypic measures of each production characteristic were obtained from the selection candidates or their relatives, and the magnitude of only one economic value was assumed to be in error of its true value. Selection response may be sensitive should indices with different selection criteria be established, andror more than one economic value be in error. The accuracy of the sensitivity analysis was largely dependent upon the phenotypic and genetic parameters used in this study. How well these parameters apply to marron populations is an obvious proviso to the sensitivity of selection response to errors in the economic values. Although the parameters used were derived from the best available sources, there is little information available for marron. Consequently, many of the parameter estimates were unreliable. This study defined a breeding objective for commercial marron production by identifying the production characteristics which influence returns and costs to production, and estimating their relative economic values. Before these estimates are chosen as those implemented in breeding programs for marron, a more thorough analysis should be conducted on the sensitivity of selection response to potential errors in the estimates. However, the evidence to date indicates that selection response is largely insensitive to errors, justifying the use of these economic values in the development of breeding programs for marron. Acknowledgements This study was funded by the Faculty of Agriculture at the University of Western Australia, Australia, and the Danish Agricultural and Veterinary Research Council, Denmark.
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