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Selection goals in oyster breeding
Aquaculture, 33 (1983) 141-148 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
141
SELECTION GOALS IN OYSTER BREEDING
G.A.T. MAHON Department
of Zoology,
University College, Galway (Ireland)
Present address: Division VI-A-2, Commission of the European Communities, 200 Rue de la Loi, B-1049 Brussels (Belgium) (Accepted 1 December 1982)
ABSTRACT Mahon, G.A.T.,
1983.
Selection goals in oyster breeding. Aquaculture,
33:
141-148.
A questionnaire on prospective breeding goals in oysters was gistributed and 49 replies received from researchers and producers in 18 countries. Respondents worked mainly with Ostrea edulis, Crassostrea gigas, C. virginica and C. rhizophorae. A score from 0 to 10 was assigned to each of 22 prospective breeding goals to indicate the emphasis that each trait should receive. Scores were standardised to a mean of 5.0 and a standard deviation of 2.0 within a respondent. The average scores for particular breeding goals ranged from 3.81 f 1.33 for number of larvae released per fertile oyster, 3.83 * 2.10 for resistance to predation (at ongrowing site), and 3.89 * 1.45 for proportion of oysters that spawn; to 6.10 f 1.60 for meat weight (at final size), 6.25 + 1.63 for proportion of spat that survive from 2 mm to 10 mm, 6.28 ? 1.88 for resistance to disease, and 6.35 ? 1.68 for growth rate from 10 mm to final commercial size. There was significant variation in the scores for some traits for geographical area and oyster species.
INTRODUCTION
A general strategy of livestock improvement which is applicable to oysters has been described by Cunningham (1974). Its first step is the definition of breeding goals for guiding selection programmes, crossing systems, breed substitution, or a combination of these three methods of improvement. The prospective breeding goals suggested by Wilkins (1981) for cultured aquatic animals and by Newkirk (1980) for oysters, include greater juvenile survival, resistance to disease and low salinity, improved growth rate, greater food conversion efficiency, higher meat : skeleton ratio, and improved shape. It is difficult to assess the relative importance of the various traits. Breeding goals can be deduced from economic and genetic analyses of oyster production, but organisations commencing breeding programmes do not have complete analyses to which to refer. A postal survey was organised to discover the opinion of oyster researchers and producers on the relative importance of a series of traits as goals in
0044-8486/83/$03.00
o 1983 Elsevier Science Publishers B.V.
142
selective breeding. The results presented here are intended to guide a complete analysis of production and aid breeders to make immediate decisions as to their goals. MATERIALS
AND METHODS
The questionnaire requested the respondent’s name and address and asked for a definition of his work with oysters (research, commercial production, or other). Information was also requested concerning the species and stage of the life-cycle with which the respondent worked. The respondent was then asked to allocate a score of 0 to 10 to prospective breeding goals, listed in Table I, according to the importance he attached to each trait. It was anticipated that respondents would give many traits a score of about 3 to 7 and provision was made to specify other goals and make additional comments. Most respondents were researchers and commercial growers who had participated in the International Workshop on Nursery Culture of Bivalve Mollusts held in Ghent in February 1981. These participants were especially suitable because they worked with larvae and spat, and were interested in TABLE I Means and standard deviations of the standardised scores for 22 prospective goals
breeding
Character
Mean * S.D.
Ease of conditioning Proportion of oysters that spawn Number of larvae released per fertile oyster Proportion of larvae that survive until settlement Proportion of larvae that settle Growth rate from settlement to 2 mm in size Proportion that survive from settlement to 2 mm Growth rate from 2 mm to 10 mm Proportion that survive from 2 mm to 10 mm Proportion that survive transfer to ongrowing site Growth rate from 10 mm to final commercial size Resistance to disease Resistance to high temperature Resistance to low temperature Resistance to predation Resistance to low salinity Resistance to de&cation Total weight Meat weight Shell diameter Ratio of maximum diameter to minimum diameter (a measure of roundness) Thickness of oyster
4.27 3.89 3.81 5.05 5.96 5.20 5.81 5.94 6.25 5.92 6.53 6.28 3.95 4.35 3.83 4.47 4.39 4.50 6.10 4.17
* f f * * + f f i f i i f f f f * t f i
1.63 1.45 1.33 2.06 1.99 1.78 1.79 1.52 1.63 1.81 1.68 1.88 1.78 1.91 2.10 1.74 1.68 1.51 1.60 1.60
3.39 + 1.69 4.42 i 1.61
143
breeding. Several participants nominated colleagues and associates who were also included in the survey. In addition, the questionnaire was sent to the first authors of all oyster papers published in Aquaculture. The supplementary group was mostly comprised of researchers and it showed a particularly wide geographical spread. Analyses of variance and covariance (Steel and Torrie, 1960) were carried out on the scores assigned by the respondents to the prospective breeding goals. The calculations were performed by computer using the LSML76 program (Harvey, 1976). RESULTS
A total of 86 questionnaires was distributed: 41 went to participants at the Ghent meeting, 16 to subsequent nominees, and 29 to Aquaculture authors. The number of completed questionnaires received from each group was 27, 11 and 11, respectively. Thus the response rate of the survey was 57%. Replies were received from respondents in the following numbers: U.S.A., 13; Ireland, 7; Great Britain, 6; Spain, 5; Denmark, 3; Belgium, 2; France, 2; The Netherlands, 1; Norway, 1; Federal Republic of Germany, 1; Israel, 1; Canada, 1; Australia, 1; Haiti, 1; Brazil, 1; Martinique, 1; French Guyana, 1; and Nigeria, 1. The numbers of respondents engaged in research, commercial production, and other activities were 31, 19 and 8 respectively. Several respondents were engaged in more than one activity. Respondents worked with one or more stages of the oyster life cycle in the following numbers: conditioning of broodstock, 22; larval rearing and settlement, 30; nursery culture of spat, 34; and final ongrowing, 32. Respondents had worked with the several oyster species to the following extents: Ostrea edulis, 38.6%; 0. lurida, 0.9%; 0. angasi, 0.4%; Crassostrea gigas, 27.5%; C. Virginia, 23.2%; C. rhizophorae, 6.9%; C. gasar, 2.0%; and C. angulata, 0.5%. The scores that the respondents assigned to the prospective breeding goals ranged from 0 to 10 but there was much variation in the average score assigned by particular respondents. The lowest such average was 1.23 and the highest was 7.68, The standard deviation of scores also varied widely from respondent to respondent, from 1.25 to 4.91. The scores were standardised to have a mean of 5.0 and a standard deviation of 2.0 for each respondent. The mean standardised score for each of the 22 prospective breeding goals is shown in Table I. The average scores ranged from 6.53 for growth rate from 10 mm to final size, to 3.81 for the number of larvae released per fertile oyster. It is clear that there were real differences between the relative importance of the various traits. The overall mean score was set to 5.0, and the standard error of the average score for a particular trait would be about 0.3. Thus in the absence of real differences almost all the averages for the traits would be in the range 4.4 to 5.6. In fact, as many as 17 out of 22 values fell outside that range. The standard deviations of the scores for the 22 traits are shown in Table
144
I, Two of the standard deviations were slightly greater than 2.0, the value to which the standard deviation within respondent had been corrected. The remaining 20 standard deviations were below 2.0. The lower standard deviation of most traits indicated that respondents were generally consistent in giving high or low scores to particular traits. The traits listed in Table I may be divided into four groups. The first five traits, those for the early stage of the life cycle up to settlement, comprise the first group. The mean score for the proportion of larvae that settle was 5.96, well above average. The scores for the remaining traits were about average or below it. The second group is made up of six traits: growth rate and survival for three successive stages from settlement to final size. These traits all had high average scores, and they include the trait with the highest score of all; growth rate from 10 mm to final size (6.53). The importance of these growth and survival traits for a breeding programme is clearly indicated. The third group of traits is made up of six characters concerned with deleterious environmental influences. One of these, resistance to disease, had a large average score of 6.25. The remainder all had scores which were below average. The fourth and final group is comprised of five traits related to the size and shape of the oyster. One trait, meat weight, had a high score of 6.10, but the remainder all had low scores. Four analyses of variance or covariance were carried out. In the first two analyses individual scores were classified by type of work (research, commercial production) and stage of oyster life cycle, respectively. In the third analysis, the scores were classified by geographical area; and in the fourth, an analysis of covariance, the scores were regressed on the proportion of the work of the respondents with each of the three main species, i.e. Ostrea edulis, Crassostrea gigas, and C. virginica. The first analysis showed general agreement among respondents, and type of work was a statistically significant source of variation for only two out of 22 traits: resistance to low salinity and resistance to high temperature. Commercial producers tended to give a lower score for resistance to low salinity but no particular pattern was apparent for resistance to high temperature. The second analysis also showed general agreement and there were significant effects for only two traits: survival from 2 mm to 10 mm and total (final) weight. No pattern was apparent for the survival trait and the effect for total weight was largely due to respondents whose work was theoretical rather than practical having given that trait a low score. For the third analysis, which concerned geographical area, the countries in which the respondents worked were combined into six group: (a) Ireland, (b) Great Britain, (c) other countries of N.W. Europe, (d) Spain, (e) U.S.A. and Canada, and (f) the remaining, generally tropical, countries. The results of the third analysis are summarised in Table II. Area was a highly significant source of variation for proportion of larvae that survive until settlement, and a significant source for the proportion of larvae that settle. Higher than average scores were assigned by respondents in Ireland and Spain for those two traits, and lower scores were assigned by respondents in the rest
145 TABLE II Mean scores, expressed as a deviation from the overall mean, for respondents from various geographical areas and for those traits for which area was a significant source of variation Ireland
Proportion of larvae that survive Proportion of larvae that settle Resistance to disease Resistance to high temperature Resistance to low temperature
Great Britain
N.W. Europe
2.1
-0.7
-1.3
1.1
1.4
-0.1
-1.3
1.4
-1.0
1.2
-1.4
0.5
0.1
1.7
Spain
0.9 -0.1 1.4
North America
0.0
-1.1
-0.2
-0.7
1.3
-0.9
0.7
-1.3
0.0
Other
-1.3 -1.6 1.2 -1.8
TABLE III Mean scores, calculated from regression coefficients and expressed as a deviation from the overall mean, for respondents who worked with the three principal oyster species. Means are tabulated only for those traits for which species was a significant source of variation
Proportion of oysters that spawn Number of larvae per oyster Proportion of larvae that settle Resistance to disease Resistance to low temperature Resistance to low salinity
Ostrea edulis
Crassostrea gigas
C. virginica
0.3 0.2 0.5 -0.4 0.2 -0.8
-0.5 -0.6 0.1 -0.1 0.7 0.6
-0.6 -0.4 -0.6 1.7 -0.1 -0.1
of N.W. Europe and in the tropical countries. Area was a highly significant source of variation in score for resistance to disease. Large scores were assigned by British and North American respondents and low scores were returned by Irish and tropical respondents. Area was significant for resistance to high temperature, and Irish and Spanish respondents gave the trait a low score while tropical ones gave it a high score. Area was highly significant for resistance to low temperature. The trait was given a high score by British and other N.W. European respondents, and a low score by Spanish and tropical workers. The fourth and final analysis concerned species of oyster. The individual scores were simultaneously regressed on the relative amount of work (on a scale from 0 to 1) that each respondent reported for the three main species: 0. edulis, C. gigus and C. uirginica. The results of the analysis are summarised in Table III. Species was a highly significant source of variation for propor-
146
tion of oysters that spawn and for number of larvae that settle. A low score was associated with C. gigas and C. virginica for both traits. There was a significant species effect for proportion of larvae that settle, and respondents who worked with C. gigas tended to give the trait a high score while those who worked with C. uirginica tended to give it a low score. Species was highly significant for resistance to disease and especially large scores were associated with C. oirginica. Species was a highly significant source of variation for resistance to low temperature and large scores were associated with C. gigas. There were significant species effects for resistance to low salinity. Low scores were assigned for that trait by respondents who worked with 0. edulis and high scores were assigned by those who worked with C. gigas. DISCUSSION
Oyster traits which might be improved by selective breeding were divided into four groups: (a) characters relating to spawning and larval development; (b) a series of survival and growth traits from settlement to final commercial size; (c) resistance to deleterious environmental influences; and (d) the final size and shape of the oyster. Respondents gave high scores to all the traits in the second group, thus indicating that survival and growth from settlement to final size should receive particular emphasis in a selective breeding programme. Occasional traits in the other groups were assigned high scores, for example, proportion of larvae that settle, resistance to disease, and meat weight (at final size). The scores given to each trait by the various respondents were remarkably consistent. The standard deviation of the scores for almost all traits was relatively low, and for most of the traits none of the sources of variation examined by analysis of variance was found to be statistically significant. The degree of unanimity among respondents is striking in view of the range of climatic conditions, diversity of oyster species, and differences in intensity of cultivation of which the respondents had experience. Furthermore, the respondents included both researchers and commercial producers who worked with various stages of the oyster life cycle. Despite the homogeneity of most of the responses, the analyses of variance showed certain criteria of classification to be significant sources of variation for several traits. In particular, geographical area and species of oyster, which were closely related, were significant for proportion of larvae that settle, resistance to disease, and resistance to low temperature. Irish and Spanish respondents, those who worked with Ostrea edulis, gave high scores to proportion of larvae that settle. Respondents in North America who worked with Crassostrea uirginica gave particularly high scores to resistance to disease. When a large number of statistics are tested a proportion of them may be expected to be significant by chance and in such cases there will be no particular pattern among the means for subclasses, However, with most of the significant F-ratios there was a clear pattern. For example, resistance to low temperature was assigned a high score by northerly respondents,
147
those from Great Britain and the rest of N.W. Europe and by those who worked with C. gigus, and the trait was given a low score by respondents from Spain and the tropical countries. Respondents suggested additional prospective breeding goals, including fast larval growth and good settlement at 14-17°C. Ongrowing phase traits included: good food conversion efficiency on a variety of good sources; ability to use non-algal feeds, especially for growth from 10 mm to final size; ability to fatten at high temperatures and also feed at low temperatures; ability to withstand stress due to fluctuations in feed or temperature; and low heavy metal uptake. Traits concerning the final marketable oyster included: shell hardness for robustness on handling; the ratio of shell diameter to thickness to be in the range 3 : 1 to 4.5 : 1; flavour of meat; good meat quality at all seasons; and appearance, especially of the inside of the shell. The importance of growth and survival, two complementary traits, was repeatedly stressed. Biomass growth was considered important because good growth may compensate for poor survival and vice versa. Growth may be relatively more important for hatcheries and survival for natural fisheries. Survival was considered increasingly important with time as the investment in each oyster would be greater. The average scores were progressively larger for survival of larvae until settlement (5.05), survival of spat from settlement to 2 mm (5.81), and survival from 2 mm to 10 mm (6.25). The respondents pointed out that selective breeding would not be a universal remedy for the problems of the oyster industry. Breeding should complement advances in husbandry methods. More research on nutrition, feeding, and mortality was requested. It was argued that many traits, for example proportion of oysters that spawn, would improve without deliberate selection as the domestication of oyster species proceeded. Respondents urged that there should be more research on the mechanism of sex determination and the control of fertility. It would be desirable to have an oyster population which bred only under specified, controllable conditions. A cross of two diverse strains that yielded a sterile hybrid would be valuable. There was a demand for distinctive, recognisable lines of oysters, the characteristics of which could become associated with quality by the consumer. Respondents’ scores identified several favoured prospective breeding goals but improvement in the pertinent traits is not possible unless there is genetic variation either within or between populations. Genetic variation for various oyster traits has recently been reviewed by Newkirk (1980). Additive genetic variation has been reported for larval survival in Crassostrea gigas (Lannan, 1972), growth rate of larvae and spat in C. virginica (Losee, 1978), and larval growth rate in C. virginica (Newkirk et al., 1977). Population differences have been reported for larval growth at different salinities in C. virginica (Newkirk, 1978), and spat growth in C. virginica (Mallet and Haley, in Newkirk, 1980). A positive response to selection has been reported for resistance to MSX disease in Delaware oysters (Haskin and Ford, 1978), low summertime mortality in C. gigus (Beattie et al., 1978), and growth rate
148
in C. uirginica (Haley, in Newkirk, 1980). The evidence for genetic variation in the traits which the survey indicated to be important favours the potential of selective oyster breeding. The clarification of breeding goals by surveys logically precedes the genetical and economic analysis of oyster production which would form the basis of a selective breeding programme. The method of index selection has been developed for the simultaneous improvement of several traits within a population. The construction of a selection index requires the estimation of the genetic variances and covariances for the traits to be improved and the net economic value of a change of one unit in each trait. The survey of breeding goals in oysters reported here indicates which traits should be included in an index. Genetic improvement might proceed by strain substitution, and the economic analysis which has been advocated would be indispensible for interpreting the results of experiments to evaluate and compare various oyster strains. ACKNOWLEDGEMENTS
The author is grateful to the many oyster researchers and producers who responded to the survey, and to Dr. P. Rodhouse for valuable discussions. The work was supported by a grant from the National Board of Science and Technology (Ireland).
REFERENCES Beattie, J.H., Hershberger, W.K., Chew, K.K., Mahnken, C., Prentice, E.F. and Jones, C., 1978. Breeding for resistance to summertime mortality in the Pacific oyster (Crassostrea gigas). Washington Sea Grant Rep. WSG 78-8, 13 pp. Cunningham, E.P., 1974. Cost-effectiveness and population structure in cattle programs. Ann. Genet. Sel. Anim., 5: 239-255. Harvey, W.H., 1976. User’s guide for LSML76. Mixed model least-squares and maximum likelihood computer program. Ohio State Univ., Columbus, OH (mimeographed). Haskin, H.H. and Ford, S.E., 1978. Mortality patterns and disease resistance in Delaware Bay oysters. Proc. Nat. Shellfish, Assoc., 68: 80. Lannan, J.E., 1972. Estimating heritability and predicting response to selection for the Pacific oyster, Crassostrea gigas. Proc. Nat. Shellfish Assoc., 62: 62-66. Losee, E., 1978. Influence of heredity on larval and spat growth in Crassostrea virginica. In: J.W. Avault (Editor), Proceedings of the Ninth Annual Meeting, World Mariculture Society, pp. 101-107. Newkirk, G.F., 1978. Interaction of genotype and salinity in larvae of the oyster, Crassostrea virginica. Mar. Biol., 48: 227-234. Newkirk, G.F., 1980. Review of the genetics and the potential for selective breeding of commercially important bivalves. Aquaculture, 19: 209-228. Newkirk, G.F., Haley, L.E., Waugh, D.L. and Doyle, R.W. Genetics of larvae and spat growth in the oyster, Crassostrea virginica. Mar. Biol., 41: 49-52. Steel, R.D.G. and Torrie, J.H., 1960. Principles and Procedures Statistics. McGraw-Hill, New York, 481 pp. Wilkins, N.P., 1981. The rationale and relevance of genetics in aquaculture: an overview. Aquaculture, 22: 209-228.
141
SELECTION GOALS IN OYSTER BREEDING
G.A.T. MAHON Department
of Zoology,
University College, Galway (Ireland)
Present address: Division VI-A-2, Commission of the European Communities, 200 Rue de la Loi, B-1049 Brussels (Belgium) (Accepted 1 December 1982)
ABSTRACT Mahon, G.A.T.,
1983.
Selection goals in oyster breeding. Aquaculture,
33:
141-148.
A questionnaire on prospective breeding goals in oysters was gistributed and 49 replies received from researchers and producers in 18 countries. Respondents worked mainly with Ostrea edulis, Crassostrea gigas, C. virginica and C. rhizophorae. A score from 0 to 10 was assigned to each of 22 prospective breeding goals to indicate the emphasis that each trait should receive. Scores were standardised to a mean of 5.0 and a standard deviation of 2.0 within a respondent. The average scores for particular breeding goals ranged from 3.81 f 1.33 for number of larvae released per fertile oyster, 3.83 * 2.10 for resistance to predation (at ongrowing site), and 3.89 * 1.45 for proportion of oysters that spawn; to 6.10 f 1.60 for meat weight (at final size), 6.25 + 1.63 for proportion of spat that survive from 2 mm to 10 mm, 6.28 ? 1.88 for resistance to disease, and 6.35 ? 1.68 for growth rate from 10 mm to final commercial size. There was significant variation in the scores for some traits for geographical area and oyster species.
INTRODUCTION
A general strategy of livestock improvement which is applicable to oysters has been described by Cunningham (1974). Its first step is the definition of breeding goals for guiding selection programmes, crossing systems, breed substitution, or a combination of these three methods of improvement. The prospective breeding goals suggested by Wilkins (1981) for cultured aquatic animals and by Newkirk (1980) for oysters, include greater juvenile survival, resistance to disease and low salinity, improved growth rate, greater food conversion efficiency, higher meat : skeleton ratio, and improved shape. It is difficult to assess the relative importance of the various traits. Breeding goals can be deduced from economic and genetic analyses of oyster production, but organisations commencing breeding programmes do not have complete analyses to which to refer. A postal survey was organised to discover the opinion of oyster researchers and producers on the relative importance of a series of traits as goals in
0044-8486/83/$03.00
o 1983 Elsevier Science Publishers B.V.
142
selective breeding. The results presented here are intended to guide a complete analysis of production and aid breeders to make immediate decisions as to their goals. MATERIALS
AND METHODS
The questionnaire requested the respondent’s name and address and asked for a definition of his work with oysters (research, commercial production, or other). Information was also requested concerning the species and stage of the life-cycle with which the respondent worked. The respondent was then asked to allocate a score of 0 to 10 to prospective breeding goals, listed in Table I, according to the importance he attached to each trait. It was anticipated that respondents would give many traits a score of about 3 to 7 and provision was made to specify other goals and make additional comments. Most respondents were researchers and commercial growers who had participated in the International Workshop on Nursery Culture of Bivalve Mollusts held in Ghent in February 1981. These participants were especially suitable because they worked with larvae and spat, and were interested in TABLE I Means and standard deviations of the standardised scores for 22 prospective goals
breeding
Character
Mean * S.D.
Ease of conditioning Proportion of oysters that spawn Number of larvae released per fertile oyster Proportion of larvae that survive until settlement Proportion of larvae that settle Growth rate from settlement to 2 mm in size Proportion that survive from settlement to 2 mm Growth rate from 2 mm to 10 mm Proportion that survive from 2 mm to 10 mm Proportion that survive transfer to ongrowing site Growth rate from 10 mm to final commercial size Resistance to disease Resistance to high temperature Resistance to low temperature Resistance to predation Resistance to low salinity Resistance to de&cation Total weight Meat weight Shell diameter Ratio of maximum diameter to minimum diameter (a measure of roundness) Thickness of oyster
4.27 3.89 3.81 5.05 5.96 5.20 5.81 5.94 6.25 5.92 6.53 6.28 3.95 4.35 3.83 4.47 4.39 4.50 6.10 4.17
* f f * * + f f i f i i f f f f * t f i
1.63 1.45 1.33 2.06 1.99 1.78 1.79 1.52 1.63 1.81 1.68 1.88 1.78 1.91 2.10 1.74 1.68 1.51 1.60 1.60
3.39 + 1.69 4.42 i 1.61
143
breeding. Several participants nominated colleagues and associates who were also included in the survey. In addition, the questionnaire was sent to the first authors of all oyster papers published in Aquaculture. The supplementary group was mostly comprised of researchers and it showed a particularly wide geographical spread. Analyses of variance and covariance (Steel and Torrie, 1960) were carried out on the scores assigned by the respondents to the prospective breeding goals. The calculations were performed by computer using the LSML76 program (Harvey, 1976). RESULTS
A total of 86 questionnaires was distributed: 41 went to participants at the Ghent meeting, 16 to subsequent nominees, and 29 to Aquaculture authors. The number of completed questionnaires received from each group was 27, 11 and 11, respectively. Thus the response rate of the survey was 57%. Replies were received from respondents in the following numbers: U.S.A., 13; Ireland, 7; Great Britain, 6; Spain, 5; Denmark, 3; Belgium, 2; France, 2; The Netherlands, 1; Norway, 1; Federal Republic of Germany, 1; Israel, 1; Canada, 1; Australia, 1; Haiti, 1; Brazil, 1; Martinique, 1; French Guyana, 1; and Nigeria, 1. The numbers of respondents engaged in research, commercial production, and other activities were 31, 19 and 8 respectively. Several respondents were engaged in more than one activity. Respondents worked with one or more stages of the oyster life cycle in the following numbers: conditioning of broodstock, 22; larval rearing and settlement, 30; nursery culture of spat, 34; and final ongrowing, 32. Respondents had worked with the several oyster species to the following extents: Ostrea edulis, 38.6%; 0. lurida, 0.9%; 0. angasi, 0.4%; Crassostrea gigas, 27.5%; C. Virginia, 23.2%; C. rhizophorae, 6.9%; C. gasar, 2.0%; and C. angulata, 0.5%. The scores that the respondents assigned to the prospective breeding goals ranged from 0 to 10 but there was much variation in the average score assigned by particular respondents. The lowest such average was 1.23 and the highest was 7.68, The standard deviation of scores also varied widely from respondent to respondent, from 1.25 to 4.91. The scores were standardised to have a mean of 5.0 and a standard deviation of 2.0 for each respondent. The mean standardised score for each of the 22 prospective breeding goals is shown in Table I. The average scores ranged from 6.53 for growth rate from 10 mm to final size, to 3.81 for the number of larvae released per fertile oyster. It is clear that there were real differences between the relative importance of the various traits. The overall mean score was set to 5.0, and the standard error of the average score for a particular trait would be about 0.3. Thus in the absence of real differences almost all the averages for the traits would be in the range 4.4 to 5.6. In fact, as many as 17 out of 22 values fell outside that range. The standard deviations of the scores for the 22 traits are shown in Table
144
I, Two of the standard deviations were slightly greater than 2.0, the value to which the standard deviation within respondent had been corrected. The remaining 20 standard deviations were below 2.0. The lower standard deviation of most traits indicated that respondents were generally consistent in giving high or low scores to particular traits. The traits listed in Table I may be divided into four groups. The first five traits, those for the early stage of the life cycle up to settlement, comprise the first group. The mean score for the proportion of larvae that settle was 5.96, well above average. The scores for the remaining traits were about average or below it. The second group is made up of six traits: growth rate and survival for three successive stages from settlement to final size. These traits all had high average scores, and they include the trait with the highest score of all; growth rate from 10 mm to final size (6.53). The importance of these growth and survival traits for a breeding programme is clearly indicated. The third group of traits is made up of six characters concerned with deleterious environmental influences. One of these, resistance to disease, had a large average score of 6.25. The remainder all had scores which were below average. The fourth and final group is comprised of five traits related to the size and shape of the oyster. One trait, meat weight, had a high score of 6.10, but the remainder all had low scores. Four analyses of variance or covariance were carried out. In the first two analyses individual scores were classified by type of work (research, commercial production) and stage of oyster life cycle, respectively. In the third analysis, the scores were classified by geographical area; and in the fourth, an analysis of covariance, the scores were regressed on the proportion of the work of the respondents with each of the three main species, i.e. Ostrea edulis, Crassostrea gigas, and C. virginica. The first analysis showed general agreement among respondents, and type of work was a statistically significant source of variation for only two out of 22 traits: resistance to low salinity and resistance to high temperature. Commercial producers tended to give a lower score for resistance to low salinity but no particular pattern was apparent for resistance to high temperature. The second analysis also showed general agreement and there were significant effects for only two traits: survival from 2 mm to 10 mm and total (final) weight. No pattern was apparent for the survival trait and the effect for total weight was largely due to respondents whose work was theoretical rather than practical having given that trait a low score. For the third analysis, which concerned geographical area, the countries in which the respondents worked were combined into six group: (a) Ireland, (b) Great Britain, (c) other countries of N.W. Europe, (d) Spain, (e) U.S.A. and Canada, and (f) the remaining, generally tropical, countries. The results of the third analysis are summarised in Table II. Area was a highly significant source of variation for proportion of larvae that survive until settlement, and a significant source for the proportion of larvae that settle. Higher than average scores were assigned by respondents in Ireland and Spain for those two traits, and lower scores were assigned by respondents in the rest
145 TABLE II Mean scores, expressed as a deviation from the overall mean, for respondents from various geographical areas and for those traits for which area was a significant source of variation Ireland
Proportion of larvae that survive Proportion of larvae that settle Resistance to disease Resistance to high temperature Resistance to low temperature
Great Britain
N.W. Europe
2.1
-0.7
-1.3
1.1
1.4
-0.1
-1.3
1.4
-1.0
1.2
-1.4
0.5
0.1
1.7
Spain
0.9 -0.1 1.4
North America
0.0
-1.1
-0.2
-0.7
1.3
-0.9
0.7
-1.3
0.0
Other
-1.3 -1.6 1.2 -1.8
TABLE III Mean scores, calculated from regression coefficients and expressed as a deviation from the overall mean, for respondents who worked with the three principal oyster species. Means are tabulated only for those traits for which species was a significant source of variation
Proportion of oysters that spawn Number of larvae per oyster Proportion of larvae that settle Resistance to disease Resistance to low temperature Resistance to low salinity
Ostrea edulis
Crassostrea gigas
C. virginica
0.3 0.2 0.5 -0.4 0.2 -0.8
-0.5 -0.6 0.1 -0.1 0.7 0.6
-0.6 -0.4 -0.6 1.7 -0.1 -0.1
of N.W. Europe and in the tropical countries. Area was a highly significant source of variation in score for resistance to disease. Large scores were assigned by British and North American respondents and low scores were returned by Irish and tropical respondents. Area was significant for resistance to high temperature, and Irish and Spanish respondents gave the trait a low score while tropical ones gave it a high score. Area was highly significant for resistance to low temperature. The trait was given a high score by British and other N.W. European respondents, and a low score by Spanish and tropical workers. The fourth and final analysis concerned species of oyster. The individual scores were simultaneously regressed on the relative amount of work (on a scale from 0 to 1) that each respondent reported for the three main species: 0. edulis, C. gigus and C. uirginica. The results of the analysis are summarised in Table III. Species was a highly significant source of variation for propor-
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tion of oysters that spawn and for number of larvae that settle. A low score was associated with C. gigas and C. virginica for both traits. There was a significant species effect for proportion of larvae that settle, and respondents who worked with C. gigas tended to give the trait a high score while those who worked with C. uirginica tended to give it a low score. Species was highly significant for resistance to disease and especially large scores were associated with C. oirginica. Species was a highly significant source of variation for resistance to low temperature and large scores were associated with C. gigas. There were significant species effects for resistance to low salinity. Low scores were assigned for that trait by respondents who worked with 0. edulis and high scores were assigned by those who worked with C. gigas. DISCUSSION
Oyster traits which might be improved by selective breeding were divided into four groups: (a) characters relating to spawning and larval development; (b) a series of survival and growth traits from settlement to final commercial size; (c) resistance to deleterious environmental influences; and (d) the final size and shape of the oyster. Respondents gave high scores to all the traits in the second group, thus indicating that survival and growth from settlement to final size should receive particular emphasis in a selective breeding programme. Occasional traits in the other groups were assigned high scores, for example, proportion of larvae that settle, resistance to disease, and meat weight (at final size). The scores given to each trait by the various respondents were remarkably consistent. The standard deviation of the scores for almost all traits was relatively low, and for most of the traits none of the sources of variation examined by analysis of variance was found to be statistically significant. The degree of unanimity among respondents is striking in view of the range of climatic conditions, diversity of oyster species, and differences in intensity of cultivation of which the respondents had experience. Furthermore, the respondents included both researchers and commercial producers who worked with various stages of the oyster life cycle. Despite the homogeneity of most of the responses, the analyses of variance showed certain criteria of classification to be significant sources of variation for several traits. In particular, geographical area and species of oyster, which were closely related, were significant for proportion of larvae that settle, resistance to disease, and resistance to low temperature. Irish and Spanish respondents, those who worked with Ostrea edulis, gave high scores to proportion of larvae that settle. Respondents in North America who worked with Crassostrea uirginica gave particularly high scores to resistance to disease. When a large number of statistics are tested a proportion of them may be expected to be significant by chance and in such cases there will be no particular pattern among the means for subclasses, However, with most of the significant F-ratios there was a clear pattern. For example, resistance to low temperature was assigned a high score by northerly respondents,
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those from Great Britain and the rest of N.W. Europe and by those who worked with C. gigus, and the trait was given a low score by respondents from Spain and the tropical countries. Respondents suggested additional prospective breeding goals, including fast larval growth and good settlement at 14-17°C. Ongrowing phase traits included: good food conversion efficiency on a variety of good sources; ability to use non-algal feeds, especially for growth from 10 mm to final size; ability to fatten at high temperatures and also feed at low temperatures; ability to withstand stress due to fluctuations in feed or temperature; and low heavy metal uptake. Traits concerning the final marketable oyster included: shell hardness for robustness on handling; the ratio of shell diameter to thickness to be in the range 3 : 1 to 4.5 : 1; flavour of meat; good meat quality at all seasons; and appearance, especially of the inside of the shell. The importance of growth and survival, two complementary traits, was repeatedly stressed. Biomass growth was considered important because good growth may compensate for poor survival and vice versa. Growth may be relatively more important for hatcheries and survival for natural fisheries. Survival was considered increasingly important with time as the investment in each oyster would be greater. The average scores were progressively larger for survival of larvae until settlement (5.05), survival of spat from settlement to 2 mm (5.81), and survival from 2 mm to 10 mm (6.25). The respondents pointed out that selective breeding would not be a universal remedy for the problems of the oyster industry. Breeding should complement advances in husbandry methods. More research on nutrition, feeding, and mortality was requested. It was argued that many traits, for example proportion of oysters that spawn, would improve without deliberate selection as the domestication of oyster species proceeded. Respondents urged that there should be more research on the mechanism of sex determination and the control of fertility. It would be desirable to have an oyster population which bred only under specified, controllable conditions. A cross of two diverse strains that yielded a sterile hybrid would be valuable. There was a demand for distinctive, recognisable lines of oysters, the characteristics of which could become associated with quality by the consumer. Respondents’ scores identified several favoured prospective breeding goals but improvement in the pertinent traits is not possible unless there is genetic variation either within or between populations. Genetic variation for various oyster traits has recently been reviewed by Newkirk (1980). Additive genetic variation has been reported for larval survival in Crassostrea gigas (Lannan, 1972), growth rate of larvae and spat in C. virginica (Losee, 1978), and larval growth rate in C. virginica (Newkirk et al., 1977). Population differences have been reported for larval growth at different salinities in C. virginica (Newkirk, 1978), and spat growth in C. virginica (Mallet and Haley, in Newkirk, 1980). A positive response to selection has been reported for resistance to MSX disease in Delaware oysters (Haskin and Ford, 1978), low summertime mortality in C. gigus (Beattie et al., 1978), and growth rate
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in C. uirginica (Haley, in Newkirk, 1980). The evidence for genetic variation in the traits which the survey indicated to be important favours the potential of selective oyster breeding. The clarification of breeding goals by surveys logically precedes the genetical and economic analysis of oyster production which would form the basis of a selective breeding programme. The method of index selection has been developed for the simultaneous improvement of several traits within a population. The construction of a selection index requires the estimation of the genetic variances and covariances for the traits to be improved and the net economic value of a change of one unit in each trait. The survey of breeding goals in oysters reported here indicates which traits should be included in an index. Genetic improvement might proceed by strain substitution, and the economic analysis which has been advocated would be indispensible for interpreting the results of experiments to evaluate and compare various oyster strains. ACKNOWLEDGEMENTS
The author is grateful to the many oyster researchers and producers who responded to the survey, and to Dr. P. Rodhouse for valuable discussions. The work was supported by a grant from the National Board of Science and Technology (Ireland).
REFERENCES Beattie, J.H., Hershberger, W.K., Chew, K.K., Mahnken, C., Prentice, E.F. and Jones, C., 1978. Breeding for resistance to summertime mortality in the Pacific oyster (Crassostrea gigas). Washington Sea Grant Rep. WSG 78-8, 13 pp. Cunningham, E.P., 1974. Cost-effectiveness and population structure in cattle programs. Ann. Genet. Sel. Anim., 5: 239-255. Harvey, W.H., 1976. User’s guide for LSML76. Mixed model least-squares and maximum likelihood computer program. Ohio State Univ., Columbus, OH (mimeographed). Haskin, H.H. and Ford, S.E., 1978. Mortality patterns and disease resistance in Delaware Bay oysters. Proc. Nat. Shellfish, Assoc., 68: 80. Lannan, J.E., 1972. Estimating heritability and predicting response to selection for the Pacific oyster, Crassostrea gigas. Proc. Nat. Shellfish Assoc., 62: 62-66. Losee, E., 1978. Influence of heredity on larval and spat growth in Crassostrea virginica. In: J.W. Avault (Editor), Proceedings of the Ninth Annual Meeting, World Mariculture Society, pp. 101-107. Newkirk, G.F., 1978. Interaction of genotype and salinity in larvae of the oyster, Crassostrea virginica. Mar. Biol., 48: 227-234. Newkirk, G.F., 1980. Review of the genetics and the potential for selective breeding of commercially important bivalves. Aquaculture, 19: 209-228. Newkirk, G.F., Haley, L.E., Waugh, D.L. and Doyle, R.W. Genetics of larvae and spat growth in the oyster, Crassostrea virginica. Mar. Biol., 41: 49-52. Steel, R.D.G. and Torrie, J.H., 1960. Principles and Procedures Statistics. McGraw-Hill, New York, 481 pp. Wilkins, N.P., 1981. The rationale and relevance of genetics in aquaculture: an overview. Aquaculture, 22: 209-228.