- Email: [email protected]
Genetic differentiation among seasonally distinct spawning populations of chum salmon, Oncorhynchus keta
Aquaculture,
211
57 (1986) 211-217
Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
Genetic Differentiation among Seasonally Distinct Spawning Populations of Chum Salmon, Oncorh ynchus keta ROSS F. TALLMAN’ Institute
of Animal Resource
1 W5 (Canada) and Pacific Biological Station,
Ecology,
Nanaimo,
University
of British
Columbia,
Vancouver,
B.C. V6T
B.C. V9R 5K6 (Canada)
Mailing address: Department of Fisheries and Oceans, Government of Canada, Pacific Biological Station, Nanaimo, B.C. V9R 5K6 (Canada)
ABSTRACT Tallman, R.F., 1986. Genetic differentiation among seasonally distinct spawning populations of chum salmon, Oncorhynchus keta. Aquaculture: 57: 211-217. Fry from three Vancouver Island chum salmon populations were reared under identical conditions from egg onward to determine if genetic divergence occurs among populations that spawn in different seasons. Comparisons of genetic traits, such as incubation rate and external morphology, revealed significant differences between the autumn spawning stock and the two winter spawning stocks. At constant 6”C, simulated autumn spawning regime and simulated winter spawning regime incubation rates of winter stock embryos were more rapid than those of the autumn spawning population. However, genotype-environment interaction occurred at constant 10’ C such that autumn stock embryos incubated more rapidly. Discriminant analysis of 10 morphometric features of the progeny separated groups corresponding to the three stocks. Environmental factors were important as the key characteristics of separation varied with temperature regime. Both genotype and genotype-environment interactions contribute to divergence among seasonally distinct spawning populations.
INTRODUCTION
Resource-use patterns of marine biota are in transition from a hunting-gathering economy to a farming economy (Towle, 1983). Generation of domesticated or semi-domesticated stock from wild organisms is an important step in this process. The success of the transition will thus depend on the genetic variability of many traits of wild organisms. Elucidation of stock distinctness, genetic flexibility and adaptations among wild populations will provide a starting point for efforts in domestication and ultimately, in the genetic engineering of aquacultural animals (Colwell, 1982).
0044-8486/86/$03.50
0 1986 Elsevier Science Publishers B.V.
212
Anadromous salmonid aquaculture programs exercise the greatest control over the freshwater phase of production. According to Gjedrem (1983), aquaculture problems can partly be overcome by choosing stock suitable for a particular system. Investigations of wild stock genetic variability in traits such as incubation rate and external morphology of the post-emergent fry will be both beneficial and tractable. Tallman and Healey (1986) hypothesized that genetic divergence in incubation rate has occurred among seasonal ecotypes of chum salmon, Oncorhynthus heta. Stabilizing selection for synchronized downstream fry migration among populations in the spring has resulted in more rapid embryonic incubation in the winter spawning populations compared to the autumn spawning stocks. In contrast, morphological characteristics of the fry did not appear to follow this pattern and might have been related to the variations in spawning environment. A critical test of this hypothesis is to compare the performance of these ecotypes under identical thermal conditions in the laboratory. Few comparisons of the external morphology of emergent fry have been made among seasonal races. Surprisingly little information exists regarding adaptation of incubation rate. This study investigates the influence of genotype on incubation rate and fry morphological variation among seasonally distinct spawning populations of chum salmon. MATERIALS AND METHODS
The wild parent stock “Autumn Bush” (AB) spawned mainly in October while “Winter Bush” (WB) and “Walker” (W) spawned in late November tomid-December (Tallman and Healey, 1986). Sample populations from each stock were reared under identical conditions during 1983-1984 to determine the influence of genotype on incubation rate to hatch and emergence. To determine the influence of thermal regime and genotype-environment interaction, sub-samples from each sample population were reared under four different temperature regimes. The protocol was as follows: five males and five females from each population were mated to produce 25 families per population. These were pooled and then distributed among four different temperature regimes: constant 6 oC; constant 10’ C; ‘early spawning’ regime which simulated a natural fall-winter-spring progression in temperature and a ‘late spawning’ regime which simulated a natural winter-spring temperature pattern. A cross-mated population consisting of the progeny from mating between five late Bush males and five Walker females was also tested. Each population-temperature regime treatment was replicated to estimate tank-to-tank variation. Observations were made on the experimental groups each week until hatch or emergence was imminent, then observations were made daily to record the number of individuals hatched or emerged each day.
213
To determine the contribution of maternal and additive genetic variance to time to hatch and emergence within the populations, samples from the 25 families produced per population were also reared individually at 8°C during 1983-1984. The sire, dam and sire plus dam heritabilities were estimated using the intraclass correlation (Becker, 1975). Maternal effects were also measured by comparing egg weights of preserved water-hardened eggs from different females by analysis of covariance, using the female length as a covariate. To compare the influence of genotype, environment and genotype-environment interaction on post-emergent juvenile morphology, temporally stratified samples of 50 emergent fry from each replicate were preserved in 5% buffered formaldehyde. Total length (TL), standard length (STL), head length (HL), snout length (SNL), pectoral fin (PFL), eye diameter (ED), head depth (HD), body depth (BD) and wet weight (WT) were measured as described by Hubbs and Lagler (1958) plus the number of parr marks (PM) on each fish. A parr mark was defined as any discrete vertical bar of dark pigment exceeding 40% of the body depth at that point. Thus, acceptable parr mark size varied with location on the fish. Two-way analysis of variance with interaction was performed to estimate the effect of population and temperature regime on the time to hatch and emerge. Heritability estimates and their standard errors were calculated by the factorial method described by Becker (1975). The lower confidence limits of the heritability estimates were generated via a Monte Carlo simulation technique similar to that of Rodda et al. (1977) assuming that the error entered when the data for hatch and emergence time were gathered. The error in the estimates of hatch time and emergence time of each family was assumed to be normally distributed. Two hundred heritability estimates were computed for hatch and emergence time of each sub-population from simulated random samples of 100 offspring per family. The tenth lowest simulated heritability estimate was taken as the lower confidence limit. Stepwise discriminant analysis was used to determine the most important traits for separating the populations as well as estimating the degree of separation. The effect of incubation environment on the separation among the populations was compared by discriminant analysis of the sample populations stratified by temperature regime. Variables were added into the discriminant function until the multiple correlation coefficient, R2, of each of the remaining variables with those already entered was greater than 0.40 (Dixon, 1981). RESULTS
The general result from the laboratory rearing experiments was that late stocks incubated more rapidly. Two-way ANOVA showed significant differ-
214
40-
8
160-
z
170.
&
160.
k B
150-
0
____---0 ______----a..
_.
/J
140P Y 8
130-
loo> l20-
IIO-
6 CELSIUS
IO CELSIUS
INCUBATION REGIME Fig. 1. Comparisons of the mean number of days to hatch and emergence among the experimental populations. Each popu!ation incubated under four temperature regimes. AB = Autumn Spawning Bush Creek Stock; WB = Winter Spawning Bush Creek Stock, W = Winter Spawning Walker Creek Stock. Temperature regimes: constant 6°C; constant 10 oC; early = fall-winter-spring progression; late = winter-spring progression.
ences among populations for time to hatch and time to emergence (P < 0.0001, Fig. 1). However, significant genotype-environment interaction occurred at 10°C such that the WB progeny incubated more slowly than the AB progeny. This interaction was especially pronounced for time to hatch. In 1983-1984 deaths were under 5% in all the within-population matings. Mortalities were from 60 to 99% in the crossed population. Families in which mortality exceeded 10% were not used to estimate heritability for time to hatch and time to emergence. To avoid missing cells in the analysis, rows and columns were dropped as necessary to maintain a factorial design. The sire by dam (S x D) design for time to hatch of AB was reduced to a 4 x 4, WB to 4 x 3 and W to a 3 x 4. The S x D design for the time to emerge was4X4forAB;2X5forWB;3X4forW. Heritability based on combined sire and dam components for time to hatch was 0.27 for AB, 0.35 for WB and 0.50 for W. Heritability for time to emergence
215 TABLE 1 F. Values testing the equality of means for each pair of populations using variables included in the discriminant functions [i.e., F(AB vs. WB) at 6°C = 89.231 Temp. regime
AB vs. WB
AB vs. W
WB vs. W
6°C 10°C Early Late Pooled
89.23** 146.47** 72.78** 74.67** 132.79**
105.09** 146.47** 229.60** 74.52** 1s5.77**
20.77** 134.46** 96.46** 16.70** 2g.93**
(Degrees of freedom: pooled= 8,590; single temperature = 7, 141). **P < 0.0001.
exceeded that for time to hatch in all populations (0.50 - AB, 0.40 - WB, 0.54 - W). In all cases, except time to emergence of AB, hg was greater than ha. The lower bounds of the 95% confidence limits estimated by simulations were greater than 0 in all cases. Analysis of covariance for egg weight differences among stock revealed that WB had significantly smaller eggs than W or AB. There was no difference between the size of the W and AB adjusted egg weights. Stepwise discriminant analysis of the populations with all temperature treatments pooled revealed that AB progeny were morphologically distinct from those of WB and W, although all paired population comparisons were significant (Table 1)Symptomatic of the relatively greater morphological overlap between WB and W was the much higher misclassification among these two groups by the discriminant functions (Table 2). The discriminators, in order of importance, were: ED, SNL, PM, PFL, HL, HD, and STL. When the discriminant functions were calculated using fry reared under a single temperature the percentage misclassifications decreased (Table 2). In general, a marked discontinuity occurred between progeny of the early spawning populations and those of the late spawning populations (Table 1). The important discriminators changed with the temperature of incubation. The discriminators, in order of importance, were: ED, SNL, HD, BD, HL, PM, WT at 6°C; ED, PM, HL, PFL, SNL, BD, HD at 10°C; ED, SNL, HL, WT, HD, PFL, BD under the “early spawning” regime; and HL, PM, PFL, STL, ED, HD, TL under the “late spawning” regime; DISCUSSION
The results show clearly that the late spawning stocks incubate more rapidly to both hatch and emergence. I believe that the relatively inefficient incubation of the WB and W progeny compared to AB at 10°C indicates adaptation to
216 TABLE 2 Jackknifed classification using discriminant functions Classified as: Temp. regime
Sample origin
AB
WB
W
6°C 6°C 6°C
AB WB W
46 0 0
2 44 2
2 6 48
10°C 10°C 10°C
AB WB W
50 0 0
0 36 6
0 14 44
Early Early Early
AB WB W
50 2 0
0 48 0
0 0 50
Late Late Late
AB WB W
48 2 0
0 44 2
2 4 48
Pooled Pooled Pooled
AB WB W
178 16 2
20 138 44
2 46 154
the season of reproduction. The late spawning populations would never encounter 10’ C in their early incubation while AB progeny are likely to experience high temperatures in some years. Differences in the kinetic efficiencies and stabilities of enzyme morphs is the probable mechanism responsible (Hochachka and Somero, 1984). The relatively high heritabilities of time to hatch and emergence suggest that incubation rate may be modified rather rapidly by selective breeding programs or by ecological forces in the wild. It confirms the hypothesis that chum salmon populations are genetically adapted to their season of reproduction with respect to incubation rate. I suspect that the higher heritability of time to emergence was due to a common environmental effect. Godin (1982) proposed that fry within a redd should behaviorally synchronize their timing of emergence in order to ‘swamp’ predators. Fry that emerge earlier or later than their fellows will have a higher probability of being eaten. To improve their chances of survival rapidly developing alevins may delay their emergence until others are prepared to leave while slower developing alevins move out of the gravel prematurely. The variance in the time to emergence would be reduced within families. This would drive heritability estimates up. Early and late spawners produce distinctly different emergent fry. Egg size
217
differences cannot account for this variation since AB and W had similarly sized eggs while their progeny had different external morphology. Therefore, the differences observed are probably a result of genetic divergence. I conclude that both incubation rate and the resulting fry morphology are dependent on the season of reproduction of the parent population. The high heritability of incubation rate is encouraging for the selective breeding prospects for this species. For example, a rapid incubating broodstock could be established using a winter spawning wild stock as seed. ACKNOWLEDGEMENTS
M.C. Healey, T.G. Northcote and B. Riddell provided. helpful advice and criticism of this study. M.C. Healey kindly supplied laboratory space and helped collect experimental stock. I was supported for this study by a Natural Sciences and Engineering Research Council of Canada Post-graduate Scholarship.
REFERENCES Becker, W., 1975. A Manual of Quantitative Genetics, 3rd edition. Washington State University, Pullman, WA, 170 pp. Colwell, R.R., 1982. Potential genetic engineering for aquaculture. ICES Council Meeting, ICES, Copenhagen, 10 pp. Dixon, W.J., 1981. Biomedical Computer Programs, P-Series. University of California Press, Berkeley, CA, 880 pp. Gjedrem, T., 1983. Genetic variation in quantitative traits and selective breeding in fish and shellfish. Aquaculture, 33: 51-72. Godin, J.-G., 1982. Migrations of salmonid fishes during early life history phases: daily and annual timing. In: E.L. Brannon and E.O. Sale (Editors), Proc. Salmon and Trout Migratory Behaviour Symposium, 3-5 June 1981. School of Fisheries, University of Washington, Seattle, WA, pp. 22-50. Hochachka, P. and Somero, G., 1984. Biochemical adaptation. Princeton University Press, 537 PP.
Hubbs, C.L. and Lagler, K.F., 1958. Fishes of the Great Lakes. University of Michigan Press, Ann Arbor, MI, 213 pp. Rodda, D., Schaffer, L., Mullen, K. and Friars, G., 1977. Measuring the precision of genetic parameters by simulation technique. Theor. Appl. Genet., 51: 35-40. Tallman, R.F. and Healey, MC., 1986. Stabilizing selection of phenotype among seasonal ecotypes of chum salmon (Oncorhynchus keta). Can. J. Fish. Aquat. Sci. (in review). Towle, J.C., 1983. The Pacific salmon and the process of domestication. Geogr. Rev., 73: 287-300.
211
57 (1986) 211-217
Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
Genetic Differentiation among Seasonally Distinct Spawning Populations of Chum Salmon, Oncorh ynchus keta ROSS F. TALLMAN’ Institute
of Animal Resource
1 W5 (Canada) and Pacific Biological Station,
Ecology,
Nanaimo,
University
of British
Columbia,
Vancouver,
B.C. V6T
B.C. V9R 5K6 (Canada)
Mailing address: Department of Fisheries and Oceans, Government of Canada, Pacific Biological Station, Nanaimo, B.C. V9R 5K6 (Canada)
ABSTRACT Tallman, R.F., 1986. Genetic differentiation among seasonally distinct spawning populations of chum salmon, Oncorhynchus keta. Aquaculture: 57: 211-217. Fry from three Vancouver Island chum salmon populations were reared under identical conditions from egg onward to determine if genetic divergence occurs among populations that spawn in different seasons. Comparisons of genetic traits, such as incubation rate and external morphology, revealed significant differences between the autumn spawning stock and the two winter spawning stocks. At constant 6”C, simulated autumn spawning regime and simulated winter spawning regime incubation rates of winter stock embryos were more rapid than those of the autumn spawning population. However, genotype-environment interaction occurred at constant 10’ C such that autumn stock embryos incubated more rapidly. Discriminant analysis of 10 morphometric features of the progeny separated groups corresponding to the three stocks. Environmental factors were important as the key characteristics of separation varied with temperature regime. Both genotype and genotype-environment interactions contribute to divergence among seasonally distinct spawning populations.
INTRODUCTION
Resource-use patterns of marine biota are in transition from a hunting-gathering economy to a farming economy (Towle, 1983). Generation of domesticated or semi-domesticated stock from wild organisms is an important step in this process. The success of the transition will thus depend on the genetic variability of many traits of wild organisms. Elucidation of stock distinctness, genetic flexibility and adaptations among wild populations will provide a starting point for efforts in domestication and ultimately, in the genetic engineering of aquacultural animals (Colwell, 1982).
0044-8486/86/$03.50
0 1986 Elsevier Science Publishers B.V.
212
Anadromous salmonid aquaculture programs exercise the greatest control over the freshwater phase of production. According to Gjedrem (1983), aquaculture problems can partly be overcome by choosing stock suitable for a particular system. Investigations of wild stock genetic variability in traits such as incubation rate and external morphology of the post-emergent fry will be both beneficial and tractable. Tallman and Healey (1986) hypothesized that genetic divergence in incubation rate has occurred among seasonal ecotypes of chum salmon, Oncorhynthus heta. Stabilizing selection for synchronized downstream fry migration among populations in the spring has resulted in more rapid embryonic incubation in the winter spawning populations compared to the autumn spawning stocks. In contrast, morphological characteristics of the fry did not appear to follow this pattern and might have been related to the variations in spawning environment. A critical test of this hypothesis is to compare the performance of these ecotypes under identical thermal conditions in the laboratory. Few comparisons of the external morphology of emergent fry have been made among seasonal races. Surprisingly little information exists regarding adaptation of incubation rate. This study investigates the influence of genotype on incubation rate and fry morphological variation among seasonally distinct spawning populations of chum salmon. MATERIALS AND METHODS
The wild parent stock “Autumn Bush” (AB) spawned mainly in October while “Winter Bush” (WB) and “Walker” (W) spawned in late November tomid-December (Tallman and Healey, 1986). Sample populations from each stock were reared under identical conditions during 1983-1984 to determine the influence of genotype on incubation rate to hatch and emergence. To determine the influence of thermal regime and genotype-environment interaction, sub-samples from each sample population were reared under four different temperature regimes. The protocol was as follows: five males and five females from each population were mated to produce 25 families per population. These were pooled and then distributed among four different temperature regimes: constant 6 oC; constant 10’ C; ‘early spawning’ regime which simulated a natural fall-winter-spring progression in temperature and a ‘late spawning’ regime which simulated a natural winter-spring temperature pattern. A cross-mated population consisting of the progeny from mating between five late Bush males and five Walker females was also tested. Each population-temperature regime treatment was replicated to estimate tank-to-tank variation. Observations were made on the experimental groups each week until hatch or emergence was imminent, then observations were made daily to record the number of individuals hatched or emerged each day.
213
To determine the contribution of maternal and additive genetic variance to time to hatch and emergence within the populations, samples from the 25 families produced per population were also reared individually at 8°C during 1983-1984. The sire, dam and sire plus dam heritabilities were estimated using the intraclass correlation (Becker, 1975). Maternal effects were also measured by comparing egg weights of preserved water-hardened eggs from different females by analysis of covariance, using the female length as a covariate. To compare the influence of genotype, environment and genotype-environment interaction on post-emergent juvenile morphology, temporally stratified samples of 50 emergent fry from each replicate were preserved in 5% buffered formaldehyde. Total length (TL), standard length (STL), head length (HL), snout length (SNL), pectoral fin (PFL), eye diameter (ED), head depth (HD), body depth (BD) and wet weight (WT) were measured as described by Hubbs and Lagler (1958) plus the number of parr marks (PM) on each fish. A parr mark was defined as any discrete vertical bar of dark pigment exceeding 40% of the body depth at that point. Thus, acceptable parr mark size varied with location on the fish. Two-way analysis of variance with interaction was performed to estimate the effect of population and temperature regime on the time to hatch and emerge. Heritability estimates and their standard errors were calculated by the factorial method described by Becker (1975). The lower confidence limits of the heritability estimates were generated via a Monte Carlo simulation technique similar to that of Rodda et al. (1977) assuming that the error entered when the data for hatch and emergence time were gathered. The error in the estimates of hatch time and emergence time of each family was assumed to be normally distributed. Two hundred heritability estimates were computed for hatch and emergence time of each sub-population from simulated random samples of 100 offspring per family. The tenth lowest simulated heritability estimate was taken as the lower confidence limit. Stepwise discriminant analysis was used to determine the most important traits for separating the populations as well as estimating the degree of separation. The effect of incubation environment on the separation among the populations was compared by discriminant analysis of the sample populations stratified by temperature regime. Variables were added into the discriminant function until the multiple correlation coefficient, R2, of each of the remaining variables with those already entered was greater than 0.40 (Dixon, 1981). RESULTS
The general result from the laboratory rearing experiments was that late stocks incubated more rapidly. Two-way ANOVA showed significant differ-
214
40-
8
160-
z
170.
&
160.
k B
150-
0
____---0 ______----a..
_.
/J
140P Y 8
130-
loo> l20-
IIO-
6 CELSIUS
IO CELSIUS
INCUBATION REGIME Fig. 1. Comparisons of the mean number of days to hatch and emergence among the experimental populations. Each popu!ation incubated under four temperature regimes. AB = Autumn Spawning Bush Creek Stock; WB = Winter Spawning Bush Creek Stock, W = Winter Spawning Walker Creek Stock. Temperature regimes: constant 6°C; constant 10 oC; early = fall-winter-spring progression; late = winter-spring progression.
ences among populations for time to hatch and time to emergence (P < 0.0001, Fig. 1). However, significant genotype-environment interaction occurred at 10°C such that the WB progeny incubated more slowly than the AB progeny. This interaction was especially pronounced for time to hatch. In 1983-1984 deaths were under 5% in all the within-population matings. Mortalities were from 60 to 99% in the crossed population. Families in which mortality exceeded 10% were not used to estimate heritability for time to hatch and time to emergence. To avoid missing cells in the analysis, rows and columns were dropped as necessary to maintain a factorial design. The sire by dam (S x D) design for time to hatch of AB was reduced to a 4 x 4, WB to 4 x 3 and W to a 3 x 4. The S x D design for the time to emerge was4X4forAB;2X5forWB;3X4forW. Heritability based on combined sire and dam components for time to hatch was 0.27 for AB, 0.35 for WB and 0.50 for W. Heritability for time to emergence
215 TABLE 1 F. Values testing the equality of means for each pair of populations using variables included in the discriminant functions [i.e., F(AB vs. WB) at 6°C = 89.231 Temp. regime
AB vs. WB
AB vs. W
WB vs. W
6°C 10°C Early Late Pooled
89.23** 146.47** 72.78** 74.67** 132.79**
105.09** 146.47** 229.60** 74.52** 1s5.77**
20.77** 134.46** 96.46** 16.70** 2g.93**
(Degrees of freedom: pooled= 8,590; single temperature = 7, 141). **P < 0.0001.
exceeded that for time to hatch in all populations (0.50 - AB, 0.40 - WB, 0.54 - W). In all cases, except time to emergence of AB, hg was greater than ha. The lower bounds of the 95% confidence limits estimated by simulations were greater than 0 in all cases. Analysis of covariance for egg weight differences among stock revealed that WB had significantly smaller eggs than W or AB. There was no difference between the size of the W and AB adjusted egg weights. Stepwise discriminant analysis of the populations with all temperature treatments pooled revealed that AB progeny were morphologically distinct from those of WB and W, although all paired population comparisons were significant (Table 1)Symptomatic of the relatively greater morphological overlap between WB and W was the much higher misclassification among these two groups by the discriminant functions (Table 2). The discriminators, in order of importance, were: ED, SNL, PM, PFL, HL, HD, and STL. When the discriminant functions were calculated using fry reared under a single temperature the percentage misclassifications decreased (Table 2). In general, a marked discontinuity occurred between progeny of the early spawning populations and those of the late spawning populations (Table 1). The important discriminators changed with the temperature of incubation. The discriminators, in order of importance, were: ED, SNL, HD, BD, HL, PM, WT at 6°C; ED, PM, HL, PFL, SNL, BD, HD at 10°C; ED, SNL, HL, WT, HD, PFL, BD under the “early spawning” regime; and HL, PM, PFL, STL, ED, HD, TL under the “late spawning” regime; DISCUSSION
The results show clearly that the late spawning stocks incubate more rapidly to both hatch and emergence. I believe that the relatively inefficient incubation of the WB and W progeny compared to AB at 10°C indicates adaptation to
216 TABLE 2 Jackknifed classification using discriminant functions Classified as: Temp. regime
Sample origin
AB
WB
W
6°C 6°C 6°C
AB WB W
46 0 0
2 44 2
2 6 48
10°C 10°C 10°C
AB WB W
50 0 0
0 36 6
0 14 44
Early Early Early
AB WB W
50 2 0
0 48 0
0 0 50
Late Late Late
AB WB W
48 2 0
0 44 2
2 4 48
Pooled Pooled Pooled
AB WB W
178 16 2
20 138 44
2 46 154
the season of reproduction. The late spawning populations would never encounter 10’ C in their early incubation while AB progeny are likely to experience high temperatures in some years. Differences in the kinetic efficiencies and stabilities of enzyme morphs is the probable mechanism responsible (Hochachka and Somero, 1984). The relatively high heritabilities of time to hatch and emergence suggest that incubation rate may be modified rather rapidly by selective breeding programs or by ecological forces in the wild. It confirms the hypothesis that chum salmon populations are genetically adapted to their season of reproduction with respect to incubation rate. I suspect that the higher heritability of time to emergence was due to a common environmental effect. Godin (1982) proposed that fry within a redd should behaviorally synchronize their timing of emergence in order to ‘swamp’ predators. Fry that emerge earlier or later than their fellows will have a higher probability of being eaten. To improve their chances of survival rapidly developing alevins may delay their emergence until others are prepared to leave while slower developing alevins move out of the gravel prematurely. The variance in the time to emergence would be reduced within families. This would drive heritability estimates up. Early and late spawners produce distinctly different emergent fry. Egg size
217
differences cannot account for this variation since AB and W had similarly sized eggs while their progeny had different external morphology. Therefore, the differences observed are probably a result of genetic divergence. I conclude that both incubation rate and the resulting fry morphology are dependent on the season of reproduction of the parent population. The high heritability of incubation rate is encouraging for the selective breeding prospects for this species. For example, a rapid incubating broodstock could be established using a winter spawning wild stock as seed. ACKNOWLEDGEMENTS
M.C. Healey, T.G. Northcote and B. Riddell provided. helpful advice and criticism of this study. M.C. Healey kindly supplied laboratory space and helped collect experimental stock. I was supported for this study by a Natural Sciences and Engineering Research Council of Canada Post-graduate Scholarship.
REFERENCES Becker, W., 1975. A Manual of Quantitative Genetics, 3rd edition. Washington State University, Pullman, WA, 170 pp. Colwell, R.R., 1982. Potential genetic engineering for aquaculture. ICES Council Meeting, ICES, Copenhagen, 10 pp. Dixon, W.J., 1981. Biomedical Computer Programs, P-Series. University of California Press, Berkeley, CA, 880 pp. Gjedrem, T., 1983. Genetic variation in quantitative traits and selective breeding in fish and shellfish. Aquaculture, 33: 51-72. Godin, J.-G., 1982. Migrations of salmonid fishes during early life history phases: daily and annual timing. In: E.L. Brannon and E.O. Sale (Editors), Proc. Salmon and Trout Migratory Behaviour Symposium, 3-5 June 1981. School of Fisheries, University of Washington, Seattle, WA, pp. 22-50. Hochachka, P. and Somero, G., 1984. Biochemical adaptation. Princeton University Press, 537 PP.
Hubbs, C.L. and Lagler, K.F., 1958. Fishes of the Great Lakes. University of Michigan Press, Ann Arbor, MI, 213 pp. Rodda, D., Schaffer, L., Mullen, K. and Friars, G., 1977. Measuring the precision of genetic parameters by simulation technique. Theor. Appl. Genet., 51: 35-40. Tallman, R.F. and Healey, MC., 1986. Stabilizing selection of phenotype among seasonal ecotypes of chum salmon (Oncorhynchus keta). Can. J. Fish. Aquat. Sci. (in review). Towle, J.C., 1983. The Pacific salmon and the process of domestication. Geogr. Rev., 73: 287-300.