Inbreeding in fish populations used for aquaculture

Aquaculture, 33 (1983) 215-227 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

215

INBREEDING IN FISH POPULATIONS USED FOR AQUACULTURE HAROLD L. KINCAID Fish Genetics (U.S.A.) (Accepted

Station,

U.S. Fish and Wildlife Service, Box 700, Kearneysville,

WV 25430

1 December 1982)

ABSTRACT Kincaid, H.L., 1983. Inbreeding in fish populations used for aquaculture. Aquaculture, 33: 215-227. The theoretical causes of inbreeding depression are reviewed. Studies to measure depression at a series of inbreeding levels in rainbow trout populations using the method of inbred-outbred half-sib families are discussed. While the actual depression estimates varied widely between populations and inbreeding levels, significant levels of depression were found in many hatchery, field, and brood stock performance traits after only one generation of brother--sister mating. Especially susceptible to inbreeding depression, were measures of growth based upon attained fish weight at a given age in each test situation examined: 147 day weight (0.0 to 19.1%). 364 day weight (6.2 to 62.8%), weight after 6 months in fishing pond (-4.6 to 25.4%), weight after 12 months in fishing pond (13.0 to 29.1%), a-year male weight (11.3 to 55.3%), and a-year female weight (10.2 to 57.0%). Other hatchery performance traits that showed inbreeding depression were: egg hatchability (-0.2 to 53.1%), fry survival (0.4 to 8.2%), feed conversion efficiency (5.0 to 9.0%), fiih length at a-years of age (4.7 to 36.9%). and egg mass produced at %-years (12.1 to 57.0%). Field performance traits that yielded inbreeding depression were percent recovery of fish stocked (-2.5 to 41.1%) and biomass index (16.2 to 47.7%). Literature on the effects of inbreeding in fish populations is reviewed and the breeding approaches for controlling the rate of inbreeding accumulation in brood stock populations are outlined and discussed.

INTRODUCTION

The increasing role of aquaculture as a source of protein and other products during the past decade has forced fishery managers to begin to address many problems associated with the intensive management of small populations. Any fishery operation that results in a limited number of fish being available to produce progeny for use as brood stock in the next generation can lead to a constriction in the gene pool of that population, either in a hatchery or a natural fishery. These constrictions can occur as a result of breeding practices such as: the selection of a small number of “superior” individuals as brood stock, the use of fish taken from a small segment of the spawning season, the use of limited numbers of individuals that survive a

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216

major collapse of the fishery, or the use of a small number of brood fish because they provide adequate numbers of eggs or progeny to meet management needs. The potential for acceleration of the natural rate of inbreeding is presented by each of these breeding practices. Current understanding of the causes and effects of inbreeding in fish populations are reviewed in this paper and general breeding methods available to the brood stock manager to minimize rates of inbreeding are outlined. During recent years the techniques of gynogenesis and sex reversal have been applied as an approach for the production and study of inbreeding effects in aquatic populations. However, further examination of sex control techniques and their application to inbreeding questions is beyond the scope of this paper. The reader is referred to the review by Yamazaki (1983, this volume) on sex control and manipulation. THEORETICAL

APPROACH TO INBREEDING

Inbreeding in an infinitely large population is defined as the mating of individuals that are more closely related to each other than individuals mating at random within a population. However, the populations actua.lly used in most aquacultural programs are finite populations because they possess a limited number of members. All finite populations experience some degree of inbreeding that is based on the number of individuals that contribute progeny to each succeeding generation. Inbreeding is measured by a value called the inbreeding coefficient which is normally represented by the symbol F. The inbreeding coefficient (F) is the probability that two alleles at any locus are identical and descended from a common ancestor. Since the inbreeding coefficient is a probability, it can assume only the values within the range from zero to 1.0. Inbreeding coefficients express the amount of inbreeding that has accumulated starting from a specific point in the ancestry of the population. Because the number of independent ancestors is limited in any finite population, all alleles of a single form would be identical and descended from a common ancestor if they were traced far enough into the past. Therefore, the inbreeding coefficient is meaningful only if a specific time in the past is chosen beyond which ancestries will not be considered, and at which time all alleles are considered to be independent. This point is called the base population, and by convention it is considered to have an inbreeding coefficient of zero. The inbreeding coefficient of any given generation therefore expresses the fraction or percent of heterozygosity that has been lost since the base population generation. When an F-value is given for a brood stock population, the specific base population is not always identified but the existence of a base population is always implied. Inbreeding depression is the effect of inbreeding normally measured as a reduction in the expected performance of the affected trait. Inbreeding depression is measured as the average performance difference between an in-

217

bred population and the base population. Traits that frequently exhibit inbreeding depression are multi-locus or quantitative traits associated with reproductive capacity (e.g. fecundity, egg size, hatchability) and physiological efficiency (e.g. fry deformities, growth rate, feed conversion efficiency, survival). Other traits may or may not show depression with inbreeding. The characteristics of inbreeding can be studied by examination of what happens to gene frequencies in a brood stock population that is randomly divided into a series of mating pairs which are then isolated for several generations to establish inbred lines. This procedure yields a series of inbred lines that collectively represent the gene pool of the original generation. Gene frequencies in the separate inbred lines would vary widely as random genetic drift caused fixation of some alleles and the loss of some alleles. This process leads to reduced heterozygosity in the individual lines, however, with a large number of inbred lines to represent the origmal gene pool and random fixation of alleles within each line, the gene frequencies collectively across all inbred lines should be the same as in the original population. This would be the case within sampling errors and provided that no lines were lost during the inbred line development process. When lines are lost, the effect is to produce a modified population that is changed by the selective loss of the “unfit” lines. The characteristics of inbreeding shown in this ideal population demonstrate that inbreeding depression is the result of differences in the genetic value of homozygotes relative to heterozygotes. The primary cause for this difference is the within locus interaction of alleles, a phenomenon known as dominance. The effects of inbreeding depression in quantitative traits can be accounted for largely by the phenotypic expression of increasing numbers of unmasked recessive alleles and the reduced frequency of heterozygous loci expressing codominance and overdominance. Inbreeding depression tends to increase in proportion to the inbreeding coefficient during the initial inbreeding generations as reproductive capacity and viability are reduced and some inbred lines are lost. However, as lines are lost, the surviving lines become a selected population and the theoretical expectations for rate of inbreeding are no longer directly applicable. As a result the prediction of increasing rate of inbreeding depression from the increasing inbreeding level is limited to the initial generations before the inbreeding coefficient reaches a high level. Traits measured during egg or fry stages, such as egg size, hatchability, fry mortality, or fry growth rate are often affected by maternal influences. Such traits are doubly sensitive to inbreeding depression because of the influence not only of the inbred genotype of the individual, but also the genotype of the inbred female parent as it effects the maternal environment provided during the egg development stage. As a result of changes in the total gene pool caused by the collective effects of natural selection and maternal influences, the relationship between inbreeding depression measured in a trait and the coefficient of inbreeding is usually non-linear, after the initial generations. A frequent problem when working with fish brood stocks is that available

I

8Nf

BN,

-+-

1

1

in inbreeding

levels shown

0.1312 0.0478 0.0312 0.0187 0.0125 0.0113 0.0088 0.0079 0.0075 0.0071 0.0069 0.0068 0.0062

20 0.1300 0.0467 0.0300 0.0175 0.0113 0.0100 0.0075 0.0067 0.0063 0.0058 0.0056 0.0055 0.0050

25

mating

N,

of table are calculated

0.1375 0.0541 0.0375 0.0250 0.0187 0.0175 0.0150 0.0141 0.0137 0.0133 0.0131 0.0130 0.0125

in body

0.1500 0.0667 0.0500 0.0375 0.0312 0.0300 0.0275 0.0266 0.0262 0.0258 0.0256 0.0255 0.0250

10

in random

where A F = increase in inbreeding coefficient per generation, to produce the next generation of brood stock.

AF=

*Change

0.1267 0.1262 0.1258 0.1256 0.1255 0.1250

75 100 150 200 250

0.1667 0.0833 0.0667 0.0541 0.0478 0.0467 0.0442 0.0433 0.0425 0.0424 0.0423 0.0422 0.0417

3

1

5

of male parents

stock*

(AF) per generation

Number

0.2500 0.1667 0.1500 0.1375 0.1312 0.1275 0.1300

of

1 3 5 10 20 ix

Number female parents

Rate of inbreeding increase used to maintain the brood

TABLE

= number

0.1267 0.0433 0.0266 0.0141 0.0079 0.0067 0.0041 0.0033 0.0029 0.0025 0.0023 0.0022 0.0017

75 0.1262 0.0425 0.0262 0.0137 0.0075 0.0063 0.0038 0.0029 0.0025 0.0021 0.0019 0.0018 0.0013

100

when different

of male parents,

by formula

0.1275 0.0442 0.0275 0.0150 0.0088 0.0075 0.0050 0.0041 0.0038 0.0033 0.0031 0.0030 0.0025

50

populations

0.1255 0.0422 0.0255 0.0130 0.0068 0.0055 0.0030 0.0022 0.0018 0.0013 0.0011 0.0010 0.0005

250

of female

0.1256 0.0423 0.0256 0.0131 C!.OO69 0.0056 0.0031 !I.0023 0.0019 0.0015 0.0013 0.0011 0.0006

200

0.1250 0.0417 0.0250 0.0125 0.0062 0.0050 0.0025 0.0017 0.0013 0.0008 0.0006 0.0005 0.0000

-

are

used

parents

parents

of male and female

and Nf = number

0.1258 0.0424 0.0258 0.0133 0.0071 0.0058 0.0033 0.0025 0.0021 0.0017 0.0015 0.0013 0.0008

150

numbers

219

information on the breeding history simply is inadequate to determine the level of inbreeding. Information on the genealogy of individual fish is almost never known, thereby eliminating most classical methods for the calculation of inbreeding coefficients. When information on the actual number of fish of each sex in the breeding population is known for several generations, an estimate of the inbreeding accumulated during that period can be calculated using approximation methods (Falconer, 1981). One formula frequently used when working with random mating populations is AF=

L+L slv,

8Nf

Where, AF = the expected increase in the inbreeding coefficient per generation, N, = number of male parents actually used to produce the next brood stock generation, and ZVf = number of female parents actually used to produce the next brood stock generation. The calculated AF value for each generation is added to the inbreeding coefficient of the preceeding generation to yield the new inbreeding coefficient. Inbreeding coefficients estimated by this approximation method yield overestimates of the actual inbreeding rate after the first generation with the magnitude of the over estimation decreasing as the effective population size increases. The estimated rate of inbreeding accumulation for a range of male and female parent number combinations is given in Table I. When historic records on breeding practices and brood stock parent numbers are unavailable, the inbreeding coefficient can not be calculated directly. INBREEDING

DEPRESSION

IN AQUATIC

ORGANISMS

The deleterious effects of inbreeding depression in domestic and laboratory animal species have long been recognized by research workers (Fisher, 1949; Robinson and Bray, 1965; Hill and Robertson, 1968; Falconer, 1981). In recent years a few reports on the effect of inbreeding in fish species have begun to appear in the published literature. Moav and Wohlfarth (1963) reported a 15% reduction in relative growth rate in inbred carp (Cyprinus carpio) produced from full-sib parents as well as an increased frequency of fish with dorsal fin anomalies. Aulstad and Kittelsen (1971) described an increase in the occurrence of fry deformities in rainbow trout (S&no gairdneri) that was associated with an inbreeding coefficient of F = 0.25. Ryman (1970) found lower recapture frequencies from inbred families of Atlantic salmon (3. s&r) suggesting that lower .survival rates were associated with inbreeding. Inbreeding depression estimates, per 10% inbreeding, of 5.12% in fish weight and 0.44% in formalin tolerance at 150 days of age was reported by Bridges (1973) in rainbow trout. Gjerde et al. (1983) working with three generations of inbreeding in rainbow trout reported inbreeding depression per 10% inbreeding in egg mortality to the eye stage (2.5%), ale-

220

vin mortality (1.9%), fry mortality (3.2%), fingerling growth rate (3.0%), and growth rate to 18 months in seawater (5.1%). Estimates of inbreeding depression in body weight of brook trout (Salvelinus fontinalis) after one generation of brothel--sister mating were found to be 27.7% at 7 months and 34.4% at 19 months (Cooper, 1961). Kincaid (1976a, b) reported that one generation of brothersister mating in rainbow trout produced an increase in fry deformities (37.6%) and decreased feed conversion efficiency (5.6%), fry survival (19%), and fish weight at 147 days of age (11.0%) and 364 days of age (23.2%). After two generations of brother-sister mating even greater changes were measured: fry deformities increased (191%) while decreases were found in feed conversion efficiency (14.9%), fry survival (29.7%), and weight attained by fish at 147 days (13.4%) and at 364 days (33.5%). This trend of increasing depression in body weight with fish age appears to be associated with the cubic nature of the growth curve magnifying the reduced growth rate of the inbred fish. Fujino (1978) reported inbreeding depression in body weight in some individuals of a wild population of Pacific abalone (Haliotis discus). Individuals expressing reduced body weight were found to have a much higher frequency of homozygosity at two e&erase loci than normal animals. Longwell and Stiles (1973) and Stiles (1981) working with the American oyster (Crassostrea uirginica) reported that progeny from full-sib matings produced significantly lower survival of larvae to metamorphosis and higher frequencies of larval abnormalities than the outbred control lines. Lannan (1979, 1980) working with the Pacific oyster (Crassostrea gigas) found no depression in larval survival through two generations of inbreeding. Mrakovcic and Haley (1979) examined half-sib and full-sib families of the zebra fish (Brachydanio rerio) and reported no effect of inbreeding on hatchability in either group. Inbreeding depression was observed in egg fertility, frequency of abnormal fry, fry survival to 30 days, and fish length at 30 days in both inbreeding levels. From the work reviewed here as well as work reported on numerous domestic and laboratory animals, it would appear that in most diploid bisexual animal species, increasing levels of inbreeding yield reduced performance in a variety of traits. The type of traits most frequently reported to show inbreeding depression in fish species have been: increased fry abnormalities, reduced survival, reduced growth rate, and lowered reproductive success. The magnitude of the depression observed in a particular strain within a species varies widely depending on genetic background, historic inbreeding, cultural history, and rearing environment. INBREEDING

DEPRESSION

IN A NATURAL

FISHERY

Studies conducted by the U.S. Fish and Wildlife Service’s Fish Genetics Laboratory between 1975 and 1980 to evaluate the performance of inbred and outbred half-sib families stocked in a l-ha spring-fed fishing pond as

221

fingerlings provide further evidence of inbreeding depression in a natural environment. Fish were stocked in the pond at approximately 7 months of age with each family identified by the use of numbered stainless steel tags inserted in the body cavity. Recovery periods were conducted at 6-month intervals after stocking. Each recovery period consisted of an angling period during which 1000 angler hours of fishing pressure was applied and a gill netting period when four experimental gillnets were placed in the pond for a total of 100 net hours. All fish captured were removed from the fishery, identified by family, and individually weighed. In addition, samples from each family were also reared to 1 year of age in a standardized rearing environment. Inbreeding depression was measured as the average difference in performance of inbred and outbred half-sib families. One experiment using two groups of fish inbred for one and three generations from the winter 1977 year class showed depression in both hatchery and field performance traits (Table II). After one generation of inbreeding, hatchery performance traits showed inbreeding depression in hatchability, fry survival, feed conversion efficiency, and 364 day weight with actual depressions being similar to that previously reported (Kin&d, 1976a). Depression in the group inbred for three generations of brother-sister mating was not markedly different than that found after one generation except for an increased effect on weight at 147 days (16.0%) and 364 days (41.7%). However, since the two groups were derived from different strains it is not possible to compare the relationship of depression to increasing level of inbreeding. Inbreeding depression in field performance traits was evident in both groups after the fish had been in the pond for 6 months (Table II). Average fish weight in inbred groups was lower at both the 6 and 12 months recovery periods compared to outbred controls. Inbreds of the F = 0.25 group had a recapture rate that was lower than outbreds in the first recovery period and equal in the second period. This recapture pattern shows the proportion of the total fish recovered during each recovery period but does not give the actual survival rate of the inbred and outbred groups for each period. Because a higher proportion of fish from the outbred group was removed during the first period, fewer fish of that group were left in the fishery to compete for survival in the pond environment and to be available for harvest during the second recovery period. Therefore, the percent recovered in the second period relative to the number of fish potentially available for recapture, was lower in the inbred group. Measures of inbreeding depression in traits such as total percent recovery of fish planted and biomass index are used to indicate the net effect of inbreeding on production in this fishery. Since each of these traits reflects the contribution of many other traits (i.e., survival, predator avoidance, feeding behavior, social behavior, adaptability to environment, and growth rate) they must be considered to be composite performance traits. Depression in total percent recovery (6.9 to 21.2%) reflects the reduction in survival during the test period while de-

222 TABLE II Effects of inbreeding by brothemister mating for one and three generations in winter spawning rainbow trout. Growth and survival traits were evaluated in the hatchery, and during a la-month field test in a l-h fishing pond after stocking as fingerlings. Fish were from the winter 1977 year class Trait

Inbreeding coefficient family pairs

and number of inbred-outbred

F = 0.50 (13 pairs)

F = 0.25 (16 pairs) Inbred mean Hatchery performance Hatchability (%) Fry survival to 84 days days (%) Weight (g), 147 days Feed conversion 147 days Weight (g), 364 days Field performance Weight (g), at planting B-month recovery percent mean weight (g) 1 a-month recovery percent mean weight (g) Total percent recovery Biomass indexb

Outbred mean

half-sib

% de- * pression

Inbred mean

Outbred mean

% de- * depression

69.1

83.5

17.2

84.2

84.0

-0.2

89.2 3.4

94.7 3.4

5.8 0.0

89.3 3.0

89.7 3.6

0.4 16.0

2.3 68.0

2.2 91.0

-6.7 25.0

2.1 85.1

2.0 145.9

-5.0 41.7

28.6

31.4

8.8

36.4

35.1

-3.6

24.5 72.4

33.0 79.9

25.8 9.4

36.5 65.2

38.4 74.5

5.0 12.5

7.2 150.7

7.2 173.2

0.0 13.0

5.9 132.6

7.1 187.0

16.9 29.1

31.7

40.2

21.2

42.4

45.5

6.9

2858.8

3883.7

26.7

3162.1

4188.5

24.5

aPercent depression is calculated as outbred mean minus inbred mean divided by outbred mean. bBiomass index is the total biomass recovered per 100 fish planted.

pression in biomass index (24.5 to 26.7%) reflects both the reduction in survival and growth rate. Both traits demonstrate a significant reduction in the productivity of the inbred groups. The effect of inbreeding on seven growth and maturity traits at first maturity was measured in inbred lines produced by one, three, four and five generations of brother--sister mating (Table III). The measures of growth evaluated in this study - weight at 1 year and length and weight at 2 years - uniformly showed large and highly significant inbreeding depression at all four inbreeding levels. In addition, the magnitude of depression increased with each generation of inbreeding. The depression mea-

69

84

165 149 340 713 344 722 112

18.1 12

74

138 109 296 484 286 447

Outbred Inbred

22.3** 20.9* 9.2** 26.2** 9.1** 1g.o**

% Dep.a

3

mating

33.9**

16.4 27.2** 13.0** 32.2** 16.8** 3a.1**

% Dep.a

113

128 118 343 748 336 689

7

68

112 92 291 496 290 491

Outbred Inbred

4

184 182 394 1081 379 963 140

40.3**

2

60

120 106 319 629 296 469

Outbred Inbred

12.a* 21.9** 15.2** 33.7** 13.9** 28.8**

% Dep.a

5

57.0s

34.8 41.8** 19.0** 41.a* 21.8* 51.2*

% Depa

aPercent depression is measured as outbred mean minus inbred mean divided by outbred mean to measure the reduced performance of inbred families relative to that of outbred half-sib families. bEgg mass is the average weight in grams of eggs removed from each female using the hand spawning method after ovarian fluid was drained off.

Number of half-sib pairs

14

104 95 303 487 299 461

134 121 334 660 329 563

364 days d weight (g) 364 days Q weight (g) 2 years d length(mm) 2 years d weight (g) 2 years Q length (mm) 2 years Q weight (g)

Egg mass (g)b

Outbred Inbred

Trait

1

Generations of brother-sister

Inbreeding depression estimates on seven growth and maturity traits from four levels of inbreeding. Fish taken from winter 1977 year class

TABLE III

224

sured in total weight of eggs produced (egg mass) was large and statistically significant in all inbreeding groups except the first generation of inbreeding where the half-sib deviations were extremely variable. Depression in egg mass weight was closely related to depression in Z-year length of the female parent suggesting that the smaller size of the inbred female provided proportionately less body cavity volume for the developing ovaries. Since the traits egg size and number in each egg mass were not evaluated, the effect of the reduced egg mass on egg number and egg size is unknown. If the smaller egg mass resulted from reduced egg size, then the expected effect would be reduced hatchability and smaller fry with reduced fry survival If the smaller egg mass results from reduced fecundity, then the expected effect would be reduced numbers of eggs and lower numbers of live fry per breeding female. It is highly probable that both reduced egg size and reduced egg numbers contribute to the smaller egg mass found in the inbred groups. Studies completed in the rainbow trout demonstrate that inbreeding depression is expressed in a variety of performance traits throughout the life cycle from the egg stage to first sexual maturity. Investigations into the effects of inbreeding have been initiated in several aquacultural species, however, only limited information has been published to date. Knowledge of inbreeding effects in rainbow trout, while not directly applicable to other species, can provide an indication of the kinds of the traits that may be susceptible to inbreeding depression and the approximate magnitude of depression to be expected in other fish species. The fishery manager needs to know as much as possible about the effects of inbreeding because he is the one who will make decisions about: What source of brood stock will be used? How many individuals are needed in the brood stock to maintain the genetic diversity of the gene pool? What are the effects of a selection program? What are the risks of stocking wild fisheries with fish taken from a small fraction of the total spawning run? The answers to these questions and many others directly affect the long term vitality and productivity of most brood stocks. BREEDING

APPROACHES

TO CONTROL

INBREEDING

Fisheries personnel responsible for maintaining brood stocks must become more aware of the serious problems that can result from inbreeding so they can adopt breeding methods that will minimize future inbreeding problems. While inbreeding can be a powerful technique for developing new and improved strains, the negative aspect of inbreeding is what is most frequently seen by the individual fish breeder. Current breeding approaches to avoid inbreeding fall into three general categories: (1) the use of large random mating populations; (2) the use of systematic line crossing schemes to eliminate the mating of close relatives; and (3) strain crossing to produce hybrid populations. The use of large random mating populations is the simplest approach and requires only that the

225

breeder take steps to insure that a large number of fish contribute progeny to the next brood stock generation. One source of confusion frequently experienced by the breeder using large random mating populations is that the breeding population is defined as the number of fish used to produce the next brood stock generation, not the total number of fish in the population. The rate of inbreeding accumulation in a random mating population may also be seriously effected by the number of fish of each sex actually used to produce the progeny generation (Table I). An important factor in determining just how large a random mating brood stock population needs to be is the expected rate of inbreeding. The methods normally used to study the effects of inbreeding involve the production of many inbred lines by mating close relatives to produce a rapid rate of inbreeding. In actual brood stock management situations, the mating system would be expected to provide a much slower rate of inbreeding accumulation. The rate of inbreeding increase is extremely important because it indicates the time in generations required for inbreeding to increase to critical levels. It also determines how effectively selection pressures can be used to mitigate the effects of inbreeding. When the inbreeding rate is low the homozygosity level increases slowly, therefore, allowing time for selection to lower the frequency of undesirable genotypes through natural mortality and the discard of individuals that fail to meet performance criteria. On the other hand, when inbreeding rates are high and homozygosity levels increase rapidly, selection would not be less effective because of the expected higher egg and fry mortality rates and general reduction in performance throughout the population. One study that applied this principal to rainbow trout inbred for three generations and simultaneously selected for rapid growth rate found that the effects of inbreeding depression and selection essentially neutralized each other (Anderson and Woods, 1979). The number of fish used each generation to maintain a brood stock by the random mating method should be the largest number of breeding adults possible with multiple spawns taken throughout the spawning season. This ideal is often limited, however, by the practical considerations of available brood stock, spawning facilities, manpower, etc. The minimum number of breeding adults for maintaining a random mating brood stock should be at least 50 pairs or 50 of the least numerous sex with spawning adults taken throughout the spawning period. A breeding population of 50 adult pairs yields an expected rate of inbreeding increase of 0.5% (Table I) per generation provided that mating adults are randomly paired and that each mating pair contributes equal numbers of progeny to the succeeding generation of brood stock. To the extent these assumptions are not fully achieved because of the range in time of individual maturity throughout the spawning season, differential egg hatchability between families, and differential survival of families to the adult stage, the actual rate of inbreeding will be somewhat higher than predicted. The recommended minimum number of 50 pairs to maintain a random mating population considers the inability of most aqua-

226

culture brood stocks to be truly random mating and the need to keep the rate of inbreeding accumulation at a low level. Brood stock managers who want to apply selection practices on the population can easily do so providing steps are taken to insure that selected fish represent many different families. The second approach - systematic line crossing schemes - serves to eliminate the mating of full sibs and therefore, effectively to reduce the rate of inbreeding accumulation below that found in random mating populations of equal population size at least during the early generations (Kimura and Crow, 1963; Robinson and Bray, 1965). The rotational line crossing system proposed by Kincaid (1977) is one mating system that uses this approach. Selection programs can be incorporated into most systematic line crossing schemes. These two approaches, large random mating populations and systematic line crossing reduce the rate of inbreeding accumulation but do not prevent further inbreeding. All breeding systems applied to a closed population will experience some inbreeding over a period of time. If the population is large, however, inbreeding will occur slowly and the normal effects of selection may prevent a serious build-up. The practice of introducing an unrelated brood stock to cross with an existing inbred brood stock to produce a new hybrid brood stock serves to break up the gene combinations (homozygosity) that cause inbreeding depression. However, if that hybrid brood stock is maintained as a closed random mating population after the initial hybridiation, then, inbreeding will again begin to accumulate. For this reason, when a different strain is introduced and hybridized with the existing brood stock, the breeder should consider incorporating a breeding scheme to minimize future inbreeding. A second problem area with strain introductions is that the new strain should be carefully chosen to complement the present strain and to avoid the introduction of undesirable characteristics. The breeding approach and specific breeding program adopted by the breeder will depend on many different factors including the rate of inbreeding and the planned use of the brood stock for present and future production.

REFERENCES Anderson, D. and Woods, D.E., 1979. Evaluation of intensive inbreeding for selection of trout broodstock. Minnesota Department Natural Resources, Completion Report, Study 208 DJ Project F-26-R, 31 pp. A&tad, D. and Kittelsen, A., 1971. AbnormaI body curvature of rainbow trout (Salmo gaircfneri) inbred fry. J. Fish. Res. Board Can., 28: 1918-1920. Bridges, W.R., 1973. Rainbow trout breeding projects. In: Progress in sport fishery research 1971. U.S. Bur. Sport Fish. Wildl. Resour. Publ. 121, pp. 60-63. Cooper, E.L., 1961. Growth of wild and hatchery strains of brook trout. Trans. Am. Fish Sot., 90: 424-438. Falconer, D.S., 1981. Introduction to Quantitative Genetics. Longman Inc., New York, 340 pp.

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