Genetic variation is the raw material in species populations which enables them to adapt to changes in their environment. New genetic variation arises in a population from either spontaneous mutation of a gene or by immigration from a population of genetically different individuals. Alternate forms of a particular gene (or locus) are called “alleles”. The number and relative abundance of alleles in a population is a measure of genetic variation, sometimes termed “heterozygosity”. Genetic variation is a measure of a population's ability to adapt to environmental change or stress, and thereby to survive.
Population geneticists have spent several decades establishing the importance of genetic variation in natural populations. It is known, for example, that the response to natural selection by experimental populations is accelerated by mutation-inducing radiation and/or the introduction of genes from different strains. In terms of the preservation of genetic resources, we can expect that benefits would be derived from maintaining the maximum level of genetic variation in a strain as well as by maintaining multiple strains that could serve as additional sources of genetic information by hybridization. It follows that the loss of genetic variation for whatever reason (e.g., prolonged selection, inbreeding, isolation) will result in a decrease of the potential adaptability of a population.
It appears that the benefits of multigene heterozygosity are universal in outbreeding organisms (see Soule, 1980, for a review). In several organisms, including some fish species, individuals possessing the most genetic variation have been shown to have better survival rates or higher relative growth rates. Relatively heterozygous individuals appear to be more resistant to environmental perturbations during development. Clearly, genetically variable populations have many advantageous characteristics that are absent from genetically impoverished ones.
Over the past several years a rather large body of evidence has accumulated on the biochemical differences between alleles of genes coding for metabolically important enzymes. These biochemical differences emphasise the relationship between genetic and functional diversity. The functional properties of different alleles often reflect a biochemical and genetic adaptation to life in a heterogeneous environment. The extent, however, to which the states of physiological and behavioural traits at the whole animal level can be correlated with genetic and biochemical data is still uncertain. Most workers in the field agree that a proportion of protein polymorphisms have no direct or measurable effect on viability or some other aspect of fitness. Nevertheless, there is significant evidence from studies of individual genes, organisms and populations to substantiate the importance of genetic variation to population adaptability.
The selection of small numbers of parents (Section 4.3) can reduce genetic variability. Equally serious is the fact that brood stock may be continually selected from closely related, perhaps full sib individuals. This leads to generation after generation of inbreeding of closely related individuals which very often results in homozygosity for unfavourable genes. The overall result is inbreeding depression.
Inbreeding depression is the loss of fitness (e.g. vigour, viability, fecundity) in connection with the loss of genetic variation due to homozygosity. The evidence that inbreeding is harmful is copious and virtually universal (Allendorf and Utter, 1979; Kincaid, 1976a, b; Kirpichnikov, 1972, and Kosswig, 1973).
A general view of the effects of inbreeding and its relationship to the conservation of genetic resources can be found in Soule (1980) and is treated more extensively by Frankel and Soule (1981). Quoting from the former: “A survey of inbreeding experiments leads to the generalization that increasing the inbreeding coefficient by 10 percent induces a 5–10 percent decline in a particular reproductive trait.” Note that an F value (inbreeding coefficient; see Section 4.2) equal to 10 percent approximates the amount of inbreeding that would theoretically occur in a population of five adults breeding at random for a single generation, or in a population of 25 adults breeding at random for five generations.
A 5–10 percent decline in fecundity might not appear to be very serious (especially when dealing with such fecund animals as fish) but if the effects of inbreeding depression on other traits (such as viability) are also considered this amount of inbreeding can lower reproductive potential as a whole by 25 percent (e.g., in fowl and swine). Gjederm (1974) showed that a 10 percent increase in the inbreeding coefficient in rainbow trout can result in a 10 percent decline in hatchability, and a 24 percent decline in viability of fingerlings. It should be noted that such independent effects are multiplicative in their impact on total, absolute survival and reproduction. Thus the unavoidable conclusion is that relatively small amounts of inbreeding can do tremendous damage to the reproductive potential and productivity of a fish stock.
Expected inbreeding depression is related to the current inbred state of a population. For example, a very small and isolated lake population of a certain fish species may be highly inbred because of its inherent demographic structure. The inbreeding of such a fish would not be expected to display an inbreeding depression as large as from a previously outcrossed group.
In some breeding systems (e.g., those using gynogenetic techniques, Section 6.2) inbreeding is the goal and its associated “depression” is an undesirable but expected result for which compensatory breeding programmes can be utilized. Using quantitative methods of Nace et al. (1970) and assuming an average recombination frequency of about 0.1, Nagy et al. (1979) estimate that one gynogenetic generation is equal to 10–12 generations of full-sib mating. In many other breeding and brood stock selection programmes, practical logistic and economic management considerations can and have resulted in the inadvertent selection of brood stock in a way which causes close inbreeding. Unfortunately, once it has occurred, inbreeding depression is not reversible except by hybridization.
Technologies exist for a direct assessment of the genetic properties of a population. In some species the genetic basis of variation in some visible characters (e.g., colour patterns) can be established by breeding experiments, and the characters, or phenotypes, can be used to directly assess gene frequencies in populations. Traits such as colour patterns are typically controlled by a small number of genes (one to three). As such, those genes may not be representative of either overall genetic variation or population structure, but probably reflect fine-scale ecological or social structuring such as family or age-class recognition. Although these characters can be conveniently assessed in a population, they will too often give misleading or unrepresentative information on genetic variation.
Electrophoresis of proteins has been widely applied for the direct study of genetic variation in fish populations. The importance of electrophoresis to the study of fish genetics resides in the ability to directly estimate genetic relationships from its results, and also because variation of electrophoretically detectable genes is often correlated with variation of other genes. To the extent that such a correlation is widespread among fish species, electrophoretic variation can be a general estimator of genetic variation.
The “state of the art” is to use electrophoretic techniques to analyze genetic variation in natural populations because, among other things, electrophoretic variation is “taxonomically congruent” with morphological variation in interpreting phylogenetic and evolutionary relationships (Mickevich and Johnston, 1976). However the use of electrophoretic variation analysis requires some qualifications regarding aquaculture. At this time there is no direct evidence in the literature to indicate that either qualitative or quantitative allozymic variation is indicative of potential economic performance in such characters as food conversion rates, tolerance to temperature extremes, low dissolved oxygen tension, etc. Because of this, caution must be exercised in using the level of allozymic variation as the only criterion for choosing stocks for desirable physiological, nutritional, or other related production performances. There is some evidence of a correlation between electrophoretic variation, meristic variation, and developmental stability in nature (Soule, 1980). It is theoretically possible to develop the use of qualitative variation as predictors or indicators of quantitative performance in laboratory or hatchery populations but this involves complex breeding systems probably beyond the practical scope of most aquaculture breeders (Soller, et al., 1976).
There are situations in which it may be desirable to electrophoretically estimate genomic variability and then to use these data as a base-line for comparing the genetic effects of a particular pattern of stock breeding or exploitation. For example, when exploitation of a species can be anticipated, base-line information on the genetic variation of pre-exploited stocks would be desirable. This would allow some direct assessment of the genetic consequences of exploitation by continuous monitoring of exploited stocks, or populations.
When re-introduction of a locally extinct population is contemplated, earlier base-line information might allow a closer matching of the introduced fish to the original population. Proper genetic matching would increase the likelihood of successful re-introduction. When base-line data is not available (the usual case), direct genetic assessment of the potential parental stocks for re-introduction allows an intelligent choice of stocks for introduction. Other things being equal, populations of maximum electrophoretic variation should be selected for introduction because this probably increases the likelihood of evolutionary adaptation to a novel environment.
A similar genetic monitoring of cultured species would be desirable to assess genetic changes that result from a particular culture scheme. It may be important to maximize outbreeding in a stock, in which case electrophoretic variation would be an important tool for monitoring the breeding programme.
There are certain dangers associated with the absence of genetic monitoring. In the south-western United States, a major breeding programme was undertaken some years ago in order to produce sterile males of the screw worm fly for introduction into the wild populations (Bush et al., 1976). During the breeding programme, a particular allele of the electrophoretically detectable gene dglycerophosphate dehydrogenase was accidently selected probably by natural selection in the culture population. This enzyme is important in flight metabolism. The properties of the selected allele militated against flight in the wild of the introduced, sterile males. Knowledge of the biochemical properties of the alleles and electrophoretic monitoring would have avoided this unfortunate situation.
We do not wish to imply or recommend that every exploited fish or every cultured stock be monitored in this way. Not only would this be expensive, but unless such studies are properly designed and controlled, the data are not likely to be very useful. We do, however, suggest that several carefully designed monitoring programmes be set up, and that these be coordinated to maximize their utility.
Fishes probably surpass all vertebrate groups in their variety of social structures and kinds of life histories. It is not surprising therefore that some controversy has arisen regarding the significance of such variables as population structure, dispersal, and genetic drift, particularly with regard to genetic integrity of populations. At one extreme there are species like the American eel in which the adult population is spread out over thousands of kilometres, yet this species apparently verges on being a single random breeding (panmictic) population. At the other extreme there are hundreds of species which are territorial, which have demersal eggs with parental protection and which have very limited vagility. In species of this latter type with a localized and fragmented type of population structure, the neighbourhood size or the local population could be as low as 100, and there may be very limited gene flow between the local populations. As a consequence of this diversity, it is hazardous to generalize about demographic, georgaphic and genetic structural characteristics of fish. Each species must be examined as a unique case, even recognizing the possibility for intra-specific variation in population structure.
A good knowledge of the population structure in the management of fisheries cannot be exaggerated, whether the purpose of the management is exploitation or preservation or both (as should often be the case). Only when stocks are properly defined can the fishery be managed optimally. For example, one might conceivably decide to artificially enhance a pink salmon fishery by introducing fry from a hatchery. But unless one knew that odd and even year pink salmon were genetically distinct populations, the resulting hybridization could cause significant genetic change and a decrease in fitness in both odd and even year stocks.
Even very closely related sympatric species can have very different population structures, and it is dangerous to generalize from one to the next (Allendorf and Utter, 1979). In the rainbow trout in the Pacific northwest, the genetic data differentiate the populations into eastern and western groups, the major division coinciding with the crest of the Cascade Mountains. Many early workers had concluded that the principal basis for genetic separation of rainbow trout populations was anadromy and time of return to fresh water. Allendorf and Utter, however, emphasize that the taxonomic units based on electrophoretic data correspond to geographic groupings rather than to the above behavioural characteristics. They emphasized the importance of glacial events in dividing these populations historically.
In coho salmon, however, glacial events are not considered to have played a major role in the present subdivision of the populations in the Pacific northwest. The discontinuous distribution of (certain) transferring alleles among coho salmon populations cannot be directly explained on the basis of glacial events.
In the chinook salmon the coastal populations appear to be genetically distinct from the inland populations in both Oregon and Washington for the fall-run salmon, but the line of geographical demarcation is different from that of rainbow trout. This information has significant management potential because it could allow the determination of the major areas of origin of ocean caught fish.
Hence, among three closely related species in the same region, there is absolutely no correspondence in the geographic distribution of racially distinct populations, i.e., the geographic barriers that separate sub-populations in one salmonid species are not relevant in another. Management generalizations based on the distribution of populations in one species could prove disastrous if applied to other species.
Population structure is thus a useful guide for a priori priority ranking with regard to genetic resource preservation, at least with regard to the extinction potential of local populations. Geographic range alone is quite useful. The majority of species which reproduce in estuaries, river systems in the temperate zones, and in coastal pelagic zones are relatively widely distributed and fairly numerous. Species residing in tropical floodplain rivers, extreme environments such as shallow desert lakes or salt lakes may be far less numerous and typically have rather limited geographic ranges.
Many such “local” species are relatively vulnerable to habitat disruption. Both from the standpoint of biological conservation and from that of genetic resource preservation. Species which are widely distributed and relatively numerous warrant less attention than do the species with very limited distribution and small population sizes. We do not mean to imply that genetic depletion is only a threat to local endemics.
The potential for genetic depletion can also be quite high for a single reproductive unit of a broadly distributed species. The obvious distinction between the two cases is that the extinction of the endemic cannot be reversed, whereas the recolonization of a reproductive habitat upon loss of the more broadly distributed species is possible. For example, the Japanese sardine Sardinops melanosticta following a massive population collapse and range contraction, has recolonized the Japan Sea from refugia on Japan's east coast. Another example is the successful, artificial reintroduction of Atlantic salmon in small coastal rivers in the northeastern U.S.A.
In the latter case, the genetic characteristics of the replacement population should be a relevant concern (Section 6.4). For example, failure to consider the particular ecological adaptations of replacement or enhancement of stock has caused severe management problems in bob-white quail (Clarke, 1954). In Scandinavia some introductions of Arctic charr and whitefish into lakes already populated by conspecifics has had deleterious consequences (Svardson, 1979).
It is a moot point whether a naturally or artificially recolonized stock becomes as well adapted as the original stock. The point is to maximize the chances of a successful restocking or recolonization by taking into account all of the relevant genetic and ecological variables.