U.S. Dept Commerce/NOAA/NMFS/NWFSC/Publications
SESSION III: Hatchery Management Strategies and Supplementation
Session Chair: C. Mahnken, National Marine Fisheries Service, Manchester, Washington
John M. Emlen
Seattle National Fisheries Research Center
Building 204, Naval Station
Seattle, Washington 98115
Of concern both to hatchery managers and advocates of protecting wild salmon stocks are three genetic processes:
1) Inbreeding. Inbreeding leads to increased homozygosity, occasionally involving loss of alleles and increased expression of deleterious and lethal genes. Inbreeding also decreases genetic variability (i.e., genetic polymorphism) via drift in depressed populations, thereby diminishing resilience to environmental stress in wild populations.
2) Local Adaptation. One way to reverse losses in heterozygosity and genetic polymorphism is to introduce fresh genetic material from other sources (Ricker 1972). Unfortunately, this remedy has drawbacks. First, each wild stock and, to a lesser degree, each hatchery stock can be expected to have adapted genetically to local conditions. Therefore, introductions result in the mixing of local fish with others, which, in their new surroundings, are almost invariably inferior. Data supporting this assertion for salmon have been published recently by Reisenbichler (1988).
3) Outbreeding. Introducing new genetic material to a gene pool usually results in developmental disruption. Picture the genome of a given stock as a well-integrated configuration of interacting genes. Much as a computer program directs a computer to perform some task, these genes, working in concert, direct development, predisposing individuals of that stock to certain physiological and behavioral responses to their environment. By analogy, think of that stock's genome as a FORTRAN-IV program, the genome of an invading stock as a program in some other language. In the first, hybrid generation, offspring will possess both programs. Development of the hybrid might benefit from the double set of instructions, one better in directing the development of one set of instructions, one better in directing another set. On the other hand, parts of one (recessive alleles) may be masked by parts of the other (dominant alleles), and one set of instructions might interfere with the reading or execution of the other. Therefore, unless the two instruction sets are very similar (two versions of FORTRAN, for example), we might expect confused development and, consequently, lower fitness.
With recombination in the second generation, bits of one program will be mingled into sections of the other, and directions to the developing embryo are likely to be hopelessly jumbled, even with similar programs. As the two languages increasingly diverge, the problem can be expected to get worse.
Mixing fish stocks, by an analogous, genetic argument, is likely to result in diminished survival, growth, reproductive capacity, or all three in the F2 generation and beyond, even if the F1 shows hybrid vigor. And unless parental stocks are quite similar, even the F1 generation may display decreased fitness.
The purpose of this paper is to explore the net effects, over time, of these three genetic processes—inbreeding, local adaptation, and outbreeding—on the following:
Consider a structural gene locus with two alleles, and suppose that the effects of these alleles are modified similarly by genes at two epistatic loci. Established genetics theory tells us that we can expect the alleles imparting greatest fitness to show dominance at all three loci. In addition, we can allow for varying degrees of overdominance at the structural gene locus. Now imagine an ecological gradient along which the relative fitnesses of the structural gene alleles change linearly. Such a model possesses only three (nonscaling) input parameters:
2) Impact of the modifier alleles on structural gene breeding values, and
3) Degree of overdominance at the structural gene locus.
The model was designed to simulate changes in gene frequency and consequent changes in fitness. Computer runs using a full, factorial array of (biologically reasonable) extremes showed output to be extremely robust to variation in input parameter values. In addition, results indicate that output would be changed only minutely by the incorporation of additional modifiers into the model. Finally, the impact of fitness is essentially the same whether the gene complex modeled acts alone or as part of a polygenic system. - Hence, the results obtained can be considered quantitatively as well as qualitatively applicable.
In support of the model, predictions of hybrid fitness as a function of genetic distance fit observed data (Reisenbichler, unpubl. manuscr.) almost perfectly.
Numeric output will be provided elsewhere in a more rigorous and detailed presentation. In brief, results are as follows:
1) If genetic distance (see above) between parental stocks is small (below about 0.4), fitness in the first, hybrid generation can be expected to be similar to that of the local stock. There is a slight depression in fitness, but this is likely to be compensated, or even overcompensated, by the reintroduction of lost alleles.
2) Genetic distance tends to cluster around two values, near zero and near one. This means that as one moves farther and farther afield (in ecological/genetic type) to find breeding stock, drop-off in F1 fitness follows very nearly a step function. Therefore, inasmuch as geographic distance correlates to ecological/genetic distance, nearby stocks generally may be mixed with no adverse effects. Mixing may even be advantageous. But beyond some critical distance, crossbreeding will result in an approximately 50% drop in fitness. Initiating such stocks would be decidedly unwise.
3) Hybrid vigor is invariably reversed in the F2 generation and beyond, even for very similar genetic stocks. For similar stocks, relative F2 fitness is about 95%; for diverse stocks, 50-60%. Gradual recovery follows due to local adaptation; 90% relative fitness is achieved, even for stocks with diverse parentage, by around generation 25. Practically, however, given the long life-histories of most anadromous salmonids, recovery is not a practical expectation.
Gene flow precludes effective adaptation. Thus a periodic influx progressively lowers fitness until about generation 5, at which point an equilibrium is reached. Here, unlike in Scenario I, strays (or donors) even from very similar stocks can produce deleterious effects. For example, if relative performance of the (pure) transplanted stocks is 80% (i.e., has a genetic distance of 20%), fitness of the recipient population drops 20% by generation 5. For very dissimilar stocks (relative performance = 20%), an equilibrium is reached at 30% of the fitness realized in the absence of mixing. In general, it is not advantageous to mix stocks on a periodic basis.
Reisenbichler, R. R. 1988. Relation between distance transferred from natal stream and recovery rate for hatchery coho salmon, Oncorhynchus kisutch. N. Am. J. Fish. Manage. 8(2):172-174.
Ricker, W. E., 1972. Heredity and environmental factors affecting certain salmonid populations. In R. C. Simon and P. A. Larkin (editors), H. R. MacMillan lectures in fisheries: The stock concept in Pacific salmon, p. 19-160. Univ. British Columbia, Vancouver, B.C.
* In the absence of the author, this paper was presented and the questions answered by Dr. Robin Waples.
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