|
<br />of efforts to minimize random genetic changes in hatchery
<br />stocks. Although Nt, can be estimated from a single pair of
<br />temporally-spaced samples, greater precision can be ob-
<br />tained from samples taken in 3-5 consecutive years. For-
<br />tunately, the temporal method is best suited to the study
<br />of populations of small effective size, or those that have
<br />experienced recent declines in population size (Waples 1989;
<br />Waples in press!). Data for hatcheries using a variety of
<br />methods for handling brood stock would help to identify
<br />those procedures that are (or are not) successful in main-
<br />taining an adequate effective population size.
<br />
<br />Developmental Instability
<br />
<br />Indirect indications of genetic changes can also be pro-
<br />vided by morphological characters. In particular, the inci-
<br />dence of fluctuating asymmetry (apparently random de-
<br />partures from bilateral symmetry of meristic characters) in
<br />a population can be used as an index of developmental
<br />instability caused by loss of genetic variability. This approach
<br />has been used in a number of studies with salmonid fishes
<br />(e.g., Leary et a1. 1985) and could be an important comple-
<br />ment to electrophoretic analyses. Allendorf and Ryman
<br />(1987) stressed that the most valuable use of fluctuating
<br />asymmetry involves monitoring populations over time.
<br />
<br />Detecting Mixtures of Gene Pools
<br />
<br />Although mixed-stock fisheries pose significant problems
<br />from a management perspective, the mixtures are temporary
<br />in the sense that they dissolve when escaping fish return
<br />to their natal streams to spawn. More permanent, and
<br />equally important, are mixtures of gene pools caused by
<br />hybridization of two or more stocks. Current and proposed
<br />management practices (transfer of eggs and progeny among
<br />hatcheries; off-site releases of fry or smolts) greatly increase
<br />the opportunities for mixtures of gene pools above the
<br />background level characteristic of natural straying rates.
<br />These effects are likely to be greatly magnified in the near
<br />future if ambitious hatchery supplementation programs
<br />currently under consideration (e.g., NWPPC 1987) are im-
<br />plemented.
<br />Concerns about the effects of such genetic mixtures center
<br />on two issues. First, to the extent that wild populations of
<br />Pacific salmon have adapted over thousands of years to
<br />local environmental conditions, extensive genetic contact
<br />with other gene pools can be expected to reduce fitness
<br />through outbreeding depression. Some studies have shown
<br />reduced fitness of locally-adapted stocks of salmonids that
<br />were allowed to hybridize with those from other areas (e.g.,
<br />R. Reisenbichler fish and Wildlife Service, unpublished
<br />data; Allendorf and Leary 1988 and references therein). In
<br />some rather specialized circumstances, intentional hybrid-
<br />ization of stocks may be an effective way of increasing
<br />intra populational genetic diversity (Krueger et al. 1981), but
<br />this strategy is experimental, does not lead to predictable
<br />results, and should be used with great caution if gene flow
<br />with other stocks is a possibility. Second, the genetic di-
<br />versity of a species as a whole is characterized not only by
<br />the amount and type of variability within populations, but
<br />also by the extent of differences among populations. Until
<br />this century, most Pacific salmon populations may have
<br />existed as essentially independent evolutionary units, each
<br />representing thousands of years of response to conditions
<br />
<br />September - October 1990
<br />
<br />in a localized !area. Over the geographic ranges of the
<br />species, this is<j>lation and adaptation led to an impressive
<br />diversity of gehotypic combinations, a diversity that pro-
<br />vides the speci~s with considerable flexibility in responding
<br />to environmen!tal changes. Permanent mixtures of gene
<br />pools erode thi$ overall genetic diversity, with likely adverse
<br />consequences fpr the long-range fitness of the species as a
<br />whole.
<br />Once these problems are recognized, identification of
<br />stocks that are ptixtures of different gene pools (as well as
<br />those that are genetically "pure") also presents difficulties.
<br />Artificial tags provide information on movement of fish but
<br />not the geneti~ consequences of such movement. For ex-
<br />ample, a fish ~ay stray to another stream but not spawn,
<br />and if it does! spawn, its offspring may not survive to
<br />reproduce in the next generation. Similarly, the success of
<br />supplementation programs designed to produce self-sus-
<br />taining populations is difficult to evaluate by traditional
<br />methods. Even!after years of repeated supplementation in
<br />a stream, it may not be clear whether (1) the natural stock
<br />has been repla~ed by the hatchery stock, (2) the hatchery
<br />stock has had Ilttle or no permanent effect on the natural
<br />stock, or (3) a n~w stock has been created by hybridization.
<br />Genetic marks ',are necessary to answer questions of this
<br />type (e.g., see Parker et aI., in press). The value of genetic
<br />marks in this cor text can be maximized through a systematic
<br />monitoring program, including collection of baseline genetic
<br />data for wild arid natural populations prior to supplemen-
<br />tation. Our la~oratory is currently involved in such a
<br />program for supplemented populations of chinook salmon
<br />and steelhead t~out (0. lIlykiss) in the Snake River drainage.
<br />Changes within! stocks and interactions between stocks will
<br />be measured by! monitoring genetic and meristic characters,
<br />and these data will be correlated with traditional biological
<br />indices (survival rates, redd counts, spawner-recruit ratios,
<br />etc.) to shed ligHt on factors leading to increased, sustainable
<br />productivity of raturally-spawning fish.
<br />If pre-supplementation baseline data are not available,
<br />power to identify and resolve mixtures of gene pools is
<br />reduced. In sudh cases, the analysis of gametic disequili-
<br />brium (correlatitms of alleles at different gene loci) may
<br />provide useful I information. Correlations across loci are
<br />expected in populations that actually are mixtures of two
<br />or more gene Bools or stocks, and the disequilibria may
<br />persist for sev~ral generations following an episode of
<br />random mating I(Nei and Li 1973). The practical usefulness
<br />of gametic disequilibrium analysis to detect mixtures of
<br />salmon populat~ons was recently evaluated by Waples and
<br />Smouse (in pre~s). They found that the technique is not
<br />suitable for dete~ting incidental straying between genetically
<br />similar stocks, \but that power to detect mixtures can be
<br />high if no single population dominates the mixture and
<br />those contributing to the mixture are genetically distinct.
<br />This approach may provide valuable information in two
<br />areas: (1) identi(ication of wild populations that have not
<br />suffered substantial introgression from exogenous fish and
<br />therefore may m~rit special management consideration; and
<br />(2) monitoring I~rge-scale supplementation efforts to de-
<br />termine whethe~ planted fish are having the desired effects
<br />on target populations (and whether they are adversely
<br />affecting other, non-target populations).
<br />
<br />(continued)
<br />
<br />23
<br />
|