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D. Restrict introductions to sites that fulfill life history requirements of the species. Adequate food, spawning, and rearing habitat for all life stages should be available. Habitat variables should be measured (Orth 1983) and water quality analyzed (U.S. Environmental Protection Agency 1976) to establish baseline habitat conditions and to determine the presence of any harmful substances. Water quality should be similar to that observed in undisturbed natural habitat. E. Restrict introductions to sites that contain sufficient habitat to support a viable population. To maintain population viability, sufficient individuals must be present to prevent serious inbreeding and loss of genetic variation by random drift. The number of individuals ac- tually contributing to recruitment of the next generation (i.e., effective population size), however, is usually only a fraction of the total population size (i.e., census population size). Alleridorf and Ryman (1987), for example, recom- mended an effective population size of 200 for sustaining hatchery stocks of salmonids. In the wild, a much larger census population would be needed to compensate for unbalanced sex ratios, age structure, etc. Sufficient habitat would be needed to maintain a viable population in the face of floods, drought or other stochastic events. Because of these factors, habitat necessary to support many thou- sands of individuals could be required to maintain an effective breeding population of 200. F. Prohibit introductions into areas where the endangered or threatened fish could hybridize with other species or subspecies. Many rare fishes, particularly those of isolated drainages in the West, have had little opportunity to develop repro- ductive isolating mechanisms to prevent hybridization with closely related taxa. Some groups of fishes, such as the suckers (family Catostomidae), readily hybridize and inter- generic hybrids are common. Introductions should not proceed when the subject species could hybridize with a fish already present in the habitat. An exception to this guideline would apply to a limited number of taxa and situations. If hybrids with the taxon to be introduced are known or are suspected to occur at the introduction site, and if the incidence of hybridization is low and is a natural occurrence in the area, then such sites can be considered for the introduction. Some catostomids and some chubs of the genus Gila, for example, exhibit limited hybridization with naturally sympatric taxa. G. Prohibit introductions into areas where other rare or endemic taxa could be adversely affected. If an introduction is proposed outside the species' historic range, pre-introduction surveys should be conducted to determine the presence of rare invertebrate, fish or other aquatic species that might be adversely affected by release of the endangered or threatened fish. Appropriate taxon- omists in entomology, malacology, or other invertebrate zoological specialities should be consulted. If an introduction is proposed within the species' historic range, the need for surveys of other rare aquatic species may be advisable, especially if physical modification of habitats is proposed as part of the introduction effort. Such sur:veys could have prevented loss of a population of hydrobiid snail species September - October 1988 endemic to the Fish Slough area in eastern California. The snail population was eliminated during habitat modification efforts associated with introduction of the Owens pupfish into Fish Slough (Landye 1983). For introductions within the species' historic range that do not include physical manipulation of habitats, surveys for other rare species, while potentially valuable, should not be required. 2. Conducting the Introduction A. Choose introduction stock from appropriate source. For rare fishes with more than one population, a source for the introduction stock must be selected. It is important to realize that each isolated population of a rare fish is likely to be a unique gene pool with specific adaptations to local conditions (Meffe 1986). Fishery managers, therefore, may have a choice of unique stocks to select from, or perhaps to mix. The availability of life history and genetic information on the candidate source stocks will greatly facilitate the proper selection. Selection criteria will vary with the intended purpose of the introduction, but consideration may be given to selecting the most genetically pure stock, the rarest stock, the stock closest geographically to the introduction site, or the stock closest ecologically. Meffe (1987) pointed out that popula- tions at the edge of a range may have 10wer genetic variance than do those near the center. It is possible that individuals from centrally located populations may display a higher fitness in characters such as growth rate, survivorship, fecundity, etc. (Meffe 1987). This phenomenon was well documented in an electrophoretic analysis of 21 populations of the Sonoran top minnow, PoeciIiopsisoccidentalis. Vrijen- hoek et al. (1985) demonstrated that the source top minnow population being used for restocking was genetically in- variant and displayed a very low fecundity. This study prompted a switch in the source population used for res- tocking efforts. Mixing of naturally isolated stocks to establish a population should be discouraged because it may reduce genetic fitness by loss of closely-linked or coadapted genes (Dobzhansky 1970). That is, genes that are coadapted within one popu- lation may be broken up by hybridization and combined into gene complexes that do not function well together (Meffe 1986). Evidence of this phenomenon was observed when isolated stocks of Atlantic salmon, Salmo salar, were mixed (Stahl 1981). Often, the first generation hybrids are robust, but subsequent generations lose fitness as the coadapted gene complexes are broken up. Meffe (1986,1987) presented good reviews of the problems of mixing isolated stocks of rare fishes and recommended against it in nearly all cases. B. Examine taxonomic status of introduction stock. Introduction stock should be examined prior to transport by an appropriate taxonomist in order to insure that only the desired form is present. If the taxonomy is questionable but the introduction nonetheless proceeds, a subsample of the stock should be preserved for future analysis. C. Examine introduction stock for presence of undesirable pathogens. 9