Upper Columbia Spring Chinook Salmon, Steelhead, and Bull Trout Recovery
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- 2.3Population Characteristics and Life Histories
- Distinct Population Segment
- Evolutionarily Significant Units
- Independent Populations
- Core Areas
2.2Community StructureSpring Chinook, steelhead, and bull trout share the aquatic environment with several other fish species in the Upper Columbia Basin. Available information (summarized in Hillman 2000; Duke Engineering 2001; subbasin plans 2005) indicates that about 41 species of fish occur within the Upper Columbia Basin (from the mouth of the Yakima River upstream to Chief Joseph Dam) (Appendix A). This is an underestimate because several species of cottids (sculpins)29 live there. Of the fishes in the basin, 15 are cold-water species, 18 are cool-water species, and 8 are warm-water species. Most of the cold-water species are native to the area; only five were introduced (brown trout (Salmo trutta), brook trout (S. fontinalis), lake whitefish (Coregonus clupeaformis), lake trout (S, namaycush), and Atlantic salmon (S. salar)). Four of the 18 cool-water species are introduced (pumpkinseed (Lepomis gibbosus), walleye (Stizostedion vitreum), yellow perch (Perca flavescens), and smallmouth bass (Micropterus dolomieu)), while all warm-water species in the Upper Columbia Basin are introduced. Anadromous species within the upper basin include spring and summer Chinook salmon, coho salmon (O. kisutch), sockeye salmon, steelhead, and Pacific lamprey. White sturgeon, which may have been anadromous historically, are present as a resident population. These fish have not been detected migrating upstream at Upper Columbia River dams. About half of the resident species in the upper basin are piscivorous (eat fish) (Appendix A). Ten cold-water species, seven cool-water species, and five warm-water species are known to eat fish. About 59% of these piscivores are exotics.30 Before the introduction of exotic species, northern pikeminnow (Ptychocheilus oregonensis), sculpin (Cottus spp.), white sturgeon, bull trout31, rainbow trout, cutthroat trout, and burbot (Lota lota) were the primary piscivores in the region (Li et al. 1987; Poe et al. 1994). Presently, burbot are rare in the upper basin (Dell et al. 1975; Burley and Poe 1994) and probably have little effect on the abundance of ESA-listed species in the region. The status of white sturgeon in the Upper Columbia Basin is mostly unknown, although their numbers appear to be quite low (DeVore et al. 2000). 2.3Population Characteristics and Life Histories2.3.1Levels of Population StructureBefore describing the population structure of spring Chinook, steelhead, and bull trout in the Upper Columbia Basin, it is important to define the different levels of population structure. Various terms have been used to define levels of population structure or ecological types. Brannon et al. (2002) stated that population structure is defined by the life-history strategies that have evolved to maximize fitness under varying environmental conditions within geographic ranges. Identified below are the levels of population structure used in this plan. Distinct Population SegmentAs amended in 1978, the ESA allows listing of distinct population segments (DPSs) of vertebrates as well as named species and subspecies. However, the ESA did not provide specific guidance on what constituted a DPS, and thus created some ambiguity (Platts et al. 1993). Because of this ambiguity, NOAA Fisheries and the USFWS created a policy in 1996 to recognize and define DPSs in relation to ESA listings (61 FR 4722). Because NOAA Fisheries had established a policy in 1991 that defined species under the ESA (56 FR 58612) for Pacific salmonids, it maintained its delineation for the ESA that a population segment would be a DPS if it were an ESU. Evolutionarily Significant UnitsWaples (1991) defined ESUs as the determining population structure for delineating whether a “species” should be listed under the ESA. An ESU is a population (or group of populations) that (1) is reproductively isolated from other related population units and (2) represents an important component in the evolutionary legacy of the species. ESUs may contain multiple populations that are connected by some degree of migration, and hence may have broad geographic areas, transcending political borders. Determining exactly what the evolutionary significance of a population is may be difficult. Independent PopulationsFollowing McElhany et al. (2000), the ICBTRT (2003) defined independent populations, as: …a group of fish of the same species that spawns in a particular lake or stream (or portion thereof) at a particular season and which, to a substantial degree, does not interbreed with fish from any other group spawning in a different place or in the same place at a different season. For our purposes, not interbreeding to a ‘substantial degree’ means that two groups are considered to be independent populations if they are isolated to such an extent that exchanges of individuals among the populations do not substantially affect the population dynamics or extinction risk of the independent populations over a 100-year time frame. Core AreasThe USFWS (2002) defined a core area to be the closest approximation of a biologically functioning unit that reflects the metapopulation structure of bull trout as described by Dunham and Rieman (1999). That is, within the metapopulation or core areas, local populations are expected to function as one demographic unit. Thus, a core area may consist of one or more local populations. Rieman and Allendorf (2001) have suggested that between 5 and 10 populations are necessary for a bull trout metapopulation to function effectively. Core areas are not necessarily synonymous with independent populations. Bull trout may be grouped so that they share genetic characteristics as well as management jurisdictions (USFWS 2002). As noted earlier, this recovery plan will focus on actions that, if implemented, should improve the VSP parameters of ESA-listed species at the “population” and “core area” level. 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