Draft TRT Document – for Discussion Purposes ... - NWIFC Access

Draft TRT Document – for Discussion Purposes – OK to circulateIndependent populations will generally be, but not necessarily, smaller than awhole DPS and will generally inhabit geographic ranges on the scale of whole riverbasins or major subbasins that are relatively isolated from outside migration.Demographically and biologically, independent populations are the primary unit forviability assessments and recovery planning.Structure Above the Population LevelJust as there may be substructuring within a population, there may be structureabove the level of a population. This is explicitly recognized in the designation of a DPSor an evolutionarily significant unit (ESU). A DPS or ESU may contain multiplepopulations that are connected by some common element. Thus organisms can begrouped into a hierarchical system in which we define the levels from individual tospecies. Although reproductive isolation forms a continuum, it probably is not a smoothcontinuum, and there is a biological basis for designating a hierarchy of levels. Theconcept of “strata” was developed by the Willamette Lower Columbia River TRT to helpdescribe, where necessary, a level of structure intermediate between populations andDPSs (McElhany et al. 2003). A similar multiple population unit was developed forChinook salmon by the Puget Sound TRT (Geographic Regions) and the InteriorColumbia River TRT (Major Population Groups). For consistency, the term MajorPopulation Groups (MPGs) has been adapted by the TRTs to describe these populationaggregates. MPGs are generally used to capture major life history differences, distinctecological zones, and/or geographic structuring. Where specific information was notavailable we considered implied life history differences to exist where populationsoccupied a suitably large geographic region with unique ecological conditions (e.g.hydrology, thermal regime, estuarine conditions, etc.). Previous TRTs have underscoredthe importance of MPGs by including them in the viability criteria. While criteria forDPS viability vary among the TRTs, there is some provision in all TRT viability criteriarequiring the viability of all extant MPGs. Previous TRTs identified MPGs inconjunction with the development of viability criteria; we have elected to concurrentlydefine DIPs and MPGs prior to establishing viability criteria.Structure Below the Population LevelBelow the population level there often will be aggregations of fish that are tosome degree reproductively isolated from other groups of fish within the population, butthat are not sufficiently isolated to be considered independent by the criteria adoptedhere. These fish groups are referred to as subpopulations. Subpopulations play animportant role in the sustainability and evolution of populations. However fewpopulations have been studied sufficiently in depth to characterize any componentsubpopulations. The presence of subpopulations can have important consequences in thecharacterization of a VSP. Additionally, subpopulations can strongly influencepopulation spatial structure, one of the four key parameters for evaluating the status of apopulation. Where possible, the TRT endeavored to describe internal variability in lifehistory, ecological, or geographic structure for each population. For example, in somesteelhead populations winter- and summer-run fish appear to co-mingle. At present thereis insufficient information to determine the degree to which these two life history types6