In the 2008 Red List assessment, a total of 75 extant and five extinct 'subpopulations' were identified across the natural range of O. nerka. In the 2011 amendment, several of these subpopulations were further subdivided based on additional input from salmon specialists (Pacific Salmon Commission, PSC, and Fisheries and Oceans Canada, DFO), resulting in a total of 98 subpopulations, of which 93 are extant. The subpopulations in this assessment are considered independent and reproductively isolated from each other.
The assessors used a combination of ecoregional (i.e., marine and freshwater habitat-based) classification and the degree of genetic divergence between known spawning 'locations', using an FST value of 0.04, representing a threshold level of gene flow between putative 'subpopulations' of less than or equal to one effective migrant per year. The five extinct subpopulations were based on Gustafson et al. 2007. While the assessors acknowledge that it is not straightforward to ascribe demographic or geographic distinctness to a subpopulation based on observed genetic distances, they feel it is the most appropriate methodology given the scale of the assessment and the paucity of observations of stray rates among the spawning locations considered in this assessment.