Four species of suckers (family Catostomidae) inhabit the Klamath River Basin of Oregon and California: Lost River suckers (LRS; Deltistes luxatus), shortnose suckers (SNS; Chasmistes brevirostris), Klamath largescale suckers (KLS; Catostomus snyderi), and Klamath smallscale suckers (KSS; Catostomus rimiculus). All but Klamath smallscale suckers are endemic and restricted to the Klamath River Basin where they occur sympatrically in large lakes and reservoirs, including the Lost River and Klamath Lake subbasins (Figure 1; USFWS 2012). Population declines, primarily due to loss or degradation of spawning, rearing, and adult habitat, have resulted in Lost River and shortnose suckers being listed as endangered throughout their entire range under the U.S. Endangered Species Act (USFWS 1988). Continued population declines coupled with failed adult recruitment prompted the USFWS to initiate an assisted rearing program in 2015 as a part of their recovery strategy (Childress et al. 2019). The program was designed to maintain as much genetic diversity as possible while improving recruitment by averting high early life stage mortality (Day et al. 2017). However, while assisted rearing efforts are targeted towards endangered LRS and SNS, species differentiation of larval and juvenile suckers is problematic in the Klamath River Basin. This, in turn, complicates the management of these species as well as the population modeling used to evaluate recovery efforts. Maintaining as much as possible of the genetic resources, or “evolutionary legacy” of a species is a goal common to conservation and endangered species recovery strategies. Inappropriate assumptions regarding species’ evolutionary lineages, and genetic characteristics may lead to the mismanagement of an endangered species through a failure to recognize and appropriately manage species boundaries and genetic population structure. Despite a considerable amount of research, the partitioning of genetic diversity within and among the four species of suckers in the Klamath River Basin remains unclear. Previously developed genetic markers are effective at differentiating some species, but fail to effectively differentiate all four species of suckers in the basin (Tranah et al. 2001; Wagman 2003; Tranah and May 2006; Hoy and Ostberg 2015; Dowling et al. 2016). Peer-reviewed publications describing the morphological characteristics of (Markle et al. 2005) and genetic relationships among (Dowling et al. 2016; Tranah and May 2006) Klamath River Basin suckers have not resolved uncertainties regarding the systematic relationships among the four currently recognized taxa. Specifically, genetic and morphological data generally support LRS and KSS as being distinct entities, but genetic evidence does not support a distinction between KLS and SNS. All three publications above refer to unpublished information regarding ecological differences between KLS and SNS as evidence to support the existence of two entities. However, the authors also acknowledge that overlap in morphological characters (Markle et al. 2005) and a lack of genetic differentiation (Dowling et al. 2016; Tranah and May 2006) between KLS and SNS raises the question of their specific identity. This is particularly problematic in the Lost River subbasin, where overlap in morphological characters between KLS and SNS is greatest. In our opinion, the basis of the strong genetic similarity between KLS and SNS has not been resolved. Morphological characters mostly support the existence of two distinct species, while genetic characters do not (i.e., genetic divergence between KLS and SNS is less than divergence among populations of each species; (Smith et al. 2015). Some have suggested that introgressive hybridization may have resulted in a lack of genetic differentiation between KLS and SNS and a breakdown of monophyletic species (Dowling et al. 2016; Tranah and May 2006), and that this hybridization may be an important process in