The species is presently considered extinct in almost all Chilean rivers with the exception of some relatively undisturbed coastal catchments (Arratia and Chang 1975, Habit et al. 2007, Vila and Habit 2015). Furthermore, various publications report that the species has undergone severe declines in abundances across its distribution range and only few remnant subpopulations exist (Arratia and Chang 1975, Arratia et al. 1981, Habit and Victoriano 2005, Vila and Habit 2015). The subpopulations seem to continue to decline. For example, its probable extinction has been recently documented in the middle course of the Biobío River (Habit et al. 2006). Furthermore, it was reported in Laguna Grande San Pedro in the Biobío Region (36° Latitude South) up to the nineties (Scasso and Campos 1998), but recent intensive monthly sampling that employed a variety of fishing techniques between March 2018 and February 2019 did not capture a single individual of the species in this location (Machino 2020). These changes may be attributed to water quality and habitat deterioration within its distribution range in recent decades (Habit et al. 2019). Habitat alteration due to water extraction, contamination and river regulation that have affected the abundance and distribution of the species are still in place and are expected to intensify in coming decades (Habit et al. 2006, 2019; Vila and Habit 2015; Díaz et al. 2021). Effects of these direct stressors will interact with projected climate change, as the species is endemic to the central regions of Chile that are highly vulnerable to climate (Araya-Osses et al. 2020, Habit et al. 2019). A population decline of 90% is suspected over the past 12 years. Therefore, this species is assessed as Critically Endangered. Urgent conservation measures are needed to slow down the decline of the populations of this species.

This species is endemic to Chile (Dyer 2000). Historically, it was distributed across central Chile between Valparaíso (33° Latitude South) and Osorno (40° Latitude South) (Arratia et al. 1981). Presently, it is considered extinct in almost all Chilean rivers with the exception of some relatively undisturbed coastal catchments (Arratia and Chang 1975, Habit et al. 2007, Vila and Habit 2015).

No specific information is available on the population size of this species. No species-specific monitoring programs have been performed. Nonetheless, various publications report that the species has undergone severe declines in abundance across its distribution range and only a few remnant subpopulations exist (Arratia and Chang 1975, Arratia et al. 1981, Habit and Victoriano 2005, Vila and Habit 2015). Presently it occupies approximately 256 kilometres of the river network within the basins it inhabits (Laborde et al. 2020). The subpopulations seem to continue to decline. For example, its probable extinction has been recently documented in the middle course of the Biobío River (Habit et al. 2006). Furthermore, it was reported in Laguna Grande San Pedro in the Biobío Region (36° Latitude South) up to the 1990s (Scasso and Campos 1998), but recent intensive monthly sampling that employed a variety of fishing techniques between March 2018 and February 2019 did not capture a single individual of the species in this site (Machino 2020). These changes may be attributed to water quality and habitat deterioration within its distribution range in recent decades (Habit et al. 2019). A population decline of 90% is suspected over the past 12 years, and an ongoing decline (2014 to 2026) and future decline (2021 to 2033) of 50% are suspected.

This species inhabits rivers (lotic systems) and it can be found in middle and lower reaches of these systems (rithron and potamon) (Arratia 1983, Habit et al. 2006, Habit and Victoriano 2005). It is characterised by ontogenic habitat shift; juveniles up to 50 mm prefer areas close to the shore with a sandy or silty bottom, whereas larger juveniles and adults occupy deeper pools with presence of aquatic vegetation and roots of shoreline trees (Arratia 1983). Nematogenys inermis feeds on benthic fauna such as crustaceans, snails, and insects (Vila and Habit 2015). Little is known about its reproductive biology, but it is characterised by low fecundity and large oocytes that mature synchronically suggesting single spawning event within reproductive cycle (Huaquín et al. 2002).

Habitat alteration due to water extraction, contamination and river regulation have likely affected the abundance and distribution of the species (Habit et al. 2006, 2019; Vila and Habit 2015; Díaz et al. 2021). These effects are expected to be superimposed by climate change, as the species is endemic to the central regions of Chile that are highly vulnerable to climate change with a 40% decrease in precipitation expected by the end of the present century (Araya-Osses et al. 2020, Habit et al. 2019). Specific interactions of Nematogenys inermis with introduced species are not known but negative effects may be expected as its distribution, habitats and diet overlap with those of highly invasive voracious Salmo trutta and Oncorhynchus mykiss (Habit et al. 2007, 2019).

This species is not utilized.

Even though it was classified nationally in Chile as Endangered in 1998 (Campos et al. 1998), no formal conservation actions are in place for Nematogenys inermis. Habitat alteration due to water extraction, contamination and river regulation and invasive species are the primary drivers of Nematogenys inermis population declines (Habit et al. 2019). Thus, river conservation actions that aim at reduction of contamination as well as preservation and restoration of natural river connectivity and control of invasive species are essential for its effective conservation. Contamination effects should be mitigated and any river regulation projects that are being planned or implemented should consider structures that allow fish movements (e.g., fish passes). The species is presently considered extinct in almost all Chilean rivers with the exception of some relatively undisturbed coastal river basins and as such conservation of these coastal river basins should be given the highest priority. Furthermore, considering severe alteration of natural habitats of the species, ex situ conservation options should be considered. Research focused on population trends is needed to provide data to inform success of conservations actions. Knowledge on the biology and ecology of the species remains limited, and as such specific studies on habitat requirements and reproductive biology are essential to allow future successful ex situ conservation.