Mass mortality of the Critically Endangered naked characin (Gymnocharacinus bergii): Possible causes and impact.

The naked characin (Gymnocharacinus bergii) is the only endemic fish of the arid Patagonia of Argentina, living only in the thermal headwaters of the Valcheta Stream in the Somuncura Plateau. This species is listed as Critically Endangered on the IUCN Red List.In April 2018, a massive mortality event was observed in the species. Parasitological studies identified the cause as an epidemic of white spot disease, caused by the parasite Ichthyophthirius multifiliis (Ich).Ich is a common disease in aquaculture settings, but it has also been reported as an epizootic in wild fish populations across tropical and temperate regions worldwide. This occurrence is associated with the introduction of fish into non‐native environments.This article presents the first recorded instance of a mass mortality event and Ich outbreak in the naked characin. It includes a mapping of the disease in the headwaters of the stream, an estimation of the impact on the species' population size, and an analysis of potential causes associated with this epidemic.The results indicate that the epizootic produced mortality of 82% of the global population of the naked characin and was linked to the presence and high abundances of the invasive fish species Uruguay tetra (Cheirodon interruptus). The disease occurred in stream sectors where both characid species coexist, particularly in areas with the highest water temperatures.The Ich outbreak represents the most recent and severe threat recorded for the Critically Endangered naked characin. Implementing control measures against the invasive Uruguay tetra is imperative to prevent future outbreaks that could push the species to the brink of extinction in the near future.This study provides crucial information for assessing current management interventions and developing new strategies within the framework of the action plan for naked characin conservation. [ABSTRACT FROM AUTHOR]