Rare but Not Gone: A Relict Population of the Black Sea Ship Sturgeon Acipenser nudiventris Persists in the Rioni River, Georgia

Historically, the ship sturgeon (Acipenser nudiventris) occurred in the Aral, Caspian, Azov, and Black Sea basins. However, its numbers decreased dramatically during the 20th century. It is now considered extirpated from the Aral, Azov, and Black Seas, and has almost disappeared in the Caspian Sea. A. nudiventris is listed as Critically Endangered on the IUCN Red List and, in Georgia, the species has been undetected for the last three decades. We collected 22 sightings, including nine genetic samples taken from fin clips of ship sturgeon from the Rioni River in Georgia during 2020–2022. For the genetic samples, the mitochondrial DNA control region was used for species identification. Because cases of sturgeon inter-species hybridization have been reported in the Rioni River, we used species-specific diagnostic markers and ship sturgeon-specific microsatellite markers for detecting hybridization with other sturgeon species. In addition, we used a sex-specific marker for sex identification. Based on the maternal identification, all nine individuals are identified as ship sturgeon, representing one haplotype, and the haplotype is different from all other A. nudiventris haplotypes available in GenBank. Based on genetic analysis, the specimens did not show signs of hybridization with other locally occurring species. We conclude that ship sturgeon still live in the Rioni River, and are a remnant of an older, preexisting Black Sea ship sturgeon population.