All five species of sturgeon in Georgia, including the Russian sturgeon Acipenser gueldenstaedtii, are categorized as Critically Endangered on the IUCN Red List. The latest study carried out by Fauna & Flora and Ilia State University shows that the Rioni River is still the only remaining sturgeon spawning river in the eastern Black Sea. From a total of 117 Russian sturgeon tissue samples (taken from individuals subsequently released into the river from where they were captured) collected from the Black Sea and the Rioni River during August 2018–June 2022, we detected juveniles only in the Rioni River and the mouth of the Black Sea, underlining the importance of the Rioni River as a spawning ground. We captured only 13 adults, all in the Black Sea.
Our findings also provided further evidence of hybridization of the Russian sturgeon and stellate sturgeon Acipenser stellatus in the Rioni River (Beridze et al., 2022, Conservation Genetics, 23, 211–216). Of the 117 samples, six were identified as hybrids (which produce infertile offspring). In all cases, stellate sturgeon males had mated with Russian sturgeon females, suggesting the stellate sturgeon may be encountering difficulty finding individuals of its own species for mating. Additionally, we found three invasive Siberian sturgeon Acipenser baerii (known to be farmed in the region) in the Rioni River. They could further hybridize with and outcompete native sturgeon species. There was an almost 1:1 sex ratio in our 117 Russian sturgeon samples (60 females, 57 males), which is common in juvenile populations but not adult populations (Fortin et al., 1993, Canadian Journal of Zoology, 71, 638–650) and suggests individuals may not be surviving to sexual maturity.
Threatened sturgeon species in the eastern Black Sea are facing critical challenges. Hybridization is a clear threat, not only to the Russian and stellate sturgeons but also to the ship sturgeon Acipenser nudiventris, which shares the same spawning habitat. Although recruitment is occurring in the Rioni River, individuals may not be surviving to maturity. Given the fact that sturgeons only reach maturity at 7–9 years old, they are extremely vulnerable to extinction. Understanding the structure and status of sturgeon populations in this region will help to target conservation measures to protect the Black Sea ecosystem and some of the evolutionarily oldest living fish species.
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