Morphospatial Characteristics of the Population of an Alien Fish Species in the Volga Delta—The European Common Bitterling Rhodeus amarus (Acheilognathinae)

For the first time, the data on meristic and plastic indices of the population of an alien fish species in the Volga Delta—the European common bitterling Rhodeus amarus—are presented. In 2018–2025, this population expanded its range in the western part of the Volga avandelta—between the Volga-Caspian Sea Shipping Canal and Zyudev Island. On the basis of the morphospatial analysis of plastic indices of samples from the Volga avandelta (Astrakhan Region), Yakhroma Reservoir (Moscow Region), and Lake Sazalnitskoye (Krasnodar Territory) according to the Euclidean metric, a hypothesis about the North Caucasian route of penetration of the bitterling population into the Volga avandelta is substantiated.