Population Dynamics and Distribution of the Lake Smelt, a Dwarf Form of the European Smelt Osmerus eperlanus (Osmeridae), and the Black Sea Sprat Clupeonella cultriventris (Clupeidae) while Cohabiting the Rybinsk Reservoir

The data for a 23-year period (2000–2022) on the population dynamics and distribution of the lake smelt (a form of the European smelt Osmerus eperlanus ) and the Black Sea sprat Clupeonella cultriventris in the Rybinsk Reservoir have been summarized. The lake smelt dominated the reservoir pelagial from the 1950s to the mid-1990s. An anomalously low water level in the reservoir in 1996 and abnormal (for this reservoir) water warming in subsequent years led to elimination of the species from the reservoir ichthyofauna in 2002. The Black-Sea sprat invaded the reservoir in the mid-1990s; its population irruption occurred in the early 2000s (during the depressed state of the smelt population). Currently, this species dominates in the pelagial of the reservoir. Since the early 2000s there were noted two periods when the lake smelt was found in practically all parts of the Rybinsk Reservoir and the sprat abundance was also high. However, high water temperatures in 2010 and 2021, as well as low level in 2014, resulted in almost complete disappearance of the lake smelt disappeared. During the periods of cohabitation of the studied species, a significant similarity in the diet of individuals of both species with the size of up to 80 mm has been noted. The larger smelt fed on larger plankton, and in the summer also on fish. Currently, the main limiting factor negatively affecting the recovery of the smelt population in the Rybinsk Reservoir is the abnormal heating of the water column in some years.