Copepod (Crustacea, Calanoida) in Large Reservoirs of Upper Volga (Russia)

The zooplankton of two large reservoirs of the Upper Volga was studied: in May–July 2016–2023 Rybinsk Reservoir (13–43 stations), July 2017 – Sheksna Reservoir (23 stations). In both reservoirs, breeding populations of the northern species (Sars, 1863), which is most widespread and numerous (890 ind./m of adults) in the Sheksninsky reservoir, were identified for the first time. A description of the structure of the species is provided with photographs of important identifying characteristics. It has been established that since 2017, has formed a small (less than 100 ind./m) self-reproducing population in the Glavnyi Reach of the Rybinsk Reservoir. In the studied reservoirs, the species is monocyclic, has a short breeding period during June, in cold years when the water temperature is below 18°C – until the first ten days of July. The structural features of the species and its life cycle are discussed.