The size structure of the Baltic river flounder Platichthys flesus trachurus (Duncker, 1892) in the coastal zone of the southeastern Baltic Sea

Two herds of Baltic river flounder (Platichthys flesus trachurus) with different life cycle features inhabit the brackish waters of the Baltic Sea. Both herds live in coastal waters during the summer and autumn period, but in spring they spawn in different biotopes: in the open sea, larger flounder spawn, sweeping out pelagic eggs, in coastal areas – slower growing smaller flounder, laying bottom eggs. According to the results of fishing in coastal waters in 2023 of the 22 fish species caught, the Baltic river flounder was dominant both in terms of numbers (39%) and biomass (55%). The mass character of the species was clearly observed at all three research stations: Svetlogorsk, Pionersky and Kulikovo stations. The maximum catches per effort in piece (5.5 fishing net/day) and weight (1.16 fishing net/day) exposures were at Pionersky station. In addition, it remained the most numerous species for almost the entire study period from April to July. The Baltic flounder is one of the components of the “core” of the coastal ichthyocenosis of the Baltic Sea. The dimensional structure of flounder was con-structed using cumulates and was considered depending on such environmental factors as tem-perature and depth. At depths of 0-5 m, a herd of small-sized flounder with coastal spawning constantly lives. As the water warms up (to 10-19 degrees or higher), a larger flounder begins to rise into the coastal zone after spawning to feed. As the depth increases (5-10, 10-15 m), the proportion of large flounders also increases. This is due to a change in the proportions of the fish's body (with an increase in linear size, the size of the mouth also increases) and the range of its nutrition: in shallow water, crustaceans predominate in the diet, while in depth – mollusks predominate.