Emigration of silver eels (Anguilla anguilla L.) from the Great Masurian Lakes complex in the southeastern Baltic region.

Obstacles encountered by downstream migrating European eels during their spawning migration are among the primary causes of their population decline. River regulation and hydrotechnical infrastructure play a crucial role in this process. Fish reaching migration barriers often interrupt their journey, while those passing through hydroelectric turbines frequently suffer fatal injuries. Using acoustic telemetry, the downstream migration of silver eels from a large lake complex in northern Poland was tracked. Along their route, the fish encountered small hydropower plant (SHP) equipped with Kaplan turbines. Of the tagged eels, 25% did not reach the dam, and an additional 47% stopped in the reservoir. Only 28% of the eels passed through the turbines, of which 7% died during passage. In total, 26% of the eels reached the Polish border-the downstream limit of the tracking range, located 41 km from the release point. Using a GLM model, we demonstrated the significance of morphological parameters such as the ocular index (EI) and total body length (TL) on the migration time after passing through the SHP. Larger EI and TL reduced migration time.