HUN-REN: South coast ports could become an ecological “trap” in winter – risk of oxygen deficiency and fish kills

Why is the southern shore the most vulnerable? As Lake Balaton’s winters are increasingly ice-free, the open water is still characterized by strong wave action and continuous water movement even in winter. Fish that feed very little in the cold season therefore seek areas sheltered from waves and move in large numbers into calmer marina basins. There, however, the water mass becomes partly isolated; weak mixing, oxygen consumption by sediments, high fish density and low winter oxygen production by algae can together easily create an “oxygen trap.”

Which species are affected? According to the researchers, the phenomenon primarily impacts Lake Balaton’s native fish species that are most significant in terms of biomass: bream species (e.g., common bream, white bream, roach), and it also affects bleak and pike-perch (zander).

Cormorants are not the main cause – and neither are water-level fluctuations Based on the statement, cormorants may at most act as an aggravating factor (by forcing already dense shoals to cluster even more), but the underlying drivers are the winter hydrodynamics and limited pass-through/flush of marina basins, combined with the effects of climate change and shoreline construction. The researchers also explicitly distinguish the phenomenon from Lake Balaton’s natural water-level fluctuations.

What does this mean for stakeholders around Lake Balaton? HUN-REN says current interventions (e.g., aeration and cormorant deterrence) are at best symptomatic measures. A lasting solution requires improving water flow and permeability in marinas, and marina planning that incorporates contemporary aquatic-ecology considerations.