increasingly frequent fish die-offs in Lake Balaton, Hungary are raising concerns about the long-term health of the region’s vital ecosystem. Researchers at the Balaton Limnological Research Institute have linked the recent losses to a combination of climate change-induced milder winters and extensive shoreline development impacting water flow [[1]]. The phenomenon is primarily affecting native species crucial to both the lake’s food web and the local fishing industry.
Fish die-offs are occurring with increasing frequency in sheltered harbors along the southern shore of Lake Balaton, Hungary, a phenomenon researchers attribute to a combination of climate change and extensive shoreline development.
The Balaton Limnological Research Institute issued a statement highlighting that these winter fish kills are not isolated incidents, but rather a systemic issue. The institute warns that the situation poses a critical risk to the lake’s overall ecological health.
Researchers say the problem isn’t limited to a single harbor or construction project. Newly constructed, expanded, or modified harbors along the southern shore present similar ecological risks, particularly those featuring large, dredged, and heavily protected bays.
Lake Balaton’s winters are becoming increasingly ice-free due to climate change, while the lake remains a shallow body of water exposed to wind. This results in strong wave action and constant water movement even during the winter months. Fish, which feed little during the winter, expend significant energy fighting the waves and seek refuge in calmer areas.
The newly created, large-scale, dredged, and well-protected harbor basins along the southern shore now function as winter havens for fish. However, the institute explains that these areas have limited water exchange, effectively isolating portions of the water.
“The high density of fish in these protected, poorly mixed harbor basins, combined with oxygen consumption by sediment and low oxygen production by algae in winter, can lead to acute oxygen deficiency,” researchers stated. “Under these circumstances, massive fish kills can occur rapidly, even without external stressors.”
“The high density of fish in these protected, poorly mixed harbor basins, combined with oxygen consumption by sediment and low oxygen production by algae in winter, can lead to acute oxygen deficiency. Under these circumstances, massive fish kills can occur rapidly, even without external stressors.”
The issue is not uniform across Lake Balaton. The deeper water and shoreline morphology on the northern shore allow for the creation of “flow-through” harbors. However, the shallow conditions on the southern shore lead to rapid sediment buildup in such designs, resulting in the prevalence of closed, dredged basins that act as ecological traps for fish. A few southern shore harbors with significant inflow, such as Balatonmáriafürdő, benefit from effective flushing and are not affected by oxygen deficiency.
Native Fish Species Affected
The HUN-REN Balaton Limnological Research Institute emphasized that the phenomenon primarily affects the lake’s native, most biomass-significant fish species, including various bream species like common bream, white bream, and silver carp, as well as pike and zander.
- These species play a crucial role in the lake’s food web,
- are key to the stability of the Balaton ecosystem,
- and are important for fisheries and recreational angling.
The problem, therefore, extends beyond fisheries management and represents a critical risk to the overall ecological functioning of Lake Balaton.
Researchers noted that the presence of cormorants may amplify the fish kills, but is not the primary cause. The birds’ mass hunting drives fish into even denser aggregations, exacerbating oxygen depletion in the confined spaces. However, a lasting solution lies in improving harbor permeability and water flow, not in cormorant deterrence.
The institute also clarified that the fish die-offs are not directly linked to natural fluctuations in Lake Balaton’s water levels. Instead, the issue stems from the combined effects of climate change (ice-free winters, altered currents) and human shoreline development (harbor construction), making it a serious, long-term risk.
Current interventions, such as aeration and cormorant deterrence, are considered symptomatic treatments that can mitigate damage in the short term but do not address the root cause. The institute argues that managing the problem requires comprehensive, professionally sound harbor design that considers the impacts of climate change, the harbors’ geometric and hydraulic characteristics, fish behavior, and the overall functioning of Lake Balaton.
(Cover image: The harbor in Balatonföldvár on June 1, 2018. Photo: György Varga / MTI)
