We have witnessed the periodic mass fish kills in Lake Palić in the last decade. The amount of dead fish varies, but these deaths always happen in late spring. This phenomenon has always been explained and related to the poor water quality and unfavourable survival conditions for fish in Lake Palić.
Gabor Mesaros, Ph.D., The Associate Project Specialist:
The most important reason for this poor state of water is that the composition of fish fauna is severely disturbed. It is dominated primarily by one species of fish that originates from Asia, i.e. the Prussian carp, known as babushka. It has been here a long time, so fishermen know it well. It does not belong to our fauna, although it is the dominant fish here in Palić. Previous analyzes proved that over 90% of the fish stock is made up of that fish. Its presence, due to excrement, etc. and its impact on the ecosystem leads to the water being as it is, i.e. algae bloom periodically, excessive algae or other plants growths, etc.
Several initiatives were launched in the past period, whose goal was to improve the water quality of Lake Palić by reducing the existing water load with wastewater from Subotica and Palić and by improving the ecological conditions in the lake and its shores. The most important and most comprehensive is certainly the project “Biodiversity and water protection of lake Palić and lake Ludaš”, within which actions were taken on selective fishing of babushka from the lake. Thanks to the fishing, there was an evident change in the composition of fish fauna in the lake and the share of babushka in the total biomass was significantly reduced. The populations of predatory species such as perch and pike are increasing. However, another fish kill occurred last year as well.
Gabor Mesaros, Ph.D., The Associate Project Specialist:
What was unique about all these fish kills was that over 99 % of the dead fish belonged to the babushka species. The lack of oxygen cannot be selectively manifested in different species of fish, especially since the perch is far more sensitive to lack of oxygen. If so, a certain amount of perch would have been found in that dead fish.
The latest research in the region has shown that one of the causes of babushka deaths in some stagnant waters in Hungary, Slovakia and Serbia, is a virus isolated from dead individuals.
For an accurate assessment of the health condition of the babushka in Lake Palić, help was requested from the associates of Veterinary Institute of Serbia from Belgrade.
Vladimir Radosavljević – Senior Research Associate at the Scientific Institute of Veterinary Medicine of Serbia:
We will try to do targeted sampling, to try to find some specimens of this population that show some symptoms of early disease, to reduce the number of samples we can take, and at the same time increase the chance of finding such a cause, followed by bacteriological, virological and other complete analyzes and, based on these results, gain some insight into the health condition. Of course, this is only the first step. We will do that sampling every month.
The research was performed from April to August 2020. Thirty specimens of babushka were taken during this period on six occasions, and the samples were analyzed in the laboratory. The results were announced during a press conference held in mid-May 2021 on Lake Palić.
Marko Stojčić PhD, Project Consultant Representative on behalf of Posch & Partners consulting engineers
The results of the research of the mass fish kill of babushka on this lake unequivocally show that the death of fish was caused by a virus. It is a cyprinid herpesvirus 2 that causes herpes virus necrosis, i.e. the tissue death. This virus is not dangerous for humans or other fish and other organisms. It attacks exclusively the Prussian carp population.
All age categories of babushka are equally affected by this virus, and the highest mortality was observed when the water temperature ranged between 20oC and 25oC. As the temperature rises above 25oC, fish mortality has been found to decline.
Marko Stojčić, Project Consultant Representative on behalf of Posch & Partners consulting engineers
Specimens were also found, that are positive for the herpes virus, but they did not show mortality. It is concluded that the virus is found in all specimens of babushka in its latent form, i.e. without manifestation and that the trigger is in some of the external stress factors. For fish, it can be the spawning situation itself, a sudden change in temperature, or overpopulation.
Fish spawning and the change of water temperature cannot be influenced. The fish kill of babushka can be prevented only by biomanipulation, i.e. by restoring the biological balance of the lake and reducing the number of overpopulated fish species.
Marko Stojčić, Project Consultant Representative on behalf of Posch & Partners consulting engineers
The ECOLACUS project, as one of its components, has selective fishing of Prussian carp, precisely for the reason of reducing the population of babushka to such a level as to allow indigenous fish species, such as perch, to multiply and continue to maintain the biological balance naturally in the lake. In this regard, the project will continue the selective fishing of Prussian carp this year and in the years to come.
The multiannual mortality of Prussian carp has long been a concern among the local population at the Palić and Ludaš lakes. This research puts an end to arbitrary conclusions about possible causes of fish kills, such as poor water quality and lack of oxygen. Since there is no known protective vaccine or effective cure against the virus that attacks babushka, the implementation of the Ecolacus project will enable the restoration of the natural balance in the disturbed ecosystem of lakes Palić and Ludaš and prevent further seasonal mass fish mortality.
