The thesis was focused on biology, ecology, habitat, and predatory pressure of the great cormorant (Phalacrocorax carbo) on a fish population in the river Hron in the winter in the years 2018 - 2020. Targeted to document the damage to the ichthyofauna in the monitored area. There was detected damage (injuries) of fish depending on the species and size parameters. Among preyed fish species were represented in particular common chub, common carp common barbel, common bleak, common nase. The thesis emphasizes the pressure of the cormorant separately on species that are original in the zone and without control of cormorant status may be found in to a group of critically endangered fish. The work lists the individual fishing grounds, on which had the highest incidence of cormorants during winter migration. In the relevant chapter is an analysis of the legislation of management (population checking) of the great cormorant and discussed exemptions for deterrence and blasting granted by the Ministry of the Environment of the Slovak Republic at the request of the Slovak Fishermen's Association. The thesis also describes the mechanism and procedures for assessment of the amount of compensation for damage caused by cormorants and their application. Part of the section in which the permitted shooting took place (exception) is also a hunting ground in hunting area J XI Vtacnik (district of University Forestry Enterprise of Technical University in Zvolen ). Biological material was obtained during field research (caught and found dead cormorants) used in routine autopsies on evaluation of preyed fish in the digestive tract of cormorants, isolation of the most common parasitic worms (roundworms), and fecal sampling for laboratory diagnosis parasitic protozoans (Protozoa) at the Institute of Parasitology of the Czech Academy of Sciences in the Czech Republic. At the time of submitting the diploma thesis, the evaluation of samples is in progress. The output of the diploma thesis are proposed feasible measures to control the numerous states of this predator.