The effect of fishing effort, fish stocking, and population density of overwintering cormorants on the harvest and recapture rates of three rheophilic fish species in central Europe

Rheophilic fish species are among the most threatened groups of organisms. There is evidence that anthropogenic activities, such as fish stocking and fishing pressure, could have a significantly negative effect on freshwater fish populations. In addition to this, high population densities of cormorants, Phalacrocorax carbo, (one of the most important avian piscivorous predators in Europe) are also partially driven by anthropogenic activities. The first goal of this study was to estimate the effects of fishery management on fish harvest in three rheophilic fish species: barbel (Barbus barbus), nase (Chondrostoma nasus), and vimba bream (Vimba vimba). The second goal was to estimate the effect of cormorant population density on the harvest of these species. Individual mandatory angling logbooks data were collected on the 22 largest rivers in central Bohemia (Czechia, central Europe) over 13 years. Cormorant density was estimated using bird census data while the diet composition of cormorants was assessed using an analysis of fish diagnostic bones extracted from regurgitated bird pellets. In total, anglers visited the selected fishing areas 3 million times over 13 years and harvested 1 000 tons of fish, of which 4.5 tons were rheophilic species. The fishing effort was the most important driver of fish harvest. Fish stocking and cormorant population density did not significantly affect the fish harvest. However, rheophilic fish species comprised 8.8 % of fish consumed by cormorants (by biomass). In conclusion, fishing pressure was the most important factor affecting the harvest of rheophilic fish. Conversely, the intensity of predation by cormorants was not significantly affecting fish harvest despite their consumption of rheophilic fish. However, there is a possibility that predation pressure of cormorants was evenly distributed among fishing areas.