Protected fish predator in a protected area: impact of the Great Cormorant ( <i>Phalacrocorax carbo</i> L.) on fish populations in the Raszyńskie Ponds Nature Reserve (Central Poland)

The Raszyńskie Ponds Nature Reserve, established in 1978, is an ornithological reserve that provides key breeding, foraging, and resting habitats for water and marsh birds. Among the protected species, the great cormorant (Phalacrocorax carbo L.) has established a significant colony. In the Raszyńskie Ponds, carp and other cyprinids represent above 90% of the annual production of fish. This study aimed to assess the size of the breeding colony and evaluate its impact on ichthyofauna. Colony size was determined in 2024 according to the Cormorant Monitoring Fieldwork Manual of the State Environmental Monitoring programme, with occupied nests recorded at the optimal counting period. Changes in cormorant abundance between 1984 and 2024 were also analysed. Diet composition was examined from regurgitated pellets (spits) collected during spring (1 April–31 June, n = 68) and summer (1 July–10 August, n = 32) 2024. In total, 218 prey fish representing eight species were identified, with prey size ranging from 10.2 to 33.6 cm total length. The largest prey included northern pike (Esox lucius, 33.6 cm) and grass carp (Ctenopharyngodon idella, 31.1 cm). A significantly higher number of fish per spit was observed in spring compared to summer (t = 3.696, p < 0.001), averaging 2.5 and 2.3 fish per spit, respectively. However, daily biomass consumption did not differ significantly between the two periods (t = 0.122, p = 0.903), averaging 300.0 g in spring and 297.0 g in summer. Based on 2024 consumption rates, the annual fish intake by the cormorant colony between 2015 and 2024 was estimated at 4.2–32.4 tonnes. While the reserve’s protection supports cormorant population growth, it simultaneously increases pressure on fish communities. Continued monitoring is therefore essential to inform sustainable management strategies that balance avian conservation with ichthyofauna protection.