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The aim of the study was to determine the food preferences of the black cormorant and to assess fisheries losses in Włocławski Reservoir. Twenty fish species were identified in the cormorant diet. The dominant species were ruff and perch (29.4% and 24.4 % of the weight share, respectively). Roach (11.9%) and Prussian carp (10.9%) were also important components of the cormorant diet. The share of invasive species from Gobiidae family was negligible (0.2%). Among the fish species important to commercial and recreational fisheries management, the most numerous in the cormorant diet were: ide (10 t; 2.7% of the weight share), pikeperch (8.6 t; 2.3%), asp (6.3 t; 1.79%), and pike (4.9 t; 1.3%). The fish biomass consumed by cormorants was a mean of 372.8 t annually, which is 46.6 kg per ha (range of 44.6 to 49.6 kg per ha). The analysis of fisheries losses in 2009 was estimated at 980,000 PLN.
The great cormorant Phalacrocorax carbo sinensis (Blumenbach, 1798) is one of the most important birds in the Vistula Lagoon and the Gulf of Gdańsk area. As a typical piscivore the bird plays a significant part in life cycles of parasites in aquatic ecosystems. Within January 2000-June 2001, a complete helminthological examination was performed on great cormorants collected in the nesting area at Kąty Rybackie on the Vistula Spit (80 specimens) and in the Vistula Lagoon (10 specimens). The infection prevalence mean intensity, and intensity range were 92.2%, 376.5, and 1-4524, respectively. The presence of 9 digenean species (Paryphostomum radiatum (Dujardin, 1845), Petasiger exaeretits Dietz, 1909, P. phalacroracis (Yamaguti, 1939), Mesorchis pseudoechinatus (Olsson, 1876), Metorchis xanthosomus (Creplin, 1846), Cryptocotyle concavum (Creplin, 1825), Hysteromorpha triloba (Rudolphi, 1819), Tylodelphys clavata (Nordmann, 1832), and Holostephanus dubinini Vojtek et Vojtkova, 1968) was recorded, P. phalacrocoracis being the most common parasile (prevalence 92.2%, mean intensity 323.8). Tylodelphys clavata proved a parasite new for the great cormorant moreover the records of P. phalacrocoracis, H. triloba, and H. dubinini are the first in Poland, while C. concavinn and M. pseudoechinatus were for the first time recorded in the great cormorant in Poland.
The history of the black cormorant, Phalacrocorax carbo sinensis (L.), at Lake Dobskie dates to 1964, and by 2008 the colony numbered 764 nests. Breeding success is 2.1 indiv./nest. Samples were collected weekly from 28.03.2008 to 22.08.2008. This material comprised more than 700 pellets and 3,104 regurgitated fish, which were used to identify the composition of the cormorant diet. Based on the regurgitated fish collected, it was determined that the cormorant diet included roach (51.5%), tench (13.0%), pike (9.6%), common and white bream (9.2%), perch (8.0%), and bleak (4.2%). The total weight of the prey taken by the colony studied in 2008 was 188 tons of fish.
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