The diet of great cormorants Phalacrocorax carbo on the Sava river between Ljubljana and Zagorje (Slovenia)

Diet of Great Cormorants Phalacrocorax carbo occurring on the Sava river between Ljubljana and Zagorje was studied in the winter 1998/99 by means of regurgitated pellets, collected at the Cormorants\u27 night roost at Hotič. Among 473 collected pellets, 69.8 % contained remains of fish. The pellets also contained worms Nematoda and tapeworms Cestoda, remains of caddis flies trichoptera, snails Gastropoda and a frog Rana sp. In separate pellets, remains of 1 to 69 fish (median 2, average 3.9) were found: in 41.8 % pellets remains of 1 fish, in 93.6 % remains of up to 10 fish. Altogether, remains of 1,288 fish were found. Length and weight were determined for 1,279 of them. Total weight of these fish was estimated at 57 kg. The diet consisted of 12 fish species (trout Salmo trutta, grayling Thymallus thymallus, chub Leuciscuscephalus, nase Chrondrostoma nasus, danube roach Rutilus pigus virgo,roach Rutilus rutilus, barbel Barbus barbus, bream Abramis brama, bleak Alburnus alburnus, pike Esox lucius, perch Perca fluviatilis and ruffe Gymnocephalus cernuus). The diet was dominated by Cyprinidae (85.8 % by number, 90.5 % by biomass). Gravling and trout were represented with 6.5 % by number and 3.6 % by mass and pike, perch and ruffe in 7.3 % by number and 3.6 % by mass. Inside Cyprinidae chub with 16.4 % by number and 38.6 by biomass and nase with 3.9 % by number and 16.5 % by biomass were most frequent. The proportion of undetermined Cyprinidae was 57.1 % by number and 28.5 % by mass. Prey size ranged from 23 to 345 mm. Most frequent length class was 70-170 mm (50 % by number and 19 % by mass), but large individuals (170 mm) were most important (25.0 % by number and 80.1 % by mass) in the diet of Great Cormorants. The numbers of specimens of Cyprinidae, Percidae and Salmonidae between months were significant, while the numbers of specimens of determined Cyprinids were not. We concluded that the differences in the investigated area depended more on random detection of fish. Chub and nase arespecies with shoaling habits, and were assumed that they were easier detectable by Great Cormorants than the non-shoaling specie