The winter diet of great crested grebe (Podiceps cristatus) was studied in the major lakes of Insubria region, N Italy, in 2000–2003 by analysing stomach and oesophagus contents of birds found dead. Winter diet of cormorant (Phalacrocorax carbo) was studied using pellet analysis. Diet was expressed in terms of numerical abundance, frequency and biomass of prey. A detailed methodology is described of how to prepare a reference collection of otoliths, pharyngeal bones and chewing pads (key-elements), essential for species-specific identification of fish remains in stomachs and pellets, and to estimate ingested fish biomass. Correlations between mass (mg) of key-elements and mass of corresponding fish were significant, and exponential regression models were used to calculate biomass of each single prey item directly from the mass of its key-elements. Advantages of this method are compared with the more traditional two step approach in which size of elements is used to estimate fish length, followed by calculating fresh mass from fish length. Cyprinids formed the largest part of the diet for both predators. Great crested grebes selected small prey, feeding mainly on bleak (Alburnus alburnus alborella), while diet composition of cormorants varied significantly in relation to prey availability.
From mass of body elements to fish biomass: a direct method to quantify food intake of fish eating birds
GAGLIARDI, ALESSANDRA;MARTINOLI, ADRIANO;PREATONI, DAMIANO;WAUTERS, LUCAS ARMAND;TOSI, GUIDO
2007-01-01
Abstract
The winter diet of great crested grebe (Podiceps cristatus) was studied in the major lakes of Insubria region, N Italy, in 2000–2003 by analysing stomach and oesophagus contents of birds found dead. Winter diet of cormorant (Phalacrocorax carbo) was studied using pellet analysis. Diet was expressed in terms of numerical abundance, frequency and biomass of prey. A detailed methodology is described of how to prepare a reference collection of otoliths, pharyngeal bones and chewing pads (key-elements), essential for species-specific identification of fish remains in stomachs and pellets, and to estimate ingested fish biomass. Correlations between mass (mg) of key-elements and mass of corresponding fish were significant, and exponential regression models were used to calculate biomass of each single prey item directly from the mass of its key-elements. Advantages of this method are compared with the more traditional two step approach in which size of elements is used to estimate fish length, followed by calculating fresh mass from fish length. Cyprinids formed the largest part of the diet for both predators. Great crested grebes selected small prey, feeding mainly on bleak (Alburnus alburnus alborella), while diet composition of cormorants varied significantly in relation to prey availability.
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