From mass of body elements to fish biomass: a direct method to quantify food intake of fish eating birds

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.