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

Abstract

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

Age-Dependent Bioaccumulation of Organochlorine Compounds in Fish and their Selective Biotransformation in Top Predators from Lake Maggiore (Italy)

“…Piersma et al 1988Martinoli et al 2003Gagliardi et al 2007). Anyway, the pelagic fish biomass in Lake Maggiore is constituted primarily by whitefish, which is, therefore, a very probable prey of great crested grebe.…”

A review of estimates of daily energy expenditure and food intake in cormorants (Phalacrocorax spp.)

“…Estimates of DFI have been based on: (1) pellet analysis and reconstructing fish sizes consumed by birds (Privileggi 2003, Gagliardi et al 2007); (2) percent of adult body weight (Johnson et al 2002, Rudstam et al 2004; (3) energy expenditure through activity time budgets and activity-specific DEE (Grémillet et al 2000(Grémillet et al , 2003; (4) allometric bioenergetic models as a function of mass and converting this to DFI through division by prey energy density and assimilation efficiency (Madenjian andGabrey 1995, Barquete et al 2008); (5) respirometry and doubly labelled water techniques (Keller andVisser 1999, Enstipp et al 2005), and (6) other methods such as automatic balances and stomach temperature tags (Grémillet et al 1999(Grémillet et al , 2000. Different studies may or may not have employed prey assimilation efficiency with values varying among studies if used (0.79 in Glahn and Brugger 1995, 0.77 in Keller and Visser 1999, 0.85 in Diana et al 2006, 0.80 in Seefelt and Gillingham 2008.…”

Dietary changes of Mediterranean ShagsPhalacrocorax aristotelis desmarestiibetween the breeding and post-breeding seasons in the upper Adriatic Sea

“…This method has been widely used to characterize the diet composition of Cormorants, Shags, Gulls, Terns and Skuas (reviewed in Barrett et al 2007) and to estimate body sizes and biomasses of the prey eaten (Duffy & Jackson 1986, Dirksen et al 1995, Velando & Freire 1999, Eschbaum et al 2003, Gagliardi et al 2007, Barquete et al 2008. However, the analysis of pellets has to consider the partial erosion of otoliths caused by the digestive process, so fish lengths derived from otoliths could be under-estimates (Johnstone et al 1990, Carss et al 1997.…”