Modelling the daily food requirements of wintering great cormorants: a bioenergetics tool for wildlife management

Summary Great cormorants Phalacrocorax carbo , Linnaeus are large piscivorous birds which occur in Asia, Australia, Africa, Europe and North America. Their European breeding population has increased by at least 15% per annum over the last 15 years, reaching a total of c . 200 000 pairs in the late 1990s. There are concerns that this increase is adversely affecting freshwater fish populations throughout Europe, but real assessment requires a detailed knowledge of cormorant food requirements. The daily food intake (DFI) of great cormorants has been measured during the breeding season, but little is known about DFI in winter when these poorly insulated birds experience consistently low temperatures. DFI is likely to vary widely according to abiotic and biotic conditions, making predictions about impact particularly difficult. We modelled DFI for great cormorants wintering at Loch Leven, Scotland, using behavioural data recorded via radio‐tracking of free‐ranging individuals, metabolic measurements obtained from captive birds, and published data. DFI was estimated to be 672 g day −1 (predicted maximum range 441–1095 g day −1 ), values similar to DFI of great cormorants breeding under temperate conditions and of other aquatic bird species. During winter great cormorants at Loch Leven decreased their average dive time and increased dive efficiency (higher proportion of time spent underwater). They nonetheless spent 130 min day −1 in the water and allocated more than a third of their daily energy budget to diving. Synthesis and applications : In view of the need for the sound management of cormorant populations, we present a general bioenergetics model, based on simple behavioural and dietary inputs, that computes an estimate of DFI outside the breeding season for a range environmental conditions and habitats. An interactive computer programme for this model is available ( http://www.cepe.c‐strasbourg.fr ) to help scientists and managers estimate local values for average, minimum and maximum DFI.