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Marine Ecology Progress Series

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MEPS 188:225-236 (1999)  -  doi:10.3354/meps188225

Intercolony and seasonal differences in the breeding diet of European shags on the Galician coast (NW Spain)

Alberto Velando1, Juan Freire2,*

1Departamento de Ecoloxía e Bioloxía Animal, Universidade de Vigo, 16200 Vigo, Spain
2Departamento de Bioloxía Animal, Bioloxía Vexetal e Ecoloxía, Universidade da Coruña, 15071 A Coruña, Spain
*Addressee for correspondence. E-mail:

ABSTRACT: The seasonal and spatial variations in the diet of the European shag Phalacrocorax aristotelis were studied during the breeding period, based on the analysis of 202 adults' pellets (February to June, 1995) and 30 regurgitations of chicks (April, May and June, 1995 and 1996) taken from 2 colonies located in close proximity to one another (<15 km) on the coast of Galicia (NW Spain; Islands of Cíes and Ons). The diet of birds from the Cíes Islands consisted mainly of sandeels (family Ammodytidae), which make up over 70% of the prey during all months. These fishes went practically unreported in a number of previous studies of fish communities in the region, probably due to the low catchability of trawl nets and the possibility that the habitats of sandeels where shags forage, shallow (<10 to 15 m) sandy bottoms, were not sampled adequately. Seasonal changes were found on the Island of Ons, where in winter (February and March) the diet was based on gobids (family Gobiidae) and sand smelts Atherina presbyter, while in spring the dominant prey were the Ammodytidae (in May and June they made up over 86%). The consumption of sandeels established an increased similarity in diet between the colonies throughout the breeding cycle, and they were the only prey whose abundance was negatively correlated with the diversity of each pellet. This study highlights the high plasticity in prey and feeding habitats (both pelagic and benthic with different types of substrates) used by the European shag. It should be noted, however, that the main prey during chick rearing are sandeels. Differences were found in the mean sizes of the different prey consumed, but the modal size for all of them throughout the season was around 9 to 11 cm in total length. The range of sizes available in the environment is greater than the range chosen by the European shag, which suggests that this bird selects a narrow range of prey sizes.

KEY WORDS: Diet · Breeding · Seasonal changes · Spatial changes · Fish prey · Prey size · European shag · Galicia (NW Spain)

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