Nascimento T., Oliveira N. & Luís A. (2021) Hey, that’s my fish – Overlap in prey composition between European Shag and local fisheries in Portugal. ARDEA 109 (1): 77-90

The European Shag Phalacrocorax aristotelis is a threatened seabird species in Portugal. Knowledge about this population is limited, and currently there is no information about dietary needs and threats posed by fisheries. We studied the diet composition of Shags in the Berlengas archipelago, the largest breeding colony in the country, through the analysis of 124 regurgitated pellets, and estimated trophic overlap with local fisheries, during the breeding and winter seasons of 2016–2017. A total of 32 fish species were identified in their diet, while their major prey consisted of sand eels (Ammodytidae) and species from the Labridae, Gadidae and Sparidae families. Diet composition varied seasonally and was more specialized during the breeding season, with sand eels comprising 70% of all prey consumed and 45% of total biomass. During winter, the consumption of heavier predatory fish increased, particularly fish from the Gadidae and Sparidae families. Shags consumed 17 species also targeted by commercial fisheries, and fish were in the same size range, highlighting the potential for resource competition. Fish species that contributed the most to prey overlap were the Pouting Trisopterus luscus, White Seabream Diplodus sargus, Common Two-banded Seabream Diplodus vulgaris, Bogue Boops boops and Black Seabream Spondyliosoma cantharus. Among different types of fishery, trophic overlap with Shags was more pronounced with polyvalent gear (longlines and gillnets), especially during winter, than with purse seine and trawling. Competition between Shags and local commercial fisheries may represent an additional pressure on foraging Shags, as well as increasing their conflict with fishing activity.