Trophic ecology of Scopoli's shearwaters during breeding in the Zembra Archipelago (northern Tunisia)

While breeding, seabirds are central-place foragers requiring resources to sustain high-energy requirements. Therefore, during this period, they are particularly sensitive to food-resource availability, which can vary within and between years. Intra- and inter-annual variations in Scopoli's shearwater (Calonectris diomedea) trophic ecology were investigated at its largest colony (Zembra Island, 37 degrees 07'33 '' N, 10 degrees 48'23 '' E, Mediterranean Sea). Carbon and nitrogen isotope values were analysed in the blood (adults and chicks) and feathers (adult wing) during pre-laying, incubation, and chick-rearing in 2015 and 2016 to assess variations in stable isotope composition, isotopic niches, trophic levels, and diet inferred from isotope mixing models. Scopoli's shearwaters showed variations in isotopes throughout the breeding season and among years, with incubation showing the highest delta C-13 and delta N-15 values, trophic levels, inter-annual isotopic niche consistency, and the most specific and narrowest isotopic niche. The difference in blood delta C-13 values between adults and chicks suggested trophic habitat segregation: adults feed inshore, while chicks are fed more oceanic prey. Stable-isotope mixing models based on three potential prey groups revealed that the diet could consist mainly of pelagic fish and crustaceans throughout the breeding season, whereas non-pelagic fish and cephalopods could be consumed more sporadically, mainly during incubation. Feather delta N-15 values suggested that the adult diets contained more zooplankton in 2014. These results demonstrated the Scopoli's shearwater trophic ecology plasticity in response to the variable nutritional demands of breeding phases and changes in prey availability. Scopoli's shearwaters may, thus, be valuable bio-indicators of small pelagic fish populations considered critically depleted in the Mediterranean.