Long‐term dietary shift and population decline of a pelagic seabird—A health check on the tropical Atlantic?

In the face of accelerating ecological change to the world's oceans, seabirds are some of the best bio‐indicators of marine ecosystem function. However, unravelling ecological changes that pre‐date modern monitoring programmes remains challenging. Using stable isotope analysis of feathers and regurgitants collected from sooty terns (Onychoprion fuscatus) nesting at a major Atlantic colony, we reconstructed a long‐term dietary time series from 1890 to the present day and show that a significant dietary shift occurred during the second half of the twentieth century coinciding with an apparent population collapse of approximately 84%. After correcting for the “Suess Effect,” δ13C in feathers declined by ~1.5‰ and δ15N by ~2‰ between the 1890s and the present day, indicating that birds changed their diets markedly over the period of population decline. Isotopic niches were equally wide before and after the population collapse but isotopic mixing models suggest that birds have grown ever more reliant on nutrient‐poor squid and invertebrates as teleost fish have declined in availability. Given that sooty terns rely heavily on associations with sub‐surface predators such as tuna to catch fish prey, the rapid expansion of industrialized fisheries for these species over the same period seems a plausible mechanism. Our results suggest that changes to marine ecosystems over the past 60 years have had a dramatic impact on the ecology of the most abundant seabird of tropical oceans, and highlight the potentially pervasive consequences of large predatory fish depletion on marine ecosystem function. We estimated contributions of each of the four main prey groups (teleost fish species [false halfbeaks, blue flying fishes, redlip blennies and Simony’s frostfishes], squid [Teuthida], marine invertebrates [Sally Lightfoot crabs and violet sea snails] and terrestrial invertebrates [locust species]) in the diet of sooty terns breeding on Ascension Island in the South Atlantic. Their diet has changed markedly in decades (a) before and (b) after their population collapsed by 84% over approximately a 30‐year period.