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Volume 48, No. 1

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Stable isotope and mercury analyses of the Galápagos Islands seabird community

Key words: foraging, δ¹⁵N, δ¹³C, mercury, ENSO events, tropical seabirds, dietary shifts

Abstract

The Galápagos Islands seabird community is directly impacted by El Niño Southern Oscillation (ENSO) cycles, which makes understanding seabird foraging behavior in response to these events important for future conservation plans. In this study, we used stable isotope analysis (δ¹⁵N and δ¹³C) to investigate trophic status and foraging location in the seabird community before, during, and after the 2015-2016 El Niño event. Mercury (Hg) analysis was also performed to provide a more thorough understanding of the relationship between contaminant exposure and foraging behavior. We analyzed breast feathers collected across five years (2011, 2014-2017) from eight nesting seabird species (Sula sula, S. granti, S. nebouxii excisa, Fregata minor, F. magnificens, Oceanodroma tethys tethys, Creagrus furcatus, and Phaethon aethereus)for δ¹⁵N and δ¹³C isotopes and total Hg (ppm). These sampling periods occurred at different points in the ENSO cycle, which allowed shifts in foraging behavior to be monitored as environmental conditions changed. Our findings indicate that higher Hg contamination is positively correlated with La Niña. Additionally, as prey abundance decreased with the onset of El Niño in 2015, most species showed more negative δ¹³C values, which indicates a shift to more pelagic foraging. Furthermore, isotopic nitrogen values revealed that while foraging by most species decreased in trophic level during the 2015-2016 El Niño, some populations, mainly Sula species, increased in trophic level. Both responses indicate a change in diet, suggestive of flexible foraging behavior.

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