Inter-specific niche partitioning and overlap in albatrosses and petrels: dietary divergence and the role of fishing discards
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Although fisheries discards are recognized as a key food source for many seabirds, there have been few thorough assessments of their importance relative to natural prey, and of their influence on the trophic structure of pelagic seabird communities during the non-breeding period. Competition for resources in Procellariiformes appears to be reduced mainly by avoiding spatial overlap, which is supposed to influence diet composition. However, artificial food sources provided by fisheries might relax niche partitioning, increasing trophic niche overlap. Using bycaught birds from pelagic longline fisheries, we combined the conventional diet and stable isotope analyses to assess the importance of fishing discards in the diet of eight species of Procellariiformes. Both methods revealed the high contribution of trawl discards to the non-breeding diet of three neritic species and a moderate contribution in several other species; discards from pelagic and demersal longline fisheries were considerably less important. There was a clear contrast in diets of neritic vs. oceanic species, which are closely related taxonomically, but segregate at sea. Niche partitioning was less clear among neritic species. They showed an unexpectedly high level of diet overlap, presumably related to the large volume of trawl discards available. This is the first study combining the conventional diet and stable isotope analyses to quantify the importance of fishery discards for a community of non-breeding seabirds, and demonstrates how the super-abundance of supplementary food generates high levels of overlap in diets and allows the coexistence of species.
We would like to thank the observers of the Programa Nacional de Observadores de la Flota Atunera Uruguaya (PNOFA). Special thanks to Martin Abreu and Rodrigo Forselledo for their invaluable cooperation in the examination of bycaught seabirds at the laboratory. Thanks also to María Salhi for the help with lipid extraction. We would also like to thank two reviewers and the handling editor for helpful comments. SJ gratefully acknowledges the support by Graham Robertson, the British Embassy (Montevideo), and the Agreement on the Conservation of Albatrosses and Petrels of three study visits to British Antarctic Survey where some of this work was carried out. SIA was partially funded by Dirección Nacional de Recursos Acuáticos (DINARA). JX was supported by the research programs CEPH, SCAR AnT-ERA, SCAR EGBAMM, and ICED and by the Investigator FCT programme (IF/00616/2013). This paper is part of the Ph.D. thesis of SJ, who received a Ph.D. scholarship from Agencia Nacional de Investigación e Innovación (ANII) and a support scholarship for the completion of postgraduate studies from Comisión Académica de Posgrado (CAP).
SJ and RAP determined the basis for the paper, with contributions of AD, OD, and AB. SJ, JX, MV, and MIL undertook the laboratory work. SJ undertook all the analyses. SJ wrote the first draft with the contribution of RAP. JX, OD, AB, and AD contributed to subsequent drafts.
Compliance with ethical standards
This study was funded in part by Dirección Nacional de Recursos Acuáticos (DINARA), Agencia Nacional de Investigación e Innovación (ANII) and Comisión Académica de Posgrado (CAP).
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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