Fatty acid signature analysis confirms foraging resources of a globally endangered Mediterranean seabird species: calibration test and application to the wild

Dietary studies of seabirds provide information on food webs and oceanographic variability. Studying fatty acid signatures (FAS), which reflect changes in the composition of the diet, has several advantages over traditional methods. In the context of the Mediterranean marine ecosystem, we studied tissue FAS in captive-fed and wild seabirds. Yellow-legged gulls Larus michahellis were experimentally fed either pelagic (sardine Sardina pilchardus) or demersal (spotted flounder Citharus linguatula) fish for 8 wk, and FAS in the birds' plasma and adipose tissue were studied. The FAS developed patterns characteristic of the sardine or flounder diet, showing the reliability of both types of samples, In addition, plasma samples of 20 endangered wild Balearic shearwaters Puffinus mauretanicas and their most likely prey items from the Mediterranean Sea were collected and analyzed for FAS. Based on FAS of Mediterranean fish, and the results from the feeding experiment, the diet of the endangered wild shearwaters was estimated. In terms of the composition of polyunsaturated fatty acids, the plasma FAS of the shearwaters resembled those of the captive gulls kept on a sardine diet, suggesting that pelagic feeding resources dominate shearwater diet. However, the high level of 20:1n-11, likely metabolized from 22:1n-11 by the birds themselves, suggested that the diet also contained a demersal component rich in 22:1n-11. Contrary to the situation in the open oceans, the levels of the C20 and C22 monounsaturated fatty acids are low in the Mediterranean food web and characteristic of most demersal species. Our results support the findings of previous field studies that the diet of the shearwaters is frequently supplemented by demersal fish from fishery discards.