Use of fatty acids to explain variability of organochlorine concentrations in eggs and plasma of common terns (Sterna hirundo).

We have studied the breeding parameters, organochlorine compounds (OCs) concentrations, and fatty acid (FA) composition of egg yolks (n = 47) and plasma (n = 90) of common terns (Sterna hirundo) from two colonies (Banya and Fangar) in the Ebro delta, NE Spain. Terns from Banya tend to have smaller clutch size and lower hatching success than terns from Fangar. p,p'-DDE and polychlorinated biphenyls (PCBs) concentrations were almost 2-fold higher in yolks from Banya in 1998 than from Fangar in 1999, and the percentage of n-6 PUFA was positively correlated with these contaminants. Differences between samplings in OCs concentrations in plasma were less evident, and were affected by breeding chronology. The highest OCs concentrations in yolks from Banya may be explained by two processes involving the increased deposition of n-6 PUFA: (1) higher mobilization of endogenous fat due to lower food resources, or (2) differences in the diet between colonies. Birds from Banya may have been feeding at a higher degree on discards of trawling fisheries composed of demersal and benthic fish species that are more exposed to contaminants from sediment and have lower n-3/n-6 ratio, whereas birds from Fangar would feed mostly on pelagic species of small clupeiformes that are less polluted and have higher n-3/n-6 ratios.