Mercury levels in the feathers of breeding seabirds in the Seychelles, western Indian Ocean, from 1996 to 2005

Abstract

We analysed mercury levels in the breast feathers of several breeding seabirds – White-tailed Tropicbird (Phaethon lepturus), Audubon’s Shearwater (Puffinus lherminieri), Lesser Noddy (Anous tenuirostris), White Tern (Gygis alba), Sooty Tern (Onychoprion fuscata) and Roseate Tern (Sterna dougallii) – on Aride Island in the Seychelles during three periods between 1996 and 2005. Mercury concentrations tended to be higher in the second sampling period (2002–03) than the first (1996–97), particularly for inshore-feeding species. El Niño events in 1997–99 and 2002–03 might help to explain this pattern, through an increase in mercury bioavailability for seabirds that forage inshore. However, levels of mercury in tissue decreased significantly from the first period (1996–97) to the third period (2004–05) for inshore-foraging species (Lesser Noddy mean ± s.e.: 0.82 ± 0.02 μg g^–1 v. 0.53 ± 0.06 μg g^–1 respectively) and more wide-ranging, offshore-feeding species (Sooty Tern: 1.16 ± 0.05 v. 0.59 ± 0.04; White-tailed Tropicbird: 1.82 ± 0.05 v. 1.43 ± 0.09). Wide-ranging species of seabirds are more likely to be useful bioindicators at large spatial scales, with such species suggesting a decreasing trend in mercury pollution in the western Indian Ocean.