Beautiful San Francisco Bay is one of my favorite places to visit, and numerous feathered types feel the same way: the Bay supports thousands of wintering and migrating shorebirds and waterbirds each year. Unfortunately, this important estuary is contaminated by residual mercury from mining and gold extraction, and the restoration of salt evaporation ponds as tidal marsh habitats may actually increase the bioavailability of mercury for aquatic wildlife. To determine current contamination levels in locally breeding shorebirds, and to correlate these levels with foraging, roosting, and breeding sites, Ackerman and colleagues measured mercury concentrations in the blood of American Avocets (Recurvirostra americana) and Black-necked Stilts (Himantopus mexicanus).
Golden Gate Bridge, San Francisco CA
Photo: Barn Owl
First, study sites in the North and South Bays were chosen, with wetland habitats including salt ponds, tidal flats, tidal marsh, and diked wetlands. Avocets and stilts are the most abundant shorebird species that breed in SF Bay, and these birds were captured during the pre-breeding season, using a net-launcher or rocket nets. Blood was collected from the brachial vein for contaminant analyses, and adult birds were fitted with radio transmitters and leg bands, for space use monitoring. Birds were tracked daily from trucks, and every two weeks from fixed-wing aircraft. Blood samples were analyzed for total mercury levels by atomic absorption spectroscopy.
Black-necked Stilt, Himantopus mexicanus
Photo: Dr. Jim Smith
Over two pre-breeding seasons in 2005 and 2006, 373 avocets and 157 stilts were examined. The highest blood concentrations of mercury were discovered in birds at the Alviso salt pond complex in South SF Bay, most likely due to contaminated sediments from the historic New Almaden mercury mine; this area is the site of over 200 avocet nests, and over 300 stilt nests. At all study sites, mercury levels were higher in stilts than in avocets. Stilts tended to use managed marshes, and vegetated areas, more than did avocets, and the researchers proposed that differences in micro-habitat selection may influence mercury exposure. Females of both shorebird species had lower mercury levels than did males, a difference that can only be partially explained by the depuration of methyl mercury into eggs, a process demonstrated to occur in loons, gulls, pelicans, and terns. Although the restoration of SF Bay wetland habitats is a positive development, these efforts may have the unintended consequence of increasing levels of toxic and bioavailable methyl mercury.
Ackerman, J.T., Eagles-Smith, C.A., Takekawa, J.Y., et al. (2007). Mercury concentrations and space use of pre-breeding American avocets and black-necked stilts in San Francisco Bay. Science of the Total Environment 384, 452-466.