Mercury (Hg) concentrations in predatory bird livers and eggs as an indicator of changing environmental concentrations: a Predatory Bird Monitoring Scheme (PBMS) report

摘要

Concern over the potential health effects of mercury (Hg) has prompted an international agreement, the Minamata Convention on Mercury, that aims to control anthropogenic releases to the environment and reduce potential impacts on humans and wildlife. Monitoring is required to determine to what extent the convention is successful. The PBMS has monitored long-term trends in environmental Hg concentration using raptors and fish-eating birds as sentinels to track changes in exposure. Overall, PBMS monitoring of Hg in predatory birds provides an evidence base by which the impact of the Minamata Convention on environmental mercury concentrations in Britain can be assessed. udThe current study consisted of four main aims that would help rationalize and inform our long-term Hg monitoring. ud(i)Updating long-term data for liver Hg concentrations in sparrowhawks, (Accipiter nisus), a sentinel for exposure in lowland terrestrial habitats. ud(ii)Exploration of the use of alternative tissues for monitoring Hg in sparrowhawks. ud(iii)Comparison of trends in liver Hg residues in sparrowhawks and kestrels (Falco tinnunculus) to examine if trends in sparrowhawks, which feed on relatively mobile avian prey, reflect those in kestrels which mainly feed on small mammals that are more likely to reflect local contamination.ud(iv)Completion of work initiated last year to explore the potential for using Hg concentrations in the eggs of inland-feeding golden eagles (Aquila chrysaetos) as a sentinel to track changes in Hg bioavailability and uptake by biota in upland terrestrial systems. udWe measured liver Hg residues in sparrowhawks that had died in 2013 and 2014 to quantify current Hg exposure in lowland terrestrial habitats and to add to previously reported long-term data. Mercury residues in birds that died in 2013 and 2014 were largely consistent with those reported in recent previous years and were below those associated with mortalities. Three birds had residues higher than those associated with potential adverse effects on reproduction. udAnalysis of long-term data (1990-2014) indicated liver Hg residues in sparrowhawks vary with age and sex; concentrations are highest in adult males. Starvation also elevates liver Hg concentrations. Taking age and sex into account and using only data for non-starved birds, we investigated temporal trends and found that, although there has been between-year variation in liver Hg concentrations, there has been no consistent upward or downward trend. We used the long-term dataset to define “current baseline” liver Hg concentrations against which levels in future years, and consistent time trends, can be quantitatively and rapidly assessed. udWe found that total Hg concentrations in sparrowhawk liver, kidney and brain were closely related. We conclude it is possible to transfer our long-term monitoring of Hg in sparrowhawks (including retrospective calculation of “current baseline concentrations”) to analysis of kidney or brain. This would preserve [what are relatively small] sparrowhawk livers for other analyses.udComparison of historic trends in liver Hg in sparrowhawks and kestrels indicated that rates of decline during 1980-1998 were similar in the two species. This is consistent with the premise that sparrrowhawks are as likely as kestrels to be representative of changes in environmental exposure to (and associated bioaccumulation of) Hg in lowland terrestrial systems. udThe conclusion of our work on Hg concentrations in golden eagle eggs enabled us to quantify a “baseline concentration” for eggs laid by females feeding predominantly on terrestrial prey. We can use this to identify significant changes in future exposure and associated bioaccumulation and thereby use our measurements as sentinel of future change in Hg bioavailability in upland habitats in northern Britain.ud