Ivory Gulls, Pagophila eburnean, are scavenging Seabirds found across the High
Arctic. Their population in Canada has declined by 80-85% since the 1980s (the
earliest dates for which records of the species are available), and currently
comprises around 400-500 breeding pairs; as such the species is considered Near
Threatened under the terms of the International Union for the Conservation of
Nature’s Red List of Threatened Species, and Endangered within Canada. The
reason for this decline is unclear, but could be related to environmental
contamination by toxic chemicals, illegal harvesting of the Birds (particularly
in Greenland, which is outside of Canada’s control) or alterations in sea ice
conditions relating to climate change.
A study published in 2006 found
that the eggs of Ivory Gulls had the highest concentrations of mercury recorded
in any Arctic Bird. Mercury is a highly toxic metal released as an atmospheric
pollutant by a variety of industrial processes, and Birds with high levels in
their blood are known to suffer both a decline in overall health and a
reduction in fertility. Mercury in its pure form is toxic, but does not tend to
accumulate within bodies of organisms, but a small amount of the mercury that
enters ecosystems is converted to methyl mercury, which is incorporated into
tissues and bioaccumulates in food chains (which is to say that level of methyl
mercury increases with each step up the food chain). Since Ivory Gulls
primarily feed by scavenging Fish and Marine Mammal carcasses they are
considered to be at or very close to the top of the food chain in Arctic
ecosystems. As environmental mercury from elsewhere at on the Earth is known to
accumulate at the poles, animals living in these environments, such as Ivory
Gulls, are considered to be particularly at risk of mercury poisoning.
Mercury is known to be deposited
in the feathers of Birds at a rate that directly corresponds to levels in the
blood, and is therefore considered to be a good proxy. It is also possible to
test mercury levels in feathers taken from living Birds, making a study of
historic mercury levels in feathers useful for comparison in future studies
(due to the declining nature of the species further tests on eggs are
undesirable).
Adult ivory gull feeding on a seal carcass, Resolute Bay, Nunavut, Canada,
10 June 1989. Keith Hobson in Bond et al.
(2015).
Firstly Bond et al. tested for ratios of stable isotopes of carbon and nitrogen
in the feathers. These reflect the diet of the living Birds, so any dramatic
shifts in these diets would result in a change in stable isotope ratios.
Previous studies of long term mercury accumulation in the feathers of other
Bird species have been hampered by shifts in feeding patterns, presumed to
relate to human-induced environmental stresses, which make it hard to access
whether the increase in mercury is due to higher levels in the environment or
the shift in the feeding behaviour of the Birds. In the case of the Ivory
Gulls, stable isotope ratios were found to remain constant throughout the
period studied, suggesting that the diet of the Birds had not changed
significantly over time.
Next a small subset of the
feathers was taken from samples collected before 1900 and after 1975 and tested
for total mercury. This is because many museum specimens were treated with
mercury chloride preservative prior to the 1940s, which would prevent the use
of analytical methods using total mercury in the study (which would be useful
as a comparison of total mercury to methyl mercury would give an indication of
the rate at which methyl mercury breaks down in museum specimens). This found
that total mercury was much higher in the nineteenth century specimens,
suggesting that they had indeed been treated with mercury-based preservatives.
Finally Bond et al. tested all the feathers for levels of methyl mercury. For
this part of the study the Birds were divided into first-winter Birds and
adults (pre-fledged Birds were not included in the study). Both first-winter
Birds and adults were shown to have increasing levels of mercury across the
time-period of the study, but this was higher in the adult Birds, suggesting that
the mercury also accumulated in the bodies of the Birds during their lifetimes.
The level of methyl mercury in the feathers of adult Birds increased from 0.09
μg per gram in 1880 to 4.11 μg per gram in 2004, i.e. 45 times as high in the
latest samples as in the earliest, an increase of roughly 1.6% per year.
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