Janelle Cook
Methylmercury Accumulation in Tissues and its Effects on Growth and Appetite in
Captive Great Egrets
Marilyn, Spalding; Frederick, McGill, Bouton and Mcdowell.
Introduction
Methylmercury contaminates wet land food chains because of this it’s been
suspected to cause reduced survival and reproduction of longlegged wading birds. In
1999 they found effects of mercury on the appetite and a large proportion of mercury
would be shunted into growing feathers. For that reason they hypothesized that the young
chicks with rapidly growing feathers would be protected from the effects of dietary
methylmercury and that chicks that continued to receive methylmercury after feather
growth ceased would be more likely to exhibit the signs of methylmercury toxicosis.
Method & Materials
They collected 23 first-hatched great egret nestlings so that they can find the
accumulation of mercury in various tissues and the effects of mercury on growth and
appetite. They put the birds in three different groups: a control group, a low dose group
and a high dose group. There were five in the control group and also five in the low
group. However there were six in the high dose group. All the birds were kept indoors
until that were five weeks old then they were kept outdoor housing. The birds stayed in
individual 65x40 cm plastic boxes with an artificial stick nests. All the birds received the
same diet of towed Atlantic silversides with capelin. Food was given four to two times
daily and then at week 10 to 14 the birds were allowed to forage on live fish.
Methylmercury dosing was based upon total daily food given, including the live fish.
Gelatin dosing capsules were made three times a week from solutions that contained 0, 3,
or 30 ug solution/g food offered. In addition to the controls they used a low dose of 15
mg/kg in fish and a high dose of 5 mg/kg. Capsules were given everyday just prior to the
evening meal by manipulating the capsules within the esophagus. The birds were dosed
every 3 days starting about 8 days of age, dosing then changed to a daily regime
beginning of day 20 and continued to the end of experiment, week 14. The birds were
examined weekly to collect blood and feather samples as well as other tests. The
department of environmental protection chemistry laboratory will determine the total and
methylmercury concentrations in tissues of the great egrets. The total mercury
concentrations were detected by cold vapor atomic absorption spectrophotometry.
Results
The proportion of the total mercury that was methylmercury varied depending on
the tissue and on the dose group. In different tissues there are the total amounts of
methylmercury. In the liver, mean was 56 % the control group was 61% the low group
was 73% and the high dose group was 69 to 80%. The kidneys were similar with a mean
of 58%. The control group was 43 to 69%, the low dose group was 49 to 70% and then
the high dose group was 74 to 145%. The methylmercury in the feathers was a mean of
120%. Mercury concentrations in growing scapular feathers and powerdown were
similar. Mercury concentrations in the blood and growing feathers increased significantly
with time over the course of the experiment for all three groups. The mean total
methylmercury chloride administered was 0.35, 8.0, and 45 mg/kg for the control group,
low dose group and the high dose group.
Discussion
Methylmercury is generally well absorbed by the intestinal tract, even though it
wasn’t able to accurately measure, there data and graphs still agreed, that there is a period
of higher risk of chronic mercury toxicity for young birds when feathers stop growing.
Only a proportion of mercury that was in the liver and kidney was methylated, and that
proportion increased with dosing level. This supports the hypothesis that the birds are
capable of demethylating mercury. There is an inverse relationship between the total
mercury concentration and the kidney. Mercury concentrations were higher in the kidney
than the liver this was because of a positive correlation between mercury and selenium in
the liver. Selenium plays an important role in the reduction of the toxic effects of
methylmercury. Mature feathers collected near the end of the experiment had a lower
mercury concentration than growing feathers collected at the same time. This might be
because the mature feathers ceased to grow at 9 week, and growing feathers were
exposed to the higher blood and other tissue concentration near the end of the
experiment. In the week of 9-11 feathers stopped growing and began to show obvious
deficits in appetite and growth. Declines in appetite were detected when feather
concentration were 49 mg/kg. this supports the hypothesis that wild nestling might be
more sensitive to methylmercury than captive nestling because of uncontrolled factors
that might interact to affect appetite. Methylmercury induced reduction in appetite.
Methylmercury affected appetite and growth of great egrets, even at the 0.5 mg/kg dose
rate. In wild birds with lower nutritional reserves, a reduction of appetite could more
quickly result in loss of weight and body condition given the rigors of completion
between siblings for limited food resources, learning to forage, and other stressors.