Environmental Engineering Reference
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“The complex series of behavioral events that lead up to mating, nest building, and
egg laying were evidently not adversely affected.” The likely mechanism of action
is inhibition of calcium transport and mineralization in the shell gland. In the brown
pelican, eggshell thickness is reduced about 15% at 75 ppm DDE on a lipid basis
(3.3 ppm fresh weight). At higher residue levels the slope of the residue-thinning
curve decreases to zero thickness at 3,000 ppm DDE (132 ppm fresh weight).
Keith et al. (
1970
) also studied the brown pelicans on the Channel Islands. Brown
pelican eggshells from Anacapa Island were 34% thinner than pre-DDT era con-
trols. DDE residues in the eggs were 29-183 ppm. DDE in brain tissue was high but
not as high as the 30-60 ppm considered lethal.
Blus (
1970
) reported a study of eggshell thinning and breeding success in brown
pelicans in Florida and South Carolina. Populations in both states were declining.
Eggshells were 6-16% thinner than pre-DDT eggshells. Brown pelicans have been
extirpated in Louisiana and other Gulf Coast localities. The reproductive failure and
population declines were attributed to eggshell thinning caused by DDE.
Risebrough et al. (
1971
) reported an account of almost complete reproductive
failure of brown pelicans on the Channel Islands in 1969. Broken and crushed eggs
were strewn about the breeding area. Eggshell thickness was reduced 50%. Only
two young were observed out of 1,272 nests.
A statistical analysis of the variability in eggshell thinning in brown pelicans
implicated DDE as the causative organochlorine (Blus et al.
1971
). Ten eggs from
California contained DDE residues as high as 135 ppm with shell thinning of
25-35%. DDE residues in eggs from nine colonies in Florida ranged from 0.2 to
6.0 ppm. Eggs from two colonies in South Carolina had DDE residues ranging
from 3.3 to 10.6 ppm. Blus et al. (
1972a
) reported that eggshell thinning of 15-20%
had been associated with declining populations of several species of birds. The dose-
response of DDE residue in eggs and eggshell thinning in brown pelicans was
log-linear (Fig.
15
). The estimated no-effect level was 0.5 ppm. The brown pelican
is unusually sensitive to eggshell thinning by DDE. Fifteen percent thinning occurs
at 4-5 ppm DDE in eggs. The herring gull showed no thinning when DDE residues
in eggs were 4-5 ppm. The level of DDE in eggs is taken as an indication of DDE
residues in the female.
The paper by Blus et al. (
1972a
) was accompanied by a letter from William
Hazeltine
challenging the assertion that the DDE eggshell thinning dose-response
was log-linear. Moreover, Hazeltine questioned whether DDE caused eggshell
thinning. He suggested scientists were acting irresponsibly to ban pesticides.
Risebrough (
1972
) also wrote a letter to the same journal. His letter defended Blus
et al. and refuted Hazeltine's comments. He stated that in some cases the log-normal
distribution provides an excellent fi t to the brown pelican data, and “In several other
cases the gamma distribution more adequately describes the observed distribution of
pollutants.”
Switzer et al. (
1972
) also wrote a letter challenging Blus et al.'s conclusion that
eggshell thinning in the brown pelican was caused by DDE. They pointed out that
museum eggs, used to establish pre-DDT era shell thickness, were often selected as
the best (and perhaps thickest) specimens for display in public exhibits.
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