DDT, Chlordane, Toxaphene and PCB Residues in Newport Bay and Watershed: Assessment of Hazard to Wildlife and Human Health - Reviews of Environmental Contamination and Toxicology - page 68

Cormorants are less sensitive to the reproductive effects of DDTs than ospreys
and brown pelicans. Residues of DDE in eggs in excess of 10 ppm, resulting in
eggshell thinning of 15% or more, appear to be necessary to produce signifi cant
hatching failure.
DDT and Black Skimmers . The black skimmer ( Rynchops niger ) feeds in estuaries,
catching small fi sh by skimming the water surface with its lower mandible; maxi-
mum lifespan is 20 years (US GS 2010 ). King et al. ( 1978 ) reported 4% eggshell
thinning in black skimmer eggs collected in 1970. Mean DDT levels in the eggs
were 9.68 ppm. No correlation was found between DDT levels and eggshell
thickness.
Blus and Stafford ( 1980 ) reported that DDE ranged from 0.81 to 12.1 ppm in
eggs from black skimmer nests that apparently failed. Eggs from successful nests
contained 0.43-3.40 ppm. Overall eggshell thinning was 5%. The authors con-
cluded that DDE and other pollutants had little infl uence on overall productivity of
black skimmers in South Carolina.
White et al. ( 1984 ) reported that 35% of black skimmer eggs collected in 1978-
1981 along the Texas coast contained DDE levels of 10 ppm or more (levels of DDE
ranged as high as 51 ppm), but no signifi cant correlation was found between residue
levels and fl edgling success. The authors state that: “Some degree of eggshell thin-
ning was detected in most colonies (9/10), ranging from 4% to 12%, but thinning
was below that (15-20%) believed to cause population declines in other avian species
(Anderson and Hickey 1972 ; Blus et al. 1972a ; Longcore and Stendell 1977 ).”
King and Krynitsky ( 1986 ) assessed DDT residues, shell thinning and reproduc-
tive success in black skimmers along the Texas coast in 1980-1982. Geometric
mean DDE levels in eggs were 1.62-3.25 ppm (range of 0.2-86 ppm). Eggshell
thinning for the years 1980, 1981 and 1982 occurred to the extent of 2-6%. Some
cracked or crushed and broken eggs were observed to be up to 36% thinner than
shells of eggs collected prior to the DDT era. Overall, DDE level did not correlate
with shell thickness.
Custer and Mitchell ( 1987 ) reported on a study of black skimmers along the Texas
coast in 1984. Geometric mean DDE levels were 5.9 ppm (range of 2.3-17.9 ppm)
in eggs from nests where the remaining eggs did not hatch and 1.9 ppm (range of
0-7.4 ppm) in eggs from nests where all of the remaining eggs hatched. DDE level
did not correlate with shell thickness. The authors concluded that: “The breeding
population of Black Skimmers in Texas does not seem to be declining nor does DDT
contamination seem to be a major infl uence on skimmer numbers.” In a follow-up
publication, King et al. ( 1991 ) reported that: “we found no evidence that shell
thinning of either tern (7%) or skimmer (5%) eggs adversely affected reproduction
in 1984.”
DDT and Black - necked Stilts . Setmire et al. ( 1993 ) collected 84 black-necked stilt
( Himantopus mexicanus ) eggs from the Salton Sea area in 1988-1990. The geometric
mean concentration of DDE was 2.57 ppm with a range of 0.05-12.1 ppm. Eggshell
thinning was estimated to be 7% at 12 ppm DDE from a plot of DDE egg residue
against eggshell thickness from an Imperial Valley data set of 33 black-necked stilt