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 65

The recovery was attributed to the fact that the DDT manufacturing plant in Los
Angeles stopped discharging wastes to the Los Angeles outfall in April, 1970.
Morrison et al. ( 1978 ) reported on DDE residues and shell thickness in cormorant
eggs collected in Texas in 1976-1977. The results were compared with an earlier
study by King, in which cormorant eggs were collected in 1970. The results of the
King study were provided by personal communication to the authors from K. A. King.
DDE residues had declined dramatically in cormorant eggs from 1970 to 1976-1977
(Table 3 ).
Eggshell thickness was not signifi cantly affected in either the 1970 or 1976-1977
studies, although the latter shells were thicker (Table 4 ).
The authors concluded that there was little difference in thickness between the pre-
DDT era shells, the 1970 shells and the 1976-1977 shells. “Most authors agree that
a 10-20% change in shell thickness is needed before reproductive failures are indi-
cated.” and “Cormorant eggshell thickness was apparently not affected by residues
in the 1970s in Texas.”
Pearce et al. ( 1979 ) reported DDE residues in cormorant eggs collected along
eastern Canadian coastal waters from 1970 to 1976. Average residues by site ranged
from 1.49 to 8.57 ppm. Individual eggs contained from 0.16 to 20 ppm DDE.
The authors reported measuring shell thickness, but no data were present in the
publication. The authors claimed that 10 ppm DDE in eggs produces 20% shell
thinning. This conclusion was based on an extrapolation of the residue—shell thinning
data. Again, no data or regression plots were present in the article.

Table 3 Marked reduction in DDE in cormorant eggs from Texas from 1970 to 1976-1977. Data
from Table 1 in Morrison et al. ( 1978 )
Residues in olivaceous cormorant eggs in Texas a
1970 (n = 5) 1976-1977 (n = 7)

Residue Mean S. E. (%) Mean S. E. (%) % Change

p , p ′ -DDE 6.22 2.08 100 0.400 0.036 100 b −93.6

a Values represent residues on a wet-wt basis
b p < 0.05

Table 4 Shell thickness in cormorant eggs collected in Texas. Table 2 in Morrison et al. ( 1978 )
reproduced with permission