In 1971 and 1972 only those pelicans bred successfully, where, referring to wet
weight, the eggs contained less than 2.5 mg/kg of DDT and less than 0.54 mg/kg of
dieldrin. During this period, however, the average concentration of harmful substances
in pelican eggs decreased from 7.3 in 1969 to 2.5 mg/kg of DDT in 1973 and from
1.16 in 1969 to 0.45 mg/kg of dieldrin in 1973, on the average. In 1973 the rate of
successful breeding of the pelicans was for the first time excellent, 2726 young pelicans fledged, that is 1.66 chicken per nest, after the number of fledglings had been
between 0.7 and 0.9 per nest from 1969 to 1972. In 1973, however, the food supply
and weather conditions had also been favorable (BIus et al. 1974, 1977). PCB concentrations in pelican's eggs remained more or less the same over the entire period.
Therefore PCB's seem not to be responsible for the catastrophic failure in breeding.
On the other hand it is difficult to conclude with significance whether DDT, dieldrin,
the weather, or the food supply really was the key factor.
In the osprey (Pandion haliai/tus) from the eastern coast of the U.S.A. (Connecticut
and Long Island) breeding was very poor during the 1960's. DDT concentrations in
the eggs decreased, however, between 1969 and 1976 to one fifth, and since they
were below 12 mg/kg (wet weight, or 60 mg/kg dry weight) breeding success improved.
In 1976 there were 1.2 fledglings per nest, equal to conditions in the 1950's (Spitzer
et al. 1978). However, in this case too, there was a parallel decrease of dieldrin, so
that it cannot be said with Significance that breeding success is only correlated with
DDT. There is a chance that DDT and dieldrin act in a similar way.
As in pelicans, the effect of DDT (or of DDT and dieldrin) on ospreys in the 1960's
was egg shells which were 15%-20% thinner than normal. In the double-crested
cormorant (Phalacrocorax auritus) from the Canadian lakes, an egg-shell thinning of
20% was critical (Vermeer and Peakall 1977). During the 1960's, cormorant populations (Phalacrocorax carbo) in the Netherlands declined, possibly as a consequence of
egg-shell thinning. Eggs collected in 1971, which had shells 0.45 mm thick, had concentrations of 2 mg/kg (wet weight) DDT and 0.3-0.5 mg/kg dieldrin, but in eggs
with thinner (0.35 mm) egg shells concentrations were higher: 12 mg/kg DDT and
5 mg/kg dieldrin (Koemann et al. 1973b).
In the meantime, there is experimental proof that birds fed with DDT lay eggs with
thinner egg shells (Fig. 89). Chickens may hatch from such thin-shelled eggs, when
the eggs are carefully treated in an incubator. But under natural conditions, in the
nest, even eggs with normal egg shells sometimes break. The chance that eggs with
20% thinner egg shells do not break is very poor. The conclusion is that relatively
small amounts of DDT which apparently have no other toxic effects upon the bird
disturb the enzymatic activity necessary for the process of egg shell formation.
It has not been completely proved that high concentrations of PCB's in blubber and
other body tissues of seal lead to a decrease of reproductive success. After the emission of DDT into the California coastal area had been stopped, stillbirths in the sealion colonies continued. This could be due to PCB effects, however, DDT is still
present in the sediment (see p. 49), and concerned females have high, unbalanced
mercury and selenium concentrations, too (see p. 135). There is no proof that PCB's
have a harmful effect on the harbor seal (Phoca vitulina) of the German North Sea
coast. In comparing the concentrations of harmful substances in dead or sick animals
and such that had been shot dead, no significant differences could be found (Drescher
180
weight, the eggs contained less than 2.5 mg/kg of DDT and less than 0.54 mg/kg of
dieldrin. During this period, however, the average concentration of harmful substances
in pelican eggs decreased from 7.3 in 1969 to 2.5 mg/kg of DDT in 1973 and from
1.16 in 1969 to 0.45 mg/kg of dieldrin in 1973, on the average. In 1973 the rate of
successful breeding of the pelicans was for the first time excellent, 2726 young pelicans fledged, that is 1.66 chicken per nest, after the number of fledglings had been
between 0.7 and 0.9 per nest from 1969 to 1972. In 1973, however, the food supply
and weather conditions had also been favorable (BIus et al. 1974, 1977). PCB concentrations in pelican's eggs remained more or less the same over the entire period.
Therefore PCB's seem not to be responsible for the catastrophic failure in breeding.
On the other hand it is difficult to conclude with significance whether DDT, dieldrin,
the weather, or the food supply really was the key factor.
In the osprey (Pandion haliai/tus) from the eastern coast of the U.S.A. (Connecticut
and Long Island) breeding was very poor during the 1960's. DDT concentrations in
the eggs decreased, however, between 1969 and 1976 to one fifth, and since they
were below 12 mg/kg (wet weight, or 60 mg/kg dry weight) breeding success improved.
In 1976 there were 1.2 fledglings per nest, equal to conditions in the 1950's (Spitzer
et al. 1978). However, in this case too, there was a parallel decrease of dieldrin, so
that it cannot be said with Significance that breeding success is only correlated with
DDT. There is a chance that DDT and dieldrin act in a similar way.
As in pelicans, the effect of DDT (or of DDT and dieldrin) on ospreys in the 1960's
was egg shells which were 15%-20% thinner than normal. In the double-crested
cormorant (Phalacrocorax auritus) from the Canadian lakes, an egg-shell thinning of
20% was critical (Vermeer and Peakall 1977). During the 1960's, cormorant populations (Phalacrocorax carbo) in the Netherlands declined, possibly as a consequence of
egg-shell thinning. Eggs collected in 1971, which had shells 0.45 mm thick, had concentrations of 2 mg/kg (wet weight) DDT and 0.3-0.5 mg/kg dieldrin, but in eggs
with thinner (0.35 mm) egg shells concentrations were higher: 12 mg/kg DDT and
5 mg/kg dieldrin (Koemann et al. 1973b).
In the meantime, there is experimental proof that birds fed with DDT lay eggs with
thinner egg shells (Fig. 89). Chickens may hatch from such thin-shelled eggs, when
the eggs are carefully treated in an incubator. But under natural conditions, in the
nest, even eggs with normal egg shells sometimes break. The chance that eggs with
20% thinner egg shells do not break is very poor. The conclusion is that relatively
small amounts of DDT which apparently have no other toxic effects upon the bird
disturb the enzymatic activity necessary for the process of egg shell formation.
It has not been completely proved that high concentrations of PCB's in blubber and
other body tissues of seal lead to a decrease of reproductive success. After the emission of DDT into the California coastal area had been stopped, stillbirths in the sealion colonies continued. This could be due to PCB effects, however, DDT is still
present in the sediment (see p. 49), and concerned females have high, unbalanced
mercury and selenium concentrations, too (see p. 135). There is no proof that PCB's
have a harmful effect on the harbor seal (Phoca vitulina) of the German North Sea
coast. In comparing the concentrations of harmful substances in dead or sick animals
and such that had been shot dead, no significant differences could be found (Drescher
180