Chemistry Reference
In-Depth Information
With the fall of
p,p
′-DDE levels, and associated eggshell thinning, populations of
these affected species have recovered in many areas. However, reproductive failure,
and physical deformities (such as crossed bills in double-crested cormorants) lasted
into the mid-1990s in some areas where PCB levels remained high. With the Caspian
tern, there was a strong correlation between TEQs (dioxin equivalents) in eggs, and
embryonic mortality. With the double-crested cormorant, there was a negative cor-
relation between TEQ values and reproductive success, and a positive correlation
between TEQ and the incidence of crossed bills (Gilbertson et al. 1998). As with the
Caspian tern, this species showed a negative correlation between TEQ values and
embryonic mortality; however, the slopes of the regression lines were very different
in the two species (Ludwig et al. 1996). There is, therefore, evidence linking the
depressed state of certain populations of piscivorous birds in the Great Lakes with
Ah-receptor-mediated toxicity caused by planar PCBs and other planar polyhaloge-
nated aromatic compounds.
It has also been suggested that seal populations in and near the North Sea (Wadden
and Baltic seas) have been adversely affected by PCBs (Brouwer 1991; Ross et al.
1995; Boon et al. 1992). Thyroid hormone antagonism, skeletal deformities, impaired
reproduction, and immunosuppression have been reported either in free-living ani-
mals or in animals dosed with fish caught in the North Sea that contained high levels
of PCBs. It was also suggested that the spread of a distemper virus that caused high
mortality in seal populations was promoted by immunosuppression due to relatively
high environmental levels of PCBs. As noted in Section 6.2.4, there is experimental
evidence to lend some support to this theory (de Voogt et al. 1996; Levin et al. 2004).
Population declines of Californian sea lions have also been linked to high tissue
levels of PCBs (Environmental Health Criteria 140).
Despite wide-ranging restrictions and limitations on their release, levels of PCBs
have been slow to come down in certain locations. It appears that redistribution of
PCBs from sinks is still going on. It has frequently been suggested that they have
had—and in some cases are still having—adverse effects on predators at the top of
food chains, for example, fish-eating birds in some parts of the Great Lakes, and in
marine mammals. The complexity of PCB pollution, with the possibility of interactive
effects between different PCB congeners and/or between PCBs and other persistent
pollutants, has made this a difficult point to prove or disprove. There is a need for the
development and application of biomarker assays that can provide evidence of cau-
sality, to link levels of pollutants, taken singly or in combination, with consequent
harmful effects, and then to relate the harmful effects to the state of populations using
population dynamic models (see Chapters 12 and 15 in Walker et al. 2000).
Apart from direct evidence of population declines that may be due, largely or
entirely, to PCBs, some studies indicate effects on reproductive success, which
may be translated into population declines. In a study of osprey populations in the
Delaware river and bay in 2002, Toschik et al. (2005), using a logistic regression
model, report that the concentration of PCBs and other persistent organochlorine
pollutants in eggs were predictive of hatching success. Arenal et al. (2004) compared
the breeding success of starlings (
Sturnus vulgaris
) at a Superfund site (high PCB
contamination) with that in a reference site. At the Superfund site, EROD levels were
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