Researchers have suggested PCBs as potential causative or contributing agents in a number of population declines. However, as discussed above, positively attributing effects on either individuals or population to PCBs is very difficult because of the impurities contained in the commercial mixtures which are the source of environmental PCBs, the co-occurrence of a number of other potential toxic agents (some of which came under controls and reduction measures at a similar time to PCBs), and the generally low incidence (even when enhanced, and potentially problematic) of the effects. Although a very substantial amount of work has been performed on the toxicity of PCBs in controlled (predominantly) laboratory studies, on a range of species, it is especially difficult to extrapolate effects seen in these studies to effects in wild populations, even when the population is studied in great depth and over an extended period.
In areas affected by acute exposure (i.e., in highly contaminated environments, such as waste outlets at PCB production or use facilities) the microbial population can be selected for PCB tolerance, and the ability to use PCBs as a source of energy (e.g., some anaerobic microbes may derive energy by dechlorinating PCBs). However, at more generally encountered environmental concentrations no effects on microbe population have been proven.
As noted above, it is difficult to identify and quantify the effects of PCBs in wild populations, and the few cases where PCBs have been accepted as potential causes of population effects have been in birds and mammals, and assessing the 'weight of evidence' from a range of sources has been very important in identifying PCBs as likely causative factors. Although few specific population declines have been attributed specifically to PCBs, it appears to be generally accepted that PCBs may cause long-term, relatively subtle effects within populations. These may affect many types of biota within an ecosystem, but any measurable evidence for adverse effects is most likely to be found in top predators, particularly birds and mammals, due to the effects of bioaccumulation. However, since PCBs are now under strict control in most countries, and effective disposal measures are in place, only populations that are exposed to relatively high concentrations of PCBs at particularly contaminated sites, or due to the effects of environmental redistribution and biomagnification, are likely to be affected in the future.
Some of the effects on bird and mammal populations for which PCBs may be contributory agents are
1. In the 1980s a range of predominantly reproductive effects (including reproductive impairment, high embryo and chick mortality, deformities including club feet and crossed bills and decreased body weight) were seen in herring gulls, Forster's terns, cormorants, and Caspian terns from the North American Great Lakes, causing population decline. The effects were linked to exposure to coplanar PCBs and dioxins. Figure 3 shows the
dramatic decline in PCB concentrations in birds around Lake Erie (North America) since the 1970s.
2. In the 1970s the populations of wild mink and otters in the North American Great Lakes region, and of otters in Europe, declined dramatically. Farmed mink fed on Great Lakes fish (containing high concentrations of orga-nochlorine chemicals) suffered reproductive failure. Laboratory studies of the effects of PCBs on mink caused embryo death and reduced survivability and growth. The population declines in wild mink and otters were accepted as being related to the effects of PCBs and dioxins.
3. Baltic ringed and gray seals suffered population declines in the 1960s due to a high incidence of abortion and sterility. There were also bone lesions and claw deformities. It was found that the seals had high concentrations of p,p'-DDE and PCBs. After controlled studies in seals and mice the effects were linked to the methylsulphonyl metabolite products of p,p'-DDE and PCBs.
4. In 1988 and 2002 common (harbor) seals in the North Sea suffered dramatic epizootics of phocine distemper virus (in 1988 around 50% of the Western Atlantic population died). The seals were found to contain high concentrations of PCBs and other organo-chlorine pollutants, and there is a suspicion that these chemicals may have suppressed the seal immune systems, making them more susceptible to the phocine distemper virus. No definite link between PCB exposure and the epizootics has been proven.
5. Harbor porpoises from the UK caught between 1989 and 2002 found to have died of infectious disease were compared to healthy porpoises that had died of physical trauma (e.g., by being caught in commercial fishing nets). A threshold for health effects in marine mammals of 17mgkg_1 lipid was proposed, and some porpoises in both groups had PCB concentrations above that. In the porpoises with PCB concentrations higher than the proposed threshold, higher PCB concentrations were found in porpoises that had died from infection than trauma, whereas for the porpoises below the proposed threshold PCB concentration there was no statistically significant difference in PCB concentrations between the two groups. It has been suggested that this is consistent with an immunotoxic effect of PCBs reducing survivability of porpoises.
6. In the 1990s, a lack of older females and high juvenile mortality were noticed in the polar bear population of Svalbard (Spitzbergen). It has not been possible to determine the cause of these effects, but there is a suspicion that they may be linked to the high concentrations of PCBs and other pollutants found in polar bears in that area. The high pollutant concentrations may be causing reproductive impairment, lowered survival rates of cubs, increased mortality of females, and immunotoxic effects.
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