An assessment of PCBs and OC pesticides in eggs of double-crested (Phalacrocorax auritus) and pelagic (P-pelagicus) cormorants from the west coast of Canada, 1970 to 2002

Eggs of double-crested and pelagic cormorants were collected between 1970 and 2002 from colonies in the Strait of Georgia, BC, Canada, and assayed for concentrations of organochlorine (OC) pesticides and polychlorinated biphenyls (PCBs). Double-crested cormorant eggs from the early 1970's contained up to 4.1 mg kg(-1) pp'-DDE and 12.5 mg kg(-1) Sigma-PCBs. Corresponding values for pelagic cormorant eggs were 1.5 mg kg(-1) pp'-DDE and 3.9 mg kg(-1) Sigma PCBs. Egg tissue concentrations of the dominant OC pesticides and Sigma PCBs dropped mainly during the 1970's, with minor declines thereafter. The data suggest that contaminant levels in cormorants have now stabilized at low levels throughout the resident population. Small but significant latitudinal gradients in several OC pesticides and PCBs indicated that areas of the southern strait were more contaminated than areas of the less populated northern strait. Interspecific differences in contamination may indicate that pelagic cormorants have a reduced capacity to metabolize chlordanes, DDT and PCBs compared to double-crested cormorants. Alternatively, the two species may have more divergent prey bases than previously thought. During the 1980's, TCDD, toxic equivalents, largely contributed by polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), were correlated with physiological and biochemical alterations. Also, from 1989 to 1990, four deformed cormorant chicks (two of each species) were found during nest visits; none were found between 1991 and 1995. The relative rates of deformed chicks were 6 per 10,000 for double-crested and 16 per 10,000 for pelagic cormorants. The findings of deformed chicks were coincident with the period of highest PCDD and PCDF contamination; however, the sample sizes were too low to derive a substantive connection. Colony-wide productivity of double-crested cormorants was poorer in the southern colonies where PCBs in particular were elevated. While of concern, these lines of evidence are insufficient to conclude that chlorinated hydrocarbon contamination was a dominant contributor to population declines. It seems more probable that ecological variables, particularly changing prey and predator dynamics, drove the reductions in population size.