Because they are at the very top of the food chain, it is not surprising that Orca are contaminated with bioaccumulative persistent organic pollutants like PCBs. Ross et al. report, however, that the actual contaminations are surprisingly high in Orca living in coastal British Columbia, particularly among one subset of Orca, a transiet population that specializes in eating marine mammals like seals and other whales. While Ross et al. performed no direct assessment of toxicity, their results suggest "that current concentrations of PCBs represent a significant toxicological risk to the populations in British Columbia."

Ross et al. used a light-weight pneumatic dart system to obtain small samples of skin and blubber from free-ranging Orca. They then subjected the samples to contaminant analysis.

The samples were obtained from individually-recognizable Orcas that have been the subjects of intense studies of natural history and behavior, including photo-identification, since 1973. Ross et al. were therefore able to analyze contamination patterns in relation to sex, age and subpopulation. Previous work had revealed that Orca in this region can be separated into two groups: resident Orca that feed primarily upon salmon, and "transient" Orca that feed exclusively on other marine mammals.

Samples were obtained from 47 individuals: 15 samples from transients and 32 from residents (subdivided into individuals from a northern and a southern pod).

Average concentrations in all groups were high, especially transient males:

The sexual differences observed are consistent with reports from other cetaceans. Females unload accumulated PCBs by transferring them to offspring, first in the womb and then via lactation. Other studies have shown that cetacean females off-load the majority of their organochlorine burden to their calf during reproduction, mostly via lactation. Males have no comparable mechanism. An analysis of contamination by age showed that contamination declines in females at the age of first calving and remain low in females until the reach approximately 50 yrs of age. Afterwards contamination once again begins to accumulate.

The greater concentrations observed in 'transients' is a result of dietary differences. Eating other marine mammals, 'transients' are one level higher in the food chain. They eat predators of salmon whereas the residents eat salmon directly.

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