Over the past year there have been a number of articles published about reproductive risks to orcas (killer whales) posed by PCBs (example articles 1, 2, and 3). Some of these articles go so far as to claim that orcas are threatened with extinction due to their reproduction being inhibited by PCBs.
But do PCBs really pose really pose an extinction risk to orcas? Or have these authors used limited information to draw conclusions that are out of proportion with the actual science? With this post I want to consider what we know about orcas and PCBs and evaluate whether the warnings in these recent articles are realistic.
Orca Characteristics and Behavior
Orcas are among the top predators in the ocean. They are not true whales, instead they are the largest member of the dolphin family. Although all orcas belong to the same species, scientists that study them group them into three subpopulations (resident, transient and offshore) based on their behavior. As you would guess, resident orcas live in near-shore locations and remain there for their entire lives, they eat either fish or other marine mammals.
The transient orcas remain close to shore, but migrate along the coast in the pursuit of prey, which consists of mostly of other marine mammals. Offshore orcas are the least well known, but they live and feed far from shore and are believed to feed on sharks and other large fish or marine mammals. Although all three groups belong to the same species, they do not interbreed or otherwise mix socially despite being considered highly social within their own group. As a species, orcas are generally not considered endangered.
Almost everything we know about orcas comes from studies of resident populations because they are relatively easy to monitor. Possibly the most carefully studied orca group (known as the Southern Resident Killer Whales), lives in the waters of the Pacific Northwest, off the coast of Washington state and southern British Columbia. Two other closely monitored resident orca groups are: a pod residing off the west coast of Scotland (known as the West Coast Community); and the Northern Resident group, which lives off the more northerly coast of British Columbia.
Orca Population Decline
While the (North American) Southern Resident and (Scottish) West Coast Community groups are different in many ways, both are now experiencing population decline. In the case of the Southern Resident group, which now has 75 individuals, scientists concluded the decline is due to the reduction in their nearly exclusive food source, Chinook salmon. Scientists have found a close link between orcas’ annual reproductive success and the abundance of Chinook salmon in their home waters. A recent decline in the salmon population is believed to have resulted in the orcas expending more effort to achieve an adequate diet and having less reproductive success. These scientists are hopeful that the key to increasing the Southern Resident group’s population is changing the salmon fishery management to ensure the orcas receive adequate nutrition.
In contrast, the West Coast Community orca group off of Scotland may indeed be on the path to extinction. This group consists of four-females and four-males, a total of 8 individuals, with no new births in the group in 25 years. In fact, the group was recently reduced when an older female died after becoming entangled in fishing lines. Scientists speculate that the remaining females may now be too old to give birth. There is no agreed upon cause for the reproductive failure of the West Coast Community orcas, however, speculation has whirled around the idea that PCBs may have caused a lack of fertility among the females as a result of its ability to mimic estrogen.
PCBs and Orcas
One report that sheds considerable light on the question of PCBs and orca reproduction is a 2000 Institute of Ocean Studies article, which describes a study in which scientists collected fat samples from 47 live, wild orcas off the coast of British Columbia and tested these samples for PCBs, furans and dioxins. The samples were collected from transient orcas as well as individuals form the Southern and Northern Resident Groups (note that the Northern Resident Group is distinct from the Southern Resident orcas, but there is some overlap of their territories of the coast of British Columbia). This study is among the most comprehensive investigations of PCBs in orcas.
The study had three important findings relative to orca reproduction:
- The study found all of the orcas sampled had surprisingly high concentrations of PCBs in their fatty tissue. The transient and Northern Resident orcas had higher PCB concentrations than the Southern Residents. This is believed to be attributable to their different diets. Transient and Northern Residents consume primarily marine mammals and Southern Residents consume primarily salmon. Salmon has significantly lower PCB concentrations than do the marine mammals preyed on by the orcas.
- In each of the three populations the adult males had significantly higher PCB concentrations (and presumably higher PCB body burdens) than did the adult females. Since their diet and thus their potential PCB exposures are the same, the only explanation for this is that the adult females transfer their PCB body burden to their young during gestation and after birth through their milk when nursing. Because the fat content of orca milk is high, and because this fat carries the PCBs accumulated by the mother, these PCBs are transferred to the nursing young. It is estimated that 60% of the mother orca’s PCB body burden is transferred to their young by nursing. The transfer of PCBs from mothers to their young is seen in other mammals as well.
- The concentrations of dioxins, furans and dioxin-like PCBs in the orca fat samples were less than anticipated based on the relatively high concentration of non-dioxin like PCBs. The authors suggest that this finding may explain how the orcas could tolerate such high PCB body burdens, without experiencing adverse effects such as reproductive failure. The authors speculate that the more toxic dioxins, furans and PCBs may have been metabolized by other animals lower on the food chain such that only the less toxic and more chemically stable PCBs are actually passed on to the orcas in their food.
The objective of this post was to consider whether the available scientific evidence supports the claim that PCBs may drive orcas to extinction as a result of reproductive failure. It is undeniably true that orcas carry high PCB body burdens, and that these PCBs are the result of their diet, whether that diet consists of fish or marine mammals. However, based on the 2000 Institute of Ocean Studies report, it does not appear that high PCB body burdens are contributing to reproductive failure in the populations studied.
As odd as it may seem, some of the strongest evidence for orca reproductive success from this study is the much lower PCB concentrations found in the adult females compared to those detected in the adult males. Since the PCB exposure of the females and males are similar (they all derive PCBs from their diet), the consistent large differences in concentration in their fat tissue can only be explained by the transfer of the adult female’s PCBs to their young. Sexually immature females and males have similar PCB concentrations. But, upon reaching reproductive age, the female’s PCB concentrations drop, while the male’s PCB concentration continues to increase with age.
The study data indicates that the orcas continue to reproduce successfully among the transient and the resident orca populations on the North American west coast despite their high PCB body burdens. In other words, the PCBs do not appear to be adversely impacting the orca’s reproduction.
It may not be appropriate to extrapolate the results of the North American orca/PCB study to the decline of Scotland’s West Coast Community group. However, the study clearly does not provide support for the contention that PCBs are causing a worldwide decline in orca population.