Arctic Science Explained: Toxins in Marine Mammals

September 12, 2022
By Liz Weinberg

By Katherine Schexneider

Welcome to Arctic Science Explained! Each month, IARPC Collaborations community member Katherine Schexneider breaks down a topic related to Arctic science. If you have a topic you’d like to see featured, please email web@iarpccollaborations.org. This month, Katherine focuses on toxicant buildup in the Arctic, particularly in orcas.

Remember DDT? That’s right; it’s the pesticide that was banned in the U.S. in 1972. I think I recall it, but no adults talked about it much, and once something was banned, that was the end of it, right? Well, it’s still around, accumulating in fish, marine mammals, and other wildlife.

I spent a week and a half in July 2020 volunteering with a research group studying the killer whale (Orcinus orca) subpopulation in Iceland. One researcher measured and analyzed contaminant concentrations and validated a technique that had been used in other marine mammals before, but not in orcas. Her lecture to the entire research team gave me a solid overview of persistent organic pollutants (POPs) and made me understand that the long-forgotten DDT, and similar compounds, can’t be forgotten at all.

What are these contaminants, how did they get to the Arctic, and why are they still around? Besides DDT (spoiler alert: I am going to spare you the unpronounceable chemical names), PCBs, a class of chemicals whose production was banned in the U.S. in 1979, has dozens of unique formulations. Less familiar to the general public, other organochlorines were used in mosquito control and PBDEs were used as flame retardants. These were only banned in the U.S. in 2001 and 2004, respectively.

POPs are very stable chemical compounds and don’t break down readily. Manufacturers in the mid-twentieth century took advantage of this, and one can understand why. Like Ford automobiles, they were “built to last.” They can also change from solid to gas phase at warm enough temperatures. As gases, they travel long distances along atmospheric currents to Arctic destinations where, in the cold, they turn back to solids. As a result, the Arctic has a disproportionate concentration of them.

What exactly do they do that’s harmful? POPs can affect the immune and reproductive systems of marine mammals. (N.B. This blog post does not cover mercury, lead, etc.) One increases the amount of a pro-inflammatory molecule, which stimulates the immune system at the wrong time. An autopsy study showed that whales dying from infectious disease had had relatively higher concentrations of PCBs than did those dying from trauma. This suggests that PCBs could have made the whales more susceptible to infection. PCBs have also been correlated with decreases in orca population numbers in multiple studies. However, food availability and noise pollution, which interferes with the sounds whales make while hunting for prey, also factor into population dynamics, so it’s hard to tease out the contribution of contaminants.

How do we detect these compounds in marine mammals? Researchers gather a small tissue sample—skin and underlying blubber—usually from the side. The tactic of firing a biopsy dart, which is about the size of a cigar, from a crossbow evokes images of Moby-Dick, but it is really very humane. The biopsy is virtually painless and causes only a brief startle response in the whale. The blubber sample is analyzed using the same laboratory techniques as in a human hospital: gas chromatography and mass spectroscopy. These provide precise measurements of the multiple forms and concentrations of DDT, PCBs, and the other POPs.

The impacts of POPs vary by the type of marine mammal, and its geographical location. So, what do we know about the Icelandic subpopulation of orcas? First, males carry four-fold higher concentrations of PCBs, the most prevalent POP in this study, than females. It’s not because males are larger; remember, concentration means milligrams of PCB per kilogram of tissue, so it’s proportional to the animal’s size. The females offload PCBs in breast milk, upwards of 60% of it, then reaccumulate it over time. Second, diet significantly affects PCB load. Orcas that eat a mixed-diet of fish (mostly herring) and marine mammals (usually seals) have several times the PCB concentrations of those eating only fish. This makes sense, as seals are higher up on the food chain and have accumulated plenty of PCBs themselves. Finally, and most concerning, the Vestmannaeyjar research team found dangerously high concentrations of PCBs in all males, regardless of diet, and in mixed-diet females. Just how these high concentrations are affecting the Icelandic subpopulation hasn’t been determined. Suffice it to say the news is probably not going to be good.

The risks to people and to other animal species stem from both biomagnification—how POPs build up in species higher on the food chain—and dietary requirements. A smaller fish can obviously ingest only a small amount of the chemicals, but then larger fish, and eventually marine mammals, animals on land, and people consume everything that’s accumulated in these smaller species. The main way to offload POPs is through breast milk, something fish species can’t do, so animals consuming a high fish diet, like whales, bears, and Arctic peoples are at greater risk. So, is the simple answer for people to limit fish in their diets? People are already doing this out of concern for mercury accumulation. Well, no, not if your diet depends on fish, whales, caribou, and other animals that have a load of POPs to begin with. This is the story for Indigenous Peoples living in the Arctic, and studies have shown higher concentrations of these toxic chemicals in Arctic communities. For them, it’s not a matter of dietary choices, but indeed of requirements.

Katherine Schexneider is a retired US Navy physician who now does volunteer work in Arctic research and climate change.

Note: Views expressed in this article are the author’s and do not necessarily reflect the views of the IARPC community.