Original public domain source by NOAA.First discovered in the 1800s, polychlorinated biphenyls (PCBs) entered mass production in the 1930s for their properties as lubricants, insulators and coolants, among other industrial uses. If you are old enough you may have had direct contact with this chemical as it was used on carbonless copy paper. Alas, the good days for PCBs came to an end after scientists linked them to cancer and health issues of the immune, reproductive and endocrine systems. Because of this, the USA and Japan banned their production in the 1970s and, eventually, the Stockholm Convention on Persistent Organic Pollutants banned them worldwide in 2004.
PCB production is estimated to have surpassed a million metric tons by 1993 and the pollutant is still being pumped into the environment today as runoff from landfills and other dumping sites. To make matters worse, PCBs do not break down easily and can enter food chains after being incorporated by microbes, accumulating in the body fat of larger animals and also being passed to the offspring during gestation/lactation in the case of mammals. In fact, the higher up in the food chain you are, as is the case of cetaceans or sharks, the more and more you will accumulate in your lifetime, a process known as biomagnification. Last year, PCBs were reported to have bioaccumulated even in the fauna found deep in the Mariana Trench, which was, believe it or not, thought of as "untouched by man" not too long ago.
With the knowledge that orca whales (commonly known as 'killer whales') had been previously found to have high levels of PCBs in their tissues (see table below) and that their populations worldwide have been shrinking, a group of researchers from multiple European and North American institutions developed a theoretical model to predict the long-term survival of the species. The results, published last month in the journal Science, were anything but encouraging.
Table showing PCB concentrations in orca whales as assessed previously in multiple studies.Using all available data on things like PCB concentration in the animals, fecundity rates, immune system effects, mother to fetus and calf PCB transfer rates and proximity to worldwide sites of past production and dumping, their model estimates that orca populations living close to industrialized countries and/or dieting on higher-level food chain mammals have a moderate to high risk of collapse within the next 100 years. We are talking about 10 out of 19 populations analysed (see table below). The remaining 9 populations, which have a low risk assessment, are estimated to probably grow in size in the coming decades.
Table displaying the risk assessment for each population where data was available.
The authors point out that even though orcas once thrived in all of Earth's oceans, now it looks like only those in Arctic and Antarctic waters will be able to sustain growth. The Northeast Pacific orcas provide an interesting case because the Northeast Pacific Bigg's population has a much higher predicted risk (and an actual PCB burden 20 to 30 times higher) compared to other Northeast Pacific populations given that it preys on marine mammals (as opposed to only feeding on lower food chain level species such as fish), which already have high levels of PCBs in them.
In their paper, the authors also point out that 80% of worldwide PCB stocks remain yet to be destroyed and that several countries are unlikely to meet their 2025 and 2028 targets as established by the Stockholm Convention on Persistent Organic Pollutants.
The study brings up the possibility that PCBs may be affecting the future reproductive success of multiple other species and that other chemical pollutants may also be currently contributing to the demise of marine animals. This story also highlights the importance of disposing of our trash properly and of taking decisive action when it's found to be noxious. A ban that comes 5 years too late can be the difference between driving countless species to extinction or not, especially with climate change bringing added or potentiated stressors with each tick of the clock.
Original paper:
"Predicting global killer whale population collapse from PCB pollution" at Science
Further reading and sources:
"Long-banned toxin may wipe out many killer whales" at Science News