The orca (Orcinus Orca) is easily the most recognisable of all cetaceans (whales, dolphins and porpoises) in the world. Its striking black and white colouration has captured the hearts of many, both young and old, over recent decades.
The orca is widely distributed throughout the world’s oceans. In the Pacific Ocean, the orca is an important natural symbol for the people of British Colombia. It is here that two harmonious populations with differing diets live. These populations consist of the transient orcas that are mostly marine mammal eating and the resident population which feed on fish. Over in the Atlantic Ocean the orca is less numerous and only a small resident population is found in industrialized regions of Europe around the Mediterranean Sea. This resident population is primarily tuna-feeding. Also, a small group of marine mammal eating orca is regularly seen around western Ireland and North West Scotland. These four populations of orca are all heavily monitored for research purposes.
However, in a recent paper in Nature Communications it has been proposed that these populations are under threat from an almost silent enemy, PCB pollution.
What is PCB?
Polychlorinated biphenyls (PCBs) are a major toxic industrial chemical that was produced during the twentieth century. PCB is well known for its environmental toxicity and its classification as a persistent organic pollutant. It was in the 1960s that PCBs where first identified as a problem. They were then subsequently banned in America in 1979 and in Europe in 1981. More recently they were banned by the Stockholm Convention on Persistent Organic Pollutants in 2001.
PCB is known to have numerous effects on humans including being carcinogenic. Not only can PCBs and other persistent toxins affect humans but they are having increasing and lasting effects on other organisms, especially those that are top predators in marine food chains.
Marine mammals that inhabit coastal waters of Europe, the Mediterranean Sea and the waters around Vancouver Island have been found to be highly contaminated with not only PCBs but also other toxic pollutants like dichlorodiphenyltrichloroethane (DDT). These chemicals accumulate in the fatty tissues of these animals.
The evidence is increasingly supportive that the increased concentration of PCBs in the ocean is leading to an increase in abnormalities seen in numerous marine species. Such abnormalities include: skeletal deformities, hormone disruption, immunotoxicity, and reproductive failure.
One majestic marine organism that is feeling the increasing effects of PCB in the world’s oceans in very different ways is the orca.
Effects of PCBs
Both the Pacific and Atlantic populations of orca are being affected by PCBs. It has been found that the PCB concentrations in individual male Pacific resident orca increases with age. This is due to them gaining PCB from their prey, which are also contaminated with this toxin. Contrastingly, the PCB concentrations of females in resident populations is greatly reduced with reproductive activity. The females pass on PCB to their offspring when in the womb and during lactation thus, reducing their own levels of PCB contamination. This is evident in many populations of orca where females are reproducing normally, and the concentrations of PCB is seen to decrease significantly with each successful pregnancy. The transient Pacific orca are particularly contaminated with PCBs due to their dietary preference of marine mammals, which themselves are high in PCB contamination. These two Pacific populations are considered to be among the most contaminated cetaceans in the world.
The two monitored populations of orca in European waters are split into almost identical categories as the Pacific orca, the Scottish population been marine mammal eating like the transients, and the southern population been predominantly fish-eating. This allows for good comparison between the Pacific and Atlantic populations.
Over the last 19 years that the Scottish population has been studied there has not been a single report of a calf seen within the group. Similarly for the resident Atlantic population between 1998-2011 only 5 calves were successfully produced and survived beyond a year. This means that over a 13 year period the southern population had a reproductive success rate of only 6.4%. This is one of the lowest recorded reproductive rates for orcas across the globe.
Adult females in the Atlantic consistently have substantially great concentrations of PCBs in their bodies than any individual Pacific orca. The adult males in the Atlantic also have higher PCB concentrations than the resident Pacific orcas and only slightly lower concentrations than the Pacific transients. The high concentrations of PCB in the adult females in the Atlantic is consistent with the reproductive failure seen in the Atlantic populations.
It is evident that the Atlantic females are either having trouble offloading PCBs to their calves during pregnancy or that they may be reaccumulating PCBs in their diets.
It is clearly evident that whilst the use of PCBs has been banned since the early 1980s, orcas are still continuing to being found heavily contaminated with this deadly toxin. Over the years the concentrations found in orca has decreased. Nevertheless this is a painstakingly slow process. The current levels of PCB in the Pacific orca has allowed them to be classified as among the most contaminated cetaceans in the world.
It is obvious that PCB levels represent an increasing toxicological risk to the Atlantic orcas. They are more heavily affected through the lack of reproduction which is having an increasingly dramatic effect on their population numbers and the future survival of these populations. It is nearing the point where if action is not taken to ascertain why PCB levels are not falling in the Atlantic Ocean that they will become under threat of extinction due to reproductive failure.
The levels of PCB in the Pacific has been declining over recent decades, however this trend has not been duplicated in the Atlantic, we must look into why this is or we may lose the Atlantic orca forever.