Marine Chemical Pollution
What are PCBs and where do they come from?
PCBs are a group of chemical compounds that have been available commercially since the 1930s. They do not occur naturally, but rather are created solely through industrial processes. Before their use was banned, they were incorporated into many different products, including flame-resistant coatings, paints, some types of paper, drilling equipment and fluorescent lighting. PCBs can be released into the natural environment as a by-product of industrial processes, and if products containing PCBs are disposed of without due care, the PCBs can then also leak out and enter the natural environment.
PCBs are prime examples of chemicals that threaten the wellbeing and survival of marine animals because:
· PCBs have entered, and continue to enter, the oceans and seas in large quantities; for example, after being released from decomposing products that have not been disposed of with due care. Scientists estimate that 1-10% of all PCBs ever produced have already entered the oceans
· PCBs tend not to quickly break down into less harmful forms after being released into the natural environment
· PCBs are toxic and even potentially lethal if absorbed by marine animals at sufficiently high levels
What problems can PCBs cause animals to suffer?
PCBs can be lethal to animals (including humans) at higher concentrations, and can cause non-lethal, yet still dangerous, effects at lower concentrations. Because of these effects, a maximum legal limit in the USA for PCBs in cow’s milk and dairy products fit for human consumption has been set at 1.5 mg per kg.
Non-lethal effects of PCBs include the disruption of reproductive and immune systems. Studies of some people exposed to PCBs have associated PCB exposure with miscarriages, low fertility and various childhood problems.
Some of the non-lethal effects of PCB exposure are caused by the disruption of hormones (natural signalling molecules that the bodies of animals use to control various important biological processes). In studies of animals, hormone disruption has been found to have many bad effects, including:
· Decreased fertility in birds, fish, shellfish and mammals
· Decreased hatching success in birds, fish and turtles
· Deformities in birds, fish and turtles
· Abnormal behaviour in birds
· Feminisation of male fish, birds and mammals (and the reverse in female birds and fish)
· Weakened immune systems in birds and mammals
Why do PCBs pose a particular threat to whales, dolphins and porpoises?
There is a lot of scientific evidence for the damaging effects of PCB exposure on a wide range of marine animals. Whales, dolphins and porpoises, however, are especially hard to study in their natural habitats.
Nevertheless, many scientists are particularly worried about the effects that PCBs may be having on the wellbeing and survival of whales, dolphins and porpoises, over and above the threats posed to other animals in the seas and oceans, for the following reasons.
Most whales, dolphins and porpoises are at the top of natural food chains, and so they are a final destination for PCBs found throughout the marine environment
Many species of whales, dolphins and porpoises are high up the natural ‘food chain’ – this means that they survive by feeding on thousands of large fish and squid over their lifetime. These large fish and squid in turn survive by eating hundreds of small fish, which in turn survive by eating tiny sea creatures known collectively as plankton. The problem is that the plankton absorb PCBs from their environment and pass these onto the small fish and squid, which in turn pass on the PCBs in their body tissues to the large fish and squid that eat them. Finally, the PCBs from all the large fish (and the small fish and the plankton) are absorbed by the whales, dolphins and porpoises that eat them. The end result of this food chain is that whales, dolphins and porpoises accumulate large amounts of PCBs from their food. In fact, the levels of PCBs that accumulate in the bodies of some whales, dolphins and porpoises are so high that under US law, they could meet the criteria for being defined as toxic waste.
Whales, dolphins and porpoises store much of the PCBs from their food in their blubber, and therefore release high levels of PCBs into the body when blubber is broken down in times of stress
The PCBs that whales, dolphins and porpoises absorb from their food often end up being stored mainly in their fatty tissue (also known as blubber). The levels of PCBs in an animal’s blubber will build up over time as it consumes more and more contaminated fish. In times of stress, when for example food is scarce, whales, dolphins and porpoises tend to break down their stores of blubber to provide them with an energy supply. Breaking down blubber in this way releases a flood of toxic PCBs into an animal’s body, adding to the stress they are already experiencing.
Whales, dolphins and porpoises are particularly ill equipped to deal with pollutants like PCBs
Scientific studies suggest that the metabolisms of whales, dolphins and porpoises are particularly ill equipped to deal with the large amounts of PCBs that often contaminate their food. Scientists have found compelling evidence that whales, dolphins and porpoises make few of the enzymes that many other species of animals use to break down dangerous chemicals such as PCBs. (Enzymes are molecules that animals’ bodies use to bring about the chemical reactions they need to stay alive).
Female whales, dolphins and porpoises sometimes pass on a large and potentially fatal proportion of their PCB burden to their first-born calves, both via the womb and through their milk
Scientists have found that the PCB burden carried by male and female whales, dolphins and porpoises tends to increase with time until they reach the age of sexual maturity. After that point, the burden in males continues to grow, as they continue to absorb PCBs from their food. However, scientists have found that the PCB burden carried by female whales, dolphins and porpoises drops off after they have their first calf. The sad fact is that it appears that female whales, dolphins and porpoises pass on most of their PCB burden to their first-born calves, while the calf is in the womb and afterwards, when the calf is suckling. It appears likely that the large amount of PCBs transferred to calves in these ways, as well as the fast rate of transfer, can sometimes prove fatal.
Specific effects of PCBs on whales, dolphins and porpoises
A population of about 500 beluga whales lives in the highly polluted Gulf and estuary of St Lawrence off the eastern coast of Canada. The size of this population has decreased dramatically over the last century.
Scientists have found PCBs (as well as other similar chemicals, known collectively as organochlorines) at much higher concentrations in this population of beluga whales, compared with in beluga whales that live in the less contaminated waters of the Arctic.
In a study of the Gulf of St Lawrence belugas started in 1982, scientists found that only about one in five female whales were either pregnant or had recently calved, compared with about two-thirds of female belugas that lived in the Arctic.
Moreover, about a third of female Gulf of St Lawrence belugas were found to have damage to their mammary glands that would have compromised their ability to successfully feed any calves they were able to give birth to.
In addition, scientists examined the bodies of 45 beluga whales from this population and found that 18 contained at least one cancerous tumour. Finding tumours in whales is very rare – the tumours found in these beluga whales account for more than half of all tumours ever found by scientists examining whales, dolphins and porpoises.
Mass ‘die-offs’ of dolphins
Mass ‘die-offs’ among marine mammals are said to have occurred when an unexpectedly large proportion of a marine mammal population dies during a short period of time.
Mass die-offs among marine mammals have been occurring more often, and have become more severe, since the middle of the 20th century.
Two examples of mass die-offs of dolphin populations include:
· The death of at least 2,500 bottlenose dolphins in 1987-1988 along the eastern coast of the USA
· The death of up to 10,000 striped dolphins in 1990 in the Mediterranean
Many scientists believe that while these mass die-offs may be caused in the first instance by viral infections, this is not the whole story. Rather, it seems likely that the weakening of the dolphins’ immune systems due to a heavy burden of dangerous chemicals such as PCBs enables viral infections to be passed on from animal to animal, to take hold within the bodies of individual animals, and to ultimately prove fatal for so many animals.
Many chemical compounds have similar properties and effects to the PCBs and these include a number of pesticides such as DDT - and essentially the problems that they create are very similar to those presented by the PCBs. We have focused on the PCBs here are they seem to be a particular problem for cetaceans but other chemicals introduced to the marine environment are likely to be creating problems too and there is a new for vigilance as new industrial and agricultural compounds also escape into the seas.
The decline in bottlenose dolphin numbers in UK waters since the 1960s
The decrease in sightings and strandings of bottlenose dolphins in the waters around the UK and on the coastline of the UK since the 1960s suggests that their numbers in these waters have declined significantly since that time.
While there are many possible reasons for this apparent decline, one of the major explanations could be the ill effects of high levels of PCBs and other organochlorines, which are absorbed by bottlenose dolphins from contaminated food.
Very high levels of PCBs have been found in the bodies of bottlenose dolphins in UK waters, levels high enough that we would expect them to badly affect the animals’ reproductive and immune systems.
Also, the contamination of the seas with PCBs from industrial processes peaked in the 1960s, at the same time as the number of bottlenose dolphins in UK waters probably began to significantly decline.
Scientists who have studied PCB levels in marine wildlife believe that these PCB levels are likely to have only just begun to decline significantly from their peak, even though the production of PCBs was effectively banned in the 1970s. Indeed, PCB levels in marine wildlife may not start to decline for decades to come.
What can be done to stop whales, dolphins and porpoises from being exposed to dangerous levels of PCBs and similar compounds?
Because of the persistence, widespread distribution and large quantities of PCBs already in the environment, they are clearly going to present toxic problems for wildlife for many years.
The manufacture of products containing PCBs has now thankfully been banned in many countries. Yet, it is still crucially important that the 2 million tonnes of PCBs that existing products in industrialised countries are estimated to contain should be disposed of safely once these products reach the end of their useful life, if the amount of PCBs leaking into the seas and oceans each year is to be reduced.
If care is not taken in their disposal, products containing PCBs may prove to be a ticking chemical ‘time-bomb’ that presents a worldwide threat to vulnerable animal species, not least to whales, dolphins and porpoises.