EXPLODING harpoons, marauding factory ships and an insatiable taste for whale meat are all well-known threats to the great whales. In the past, the International Whaling Commission (IWC), which aims to conserve whale stocks has focused its efforts on staying the hand of the hunter. But last month鈥檚 annual meeting of the IWC saw a sea change.
For the first time, the commission鈥檚 scientific committee reviewed research into the effects of industrial pollutants on cetaceans and called for more studies. Researchers have long suspected that some chlorine-containing compounds are having a slow, subtle and sometimes lethal effect on marine mammals. The evidence gathered so far 鈥� on seals as well as cetaceans 鈥� suggests that these suspicions are well founded.
In the past decade, there have been at least five massive 鈥渄ie-offs鈥� of marine mammals, after only four in the preceding 80 years. Some 8000 seals died at Lake Baikal in Siberia in 1987 and 1988, and more than 18 000 were found dead in and around the North Sea, also in 1988. Over the next few years, almost 3000 bottlenose dolphin carcasses were found in two more episodes on the eastern seaboard of the US and in the Gulf of Mexico, and in 1990 striped dolphins began to wash up in their hundreds around the Mediterranean.
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In each of these events, marine scientists isolated strains of morbillivirus, a family of viruses that includes measles and canine distemper. But outbreaks of infection alone are unlikely to have accounted for the huge death toll, says Mark Simmonds of the School of Environmental Sciences at the University of Greenwich. 鈥淓pidemics are always multifactorial,鈥� he says.
The events all had something else in common. They took place along polluted coastlines. Suspicion fell on compounds such as PCBs, used chiefly as insulators in transformers, and pesticides, including DDT and dieldrin. These compounds, collectively called organochlorines, persist for many years in the environment and have been closely linked to reduced immunity and fertility in mammals.
Vicious circle
At each step up the food chain, from plankton to fish, these contaminants become more concentrated. As 鈥渢op-feeders鈥� in the aquatic system, dolphins, porpoises and the toothed whales, such as sperm whales, killer whales and belugas, tend to accumulate high levels in their tissues. Baleen whales, which consume plankton, may be at lower risk from the chemicals.
What makes organochlorines an especially serious problem for sea mammals is that they have large fat stores 鈥� up to 40 per cent of a whale鈥檚 weight is blubber. Organochlorines dissolve in fat and stay there until those fat reserves are needed as energy. Here begins a vicious circle.
During long journeys or during food shortages, marine mammals call on their reserves of blubber, releasing the chemicals into their systems. Worse still, a sick animal will mobilise still more fat in order to fight off the infection and release even more of the toxins. During the Mediterranean die-off, scientists took tissue from dolphins as the infection struck and found that those with higher burdens of organochlorines were more likely to die.
To firm up the link between organochlorines and immune depression, Dutch researchers spent two years studying harbour seals. They fed one group of 11 seals on herring from the Baltic Sea, which contains high levels of organochlorines, and another group with herring from a reasonably clean area of the Atlantic.
鈥淭he outcome was very clear,鈥� says Peter Reijnders, of the Institute for Forestry and Nature Research in Den Burg. Blood tests revealed that the group given contaminated fish had a significantly higher white blood cell count than the control group. This, says Reijnders, indicates that seals with the pollution-rich diet had a harder time dealing with common infections. The activity of key immune cells, called natural killer cells, was lower than in the animals fed unpolluted food.
鈥淭he important message is that there鈥檚 a sublethal effect going on, lowering the resilience of the animal,鈥� says Reijnders. While he does not blame pollution alone for the die-offs, 鈥渢here is enough evidence from this study to see that organochlorines like PCBs have contributed to the outbreaks鈥�, he says.
Loaded with toxins
Pierre B茅land of the St Lawrence National Institute of Ecotoxicology in Montreal is also studying the effect of organochlorines on immunity, this time in beluga whales. Little was known about these animals in 1982 when B茅land and his colleague Daniel Martineau, now of the University of Montreal, chanced upon a carcass washed up on a beach. When they found that the animal was 鈥渓oaded with toxic chemicals鈥�, the two men became intrigued.
The pair decided to look at other carcasses. 鈥淭he more we looked, the more we found,鈥� says B茅land. They discovered lesions all over the corpses, including cancerous growths in the liver and stomach, and in the salivary and mammary glands. PCB levels were as high as 500 parts per million (ppm), enough to classify them as toxic waste under Canadian law. The belugas of the St Lawrence estuary are now recognised as some of the most contaminated creatures on Earth.
B茅land, Martineau and Sylvain De Guise and Michel Fournier of the University of Quebec at Montreal, are now halfway through a five-year project to compare the immune response of belugas in the St Lawrence with those of their cousins in less-polluted Arctic waters.
The scientists have already taken samples from belugas in the Arctic. After driving a pod of whales to shore, they scoop a single animal into a hoop net and take a blood sample from under the tail. In 1996, they plan to take samples from belugas in the St Lawrence and use the two sets of data to investigate how immune response varies with levels of PCBs and DDT in the blood.
Garet Lahvis of the University of Maryland recently published the results of a similar study on five male bottlenose dolphins (Environmental Health Perspectives, vol 103, p 67). When an infection strikes, immune cells called lymphocytes proliferate rapidly, and the rate at which they divide gives a measure of the health of the immune system. Lahvis and his team incubated the dolphins鈥� lymphocytes with a mitogen, a chemical that stimulates cells to divide. They found that the higher the levels of PCBs and DDT in the dolphins鈥� blood, the lower the rate of proliferation (see Diagrams).
Researchers are not just worried about the effects of organochlorines on immunity. They also fear that the chemicals are disrupting cetaceans鈥� reproductive systems. Some organochlorines are known to mimic or block the actions of oestrogen and other sex hormones (This Week, 12 February 1994). Research on Dall鈥檚 porpoises has shown that high levels of organochlorines in their blubber correlated with reduced levels of testosterone.
Evidence of the effects of organochlorines on fertility comes once again from work on seals. Reijnders has shown that seals fed on contaminated fish from the Waddenzee off the Dutch mainland produced about half as many pups as those fed cleaner fish. Conducting similar studies with cetaceans has proved particularly difficult to organise, largely because of their size and habitat, but also because of popular disapproval.
But B茅land says there is a lot of circumstantial evidence that pollution has reduced the belugas鈥� fertility. Between 5000 and 10 000 whales thrived in the St Lawrence estuary until hunting depleted numbers to around 1500 in the 1950s. Although belugas have been a fully protected species since 1979, the population in the estuary has failed to recover. 鈥淭hey have no predators,鈥� says B茅land, 鈥渟o they should have increased.鈥� A sample of oil taken from a beluga in about 1952 鈥� just six years after DDT was introduced to North America 鈥� already contained 30 ppm of the substance, says B茅land.
Today, the proportion of young belugas in the estuary is also a cause for concern. They make up less than 30 per cent of the population, compared with about 45 per cent in the Arctic. Examination of the uteruses of dead female whales also suggests all is not well in the St Lawrence. In Arctic belugas, B茅land found that 66 per cent of the females were either pregnant or had recently given birth. In the St Lawrence, however, only 20 per cent had been carrying young. 鈥淭his suggests that there鈥檚 a segment of the population that鈥檚 not reproducing,鈥� B茅land says. 鈥淚t鈥檚 reasonable to assume that chemicals are interfering.鈥�
Cetacean calves face a particular danger from organochlorines. Not only can the chemicals pass from mother to fetus via the placenta, but they are also passed on in mothers鈥� milk. A mother transfers a huge part of her chemical load to her suckling young. According to a 1989 study of bottlenose dolphins off the coast of South Africa, mothers pass almost 80 per cent of their burden of organochlorines to their first-born. In the case of dolphins, this takes just seven weeks.
B茅land has studied the carcasses of four dead females from the St Lawrence which were lactating when they died. The milk contained an average of 10 ppm of PCBs, with the worst single case being 30 ppm. Anything over 1 ppm is considered unfit for human consumption.
B茅land says that the evidence shows that people are still putting economic growth 鈥� with the pollution that accompanies it 鈥� above the health of the environment. 鈥淲e have to convince people to change their way of life,鈥� he says. But change will be a mammoth task. DDT and other persistent organochlorine pesticides, which are banned in the industrialised world, are still used in developing countries. Industry has also turned out some 1.8 million tonnes of PCBs around the world and 60 per cent is still in use, says Paul Johnston, an environmental researcher at the University of Exeter. 鈥淭here鈥檚 no way to deal with the PCBs already out there,鈥� he says. 鈥淏ut if we want the problem to get three times worse we can adopt a do-nothing attitude.鈥� Safer methods of disposal have been developed, he says.
Reform may come too late for the St Lawrence belugas, however. 鈥淚 think they鈥檙e doomed,鈥� says B茅land. The question is how many other species will be lost before we change our ways?