By Heather Robison
Belugas in the St. Lawrence area of Quebec live at the southernmost edge of the species’ range and are isolated from other belugas which are found in northern and Arctic waters. Before 1885, there may have been as many as 10,000 St. Lawrence belugas, but over-harvesting greatly depleted the population size. The species did not recover when the harvest ceased in 1950, and the population remained between 650 and 1000 individuals.
In 1983, scientists at the University of Montreal began studying the beluga carcasses found in the estuary to find out why the population wasn’t recovering from decades of over-harvesting. It was discovered that about 21 per cent of the adult beluga carcasses that had washed ashore and could be examined post mortem had cancer, mostly of the gastrointestinal tract and mammary glands.
Stéphane Lair, Professor of Zoological Medicine at the University of Montreal, said that every year, between six and 12 beluga carcasses are examined by his department, a rate that has remained fairly steady for the past 27 years. Although a total of 193 carcasses have been studied by the University of Montreal over the years, about 400 St. Lawrence beluga carcasses have actually been found washed ashore since 1983. Half of the carcasses found are too decomposed to be used for post-mortem exams, Lair said. Some carcasses never wash ashore. There is a relatively equal mix of genders amongst the sample that could be examined, with 97 females and 94 males ending up in the study group. Two mammals had both male and female gonads, which is extremely rare and leads scientists to wonder about the consequences of contaminant exposure.
Over the years, there have also been single cases of uterine, lung, bladder and thyroid cancers found in the beluga carcasses, as well as a case of lymphoma and a couple of cases of skin cancers. In a few of the whales, the cancer had metastasized and the source of the disease could not be isolated. The animals that died of cancer were adult belugas, and since cancer typically takes years to develop, their mortality is proposed to be from lifelong exposure to toxins, Lair said.
How did the adult St. Lawrence belugas get cancer in the first place? The belugas feed on invertebrates in the river sediment. In the 1950s and 1960s, the level of to polycyclic aromatic hydrocarbon (PAH) toxins released into the river from aluminum smelters was very large compared with today. These toxins settled in the sediment, and when the belugas dug in they were probably exposed PAHs.
PAHs occur in oil, coal, and tar deposits, and are produced as byproducts of fuel burning. At high enough concentrations, PAHs are known to have a number of negative effects on the biological functions of organisms including reproduction problems.
While adult belugas in the St. Lawrence Estuary have the highest rates of cancer detected in wildlife, the leading cause of death of younger whales is actually parasitic pneumonia. Veterinarians at the University of Montreal are wondering if the immune systems of these young cetaceans were compromised by exposure to PCBs, or whether the stress from boat activities plays a role in disease resistance. Other possibilities are a shift in the ecosystem that may have resulted in a lack of balance between parasites and beluga hosts or a change in the beluga diet.
Juvenile belugas, ranging in age from one to 14 years, are still growing. About 12 per cent of the carcasses studied by the University of Montreal over the years were juvenile belugas, and verminous pneumonia was the leading cause of their death, Lair said. The belugas would have acquired the disease by eating fish with high levels of parasites. It is not known exactly which fish species is the source, and it is possible that several fish species have the parasites. Before the age of one and half, belugas do not eat solid foods, so they do not ingest these worms. Once they start feeding on fish, however, they are exposed to the parasitic larvae. The worms infect the lungs, and the belugas that are unable to fight off the respiratory disease die. Adult belugas have an immune response to the parasites and do not suffer the same fate as their young.
It is normal for fish to have parasites, Lair said, but the immune systems of the young belugas may have been compromised, which may be why they cannot fight the disease.
“It is known that these belugas are exposed to contaminants like PCBs that have been shown to cause immunosuppression in different animal species,” said Lair. PCBs accumulate in the fat and biomagnify up the food chain, so the big predators at the top of the food chain, like belugas, are highly exposed. “Therefore, the mortality observed in these young belugas might be linked to the exposure to these contaminants.”