Mysteriously high mortality of the Southern Right Whale

The largest mammals of modern day reside in the oceans and are often referred to as one of the world’s gentlest creatures. With their grandeur and modesty aside, whales suffer from extremely high mortality rates. Much of the depletion of most whales was caused by excessive, commercial whaling as early as the 1600s, but for one species, there is far more at play.

The southern right whale, a species of baleen whales native to the southern hemisphere, rebounded from near extirpation in most of their southern habitats after centuries of hunting. Now that more regulation has been enforced and whaling is internationally illegal, their population bounced back almost to the size of the pre-whaling era.

But oddly enough, since the 1970s, researchers have seen some sharp declines in population around Peninsula Valdes in Patagonia, Argentina that isn't due to human intervention. Multiple hypotheses have been tossed around, but no one can respond with a reason as to why they are seeing some of the worst baleen whale die-offs in recent years.

“There has not been a definitive answer in terms of a single cause for the ongoing die-offs each year,” said Katie Moore, program director of animal rescue at the International Fund for Animal Welfare.

Moore conducted necropsies, autopsies performed on animals, in Argentina to find out if there is a very specific cause of death. “The reality is that many [whales] were very decomposed and gross evidence of a cause of death was not apparent,” she said.

The International Whaling Commission, one of the largest groups to control and regulate whaling, recognized the unique situation of the southern right whale deaths in Argentina as a serious problem, even though there has not been a consistent cause.

Victoria Rowntree, cofounder of the Ocean Alliance and director of the Right Whale Program, said she has also worked on necropsy analysis of the southern right whale. According to Rowntree and her colleagues’ statistical analysis report, a part of the Marine Ecology Progress Series in 2013, the researchers found that 89 percent of the whales that were dying each year were calves.

Stranded, deceased southern right whale calf.  (Courtesy of the Southern Right Whale Health Monitoring Program)

Stranded, deceased southern right whale calf. (Courtesy of the Southern Right Whale Health Monitoring Program)

Rowntree has studied this issue since 1976, and every year, a group of researchers took aerial surveys of the whales in the Gulfo San Jose and Gulfo Neuvo nursing grounds in the Peninsula Valdes. Right whales are unique in that they can be identified based on their callosities, white marks on their snouts.

When these studies began, Roger Payne, the founder and president of Ocean Alliance, decided that whale studies and observation were not sufficient and thorough enough when looking at large-scale issues like these annual die-offs.

“In the 1970s, the only way we learned about the whales was by studying the dead bodies during the killing from the whaling industry,” Rowntree said. “Rogers said if you could find the individuals without killing them, you can follow them throughout their lives and learn much more about the whales.”

By changing the methods, the team learned that mother right whales did not return to the Peninsula Valdes every year. They returned every three years when they were ready to give birth and nurse the newborn. The white marks were the clues to differentiate individuals.

After realizing each year that a different set of mothers were coming to have babies, Rowntree and her team were better able to understand population density and growth. But in reality, they were also seeing huge losses in calf population.

According to the statistical analysis report, “During the first 30 years of the study, deaths appeared to increase at a rate similar to the increase in number of whales using the calving grounds, but an unexpectedly large number of whales, 47, died in 2005, and high mortality events have continued annually on average of 75 whales dying each year from 2007 through 2011.”

Some important concerns, Moore said, have to do with toxins from the occurrence of algal blooms, possible unknown diseases and a form of parasitic harassment.

“One hypothesis to explain the die-offs is the poor nutritional condition of the mothers, and that may be related to where some animals are feeding,” said Alex Zerbini, a whale telemetry expert with the National Oceanic and Atmospheric Administration’s National Marine Mammal Laboratory. “We have to tag quite a bit more whales to have a better idea of the variability in movements and feeding ground choices.”

Zerbini’s work at NOAA will help determine where the mother right whales travel for food, so researchers can test the water and food supply for toxins that the mothers are ingesting.

The idea behind this hypothesis is known as biomagnification. When small organisms consume toxins, the concentration increases the higher up the food chain when those smaller organisms are later eaten. Southern right whales eat krill and copepods. Copepods are microscopic, and they eat diatoms, such as phytoplankton.

According to a report from the Southern Right Whale die-off at Peninsula Valdes, Argentina workshop in 2013, there was evidence of “blooms of chlorophyll that began to occur in Gulfo San Jose and Gulfo Neuvo in 2004, around the time calves began dying in large numbers.”

Rowntree said there were two years where the calves died early in the season, just after birth. And in other years, they died later, indicating that these calves were not born ill or weak because they were growing and living longer. “It is possible that the mothers ingested these toxins when the calves were in utero when they were two months in, and the gestation period is 12 months,” she said. “It would gather in the amniotic fluid and affect the development of the calve's spinal cord.”

Researchers have not found enough evidence to prove that this may be the case, but it is possible that this may be occurring out in the ocean where they cannot gather samples.

Another issue that is hard to prove is disease and viral infection. According to the workshop report, “necropsies and histological analyses were conducted on 2 to 8 week-old calves.” There were no substantial patterns present in the analyses, but Rowntree and others believe there is a possibility of disease in whales that they cannot determine.

“We’re trying to get deeper evidence by doing viral chip analyses of blood or lung tissue on whether there's a virus that was causing a certain disease,” she said. “We have done things like captured exhalation, the blows of the whale and analyzed the exhalation for any bacteria that would indicate disease. We haven’t found any common disease.”

These are methods that can easily be done on humans, but for whales, it is a much heftier task. To combat this, Rowntree observes and records the times at which a healthy mother and calf blows when they come up to breathe. If she notices a whale that is coming up frequently, that would be a good indication that the whale may be sick. “Some of the whales die of disease, but it still doesn’t say why 116 whales died in 2012, the highest mortality yet,” Rowntree said.

The other contender that may yield the most information on the die-offs is the kelp gull attacks on the young whales. According to the workshop report, these incidents are compared to the “fly worry” phenomenon, insect harassment with land mammals like horses and hippopotamuses. Kelp gulls live on the coasts of the peninsula and eat the blubber from the backs of southern right whales.

The reports stated that mother whales learn to keep their back arched or hold their breath for longer periods to avoid these attacks. Rowntree said that when the calves are first born, they are the most vulnerable victims of these attacks because they have not learned the new behavior and need to come up more often to breathe.

“So if calves get one lesion and one gull breaks open an area, then more gulls will come back and keep picking at that area and make bigger lesions,” Rowntree said. “If [the calves keep coming up] again and again, they may learn that ‘something is going to get me,’ so they may stick their snouts out of the water so their whole back isn’t exposed.”

The reason why this is a bigger problem than most people think is because the gulls are relentless. Gulls attack mother-calf pairs for up to an hour, so the whales move to another area but run into the same situation. Rowntree said mothers and calves rest for about 80 percent of the day, but if they are constantly bombarded by gulls, they expend too much energy. “There’s no normal, happy growth and that can affect brain development, muscle, and body development of the calves,” she said.

Mothers fast during the nursing period to give most of their stored nourishment to their young. “When the calves are first born, they don’t have enough blubber to float, so the mother gives a lot of milk and they get fat rapidly, which helps them float,” Rowntree said. “That means they don’t have to keep on swimming to come to the surface to breathe.”

If the pair is constantly fleeing an attack, both would lose vitality, but the calves that are barely developed would suffer the most.

There are many factors at play with the case of the southern right whale die-offs, and it seems to be worsening. Researchers may have to concede and say it is more than one cause, but it still does not make sense as to how hundreds of southern right whales are dying each year.

“It’s totally frustrating, and we wish we could find a cause,” Rowntree said. “It’s very rare for this many baleen whales to die, you get many hundreds of whales washed up on a beach, but those are toothed whales. It never really happens with baleen whales, and it’s unique in the world.”