Mystery of marine mammal stranding solved

In a study in Proceedings of the Royal Society B scientists explain the stranding of 40 marine mammals that washed up on the Chilean coast millions of years ago.

While slicing through the Chilean desert to extend the Pan-American Highway workers did not expect to uncover a massive marine mammal graveyard. The building works opened up a 250 metre wide quarry site revealing the skeletons of more than 40 marine mammals including 31 large baleen whales, seals, a walrus-like toothed whale, an aquatic sloth and an extinct species of sperm whale. The site, Cerro Ballena, from which the skeletons were removed for further study and care, has now been paved over, however while the quarry was open, a team of scientists led by Nicholas Pyenson  travelled to the site to unearth the mystery of the ancient stranding.

Mammal strandings have occurred for millions of years but their origins are still not completely understood. Alongside human causes, strandings are sometimes attributed to herding behaviour and harmful algae blooms. The scientists behind this study say that because algal toxins can cause organ failure in marine mammals, harmful blooms could be the most common cause of mass strandings.

The team gathered evidence from Cerro Ballena, observing that the mammals were all belly side up and close together. They also noticed that the mammals were at right angles to the direction that the current flow would have flowed at the time. These clues suggest that the animals died at sea and were washed up on the beach rather than becoming stranded due to herding behaviour while alive. Many of the skeletons were also almost completely intact indicating that after the animals died they were deposited on the coast within hours or days before being buried.

The team also noticed that as well as being from a number of species, the skeletons covered a range of ages from calves to large adult baleen whales and had washed up during 4 different events. The wide array of animals buried at the site indicated that a broad death mechanism was at work which didn’t differentiate between the young and old or between species.  This suggests that harmful algae blooms might be responsible. Other causes, like tsunamis, which could have occurred periodically at the site, were ruled out by the team because they would have produced a range of skeletons including much smaller species, rather than the primarily large mammals found at Cerro Ballena.

The researchers say all the signs point towards poisoning by toxic algal blooms and that the evidence from Cerro Ballena is similar to a modern stranding in the late 1980s where 14 humpback whales were washed up at Cape Cod. A study at the time testing the stomach contents of Atlantic mackerel in the area showed high concentrations of neurotoxins. The strange behaviour of one of the dying whales helped scientists make a link between the stranding and major algal toxins which could have killed the humpbacks. The whales from Cerro Ballena might have met a similar fate say the researchers.

Delivering the cause of death the team say that while harmful algae blooms are not fully understood they could be tied to upwelling currents on the westerly edge of continental coastlines. The researchers say that ‘iron-rich runoff’  from the Andes into the Pacific ocean could have led to increased iron in the ocean, boosting the productivity of phytoplankton whose growth is limited by lack of iron. The excess iron could also have promoted the bloom of harmful algae consumed or inhaled by the creatures buried at Cerro Ballena.