Spike in Gray Whale Deaths Triggers Investigations

This adult female gray whale washed ashore at Limantour Beach in Point Reyes National Seashore on April 30. Scientists found that it died due to a propeller strike to its tail fluke. It was also malnourished.

NPS / Sarah Codde

June 2019 - Eastern Pacific gray whales have generally been doing well in recent years. They recovered successfully from the brink of extinction with the help of conservation legislation, and were taken off of the US endangered species list in 1994. These days, it’s not hard to spot them off of the California coast in the winter and spring. They pass close by as they undertake one of the longest migrations of any whale, from their summer feeding grounds in Alaska to their winter breeding grounds off of Southern California and Baja California, Mexico, and back. It is far less common to find a dead gray whale washed up on shore. On average, fewer than 15 gray whales are found dead along western US coastlines each spring. But this spring, 81 gray whale carcasses have turned up dead on US shores, including 13 in the San Francisco Bay Area alone. More have been found in Mexico (78) and Canada (8), and hundreds of others have likely perished unseen in the ocean. The question is, why?

The California Academy of Sciences and The Marine Mammal Center in the Marin Headlands has investigated the deaths of the gray whales found locally in an effort to understand what is going on. They found malnutrition (8) and ship strikes (5) to be the immediate causes of death. Some of the whales struck by ships also showed signs of malnutrition. For example, the whales that washed up on Limantour Beach in Point Reyes National Seashore on April 30th and on Ocean Beach in Golden Gate National Recreation Area on May 6th were both killed by ships. Both also had unusually thin layers of blubber. Other gray whale death investigations up and down the coast have also implicated malnutrition as a likely cause or contributing factor.

If malnutrition is the biggest common thread in these recent whale deaths, its underlying causes are still very much a mystery. At the end of May, NOAA Fisheries designated the spike in deaths as an “Unusual Mortality Event,” which could help us find answers. The designation, which was applied only once before for gray whales during a surge in deaths in 1999-2000, means the launch of an investigation by a broad coalition of scientists. The scientists will explore a wide range of possibilities. For instance, did ocean conditions like the dearth of arctic sea ice in recent years hurt the whales’ food supply? Is the gray whale population now near carrying capacity, and therefore more vulnerable to normal environmental shifts? A thorough investigation could take years, but the answers will be invaluable as we seek to better understand what we can do to keep our oceans and their inhabitants in good health.

Check out NOAA’s 2019 Gray Whale Unusual Mortality Event page to learn more.

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