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Unusual Mortality Event Declared for Gray Whales as More Strandings Linked to Malnutrition and Ship Strikes

Researchers say gray whales that started their annual migration underweight this year are more vulnerable to human impacts such as ship strikes, especially in high-traffic areas like San Francisco Bay.

UPDATED May 31, 2019

Since January 1, more than 70 gray whales have stranded along the west coast of North America from Mexico through Alaska, and NOAA officials have just declared the elevated strandings an Unusual Mortality Event.

Scientists at The Marine Mammal Center have investigated 12 gray whale strandings in the San Francisco Bay Area since March, and have linked all of the deaths to malnutrition and ship strikes.

Increased numbers of gray whales have been sighted in the San Francisco Bay as the population makes their northerly migration this spring. Historically, one or two gray whales would temporarily pass underneath the Golden Gate Bridge during this time period. This year, experts have counted up to five entering the Bay at one time.

Biologists have observed gray whales in poor body condition during their annual migration this year, potentially due to anomalous oceanographic conditions over the past few years that have impacted their food source.

Gray whales complete the longest annual migration of any whale on earth, traveling from their feeding grounds in Alaska in the fall to their breeding grounds in the warmer waters off the coast of Baja California, Mexico, and returning in the spring months.

As gray whale migration season enters its final stages of the season, adult female gray whales and their calves with low body reserves are the last to migrate northward to their feeding grounds in Alaska. These mother whales are worn out and running on empty, making them even more susceptible to negative human interaction such as ship strikes and entanglements.

“By investigating deaths like this, we are able to identify and respond to rapidly changing environmental trends that are impacting marine mammal populations,” said Dr. Padraig Duignan, Chief Research Pathologist at The Marine Mammal Center. “These animals are representative of a growing issue for migrating gray whales who appear unable to sustain themselves due to shifting food source availability.”

It is critical for scientists to conduct in-depth necropsies to better understand why animals are dying. Whales and other marine mammals face numerous human-caused threats and solutions must be found to protect healthy and vulnerable species alike. Climate change affects water temperatures and prey availability, leading to shifting food sources for marine mammal populations and other marine species.

The death of 12 gray whales in the San Francisco Bay Area this year is a cause for serious concern as it speaks to the broader challenges this species continues to face.

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