According to a new NOAA Fisheries assessment, the number of gray whales migrating along North America’s West Coast has continued to decline over the last two years. As researchers investigate the underlying causes, the population is now down 38% from its peak in 2015 and 2016. According to an accompanying report, the population also had the fewest calves on record this year since counts began in 1994.
The 38 percent decline from a peak of approximately 27,000 whales in 2016 to 16,650 this year is consistent with previous fluctuations in the eastern North Pacific population. Southwest Fisheries Science Center researchers say it’s worth keeping an eye on. The population of gray whales in the eastern North Pacific is typically counted every two years. However, NOAA Fisheries will add a third year counting gray whales that pass along the Central California Coast to this survey, from late December to mid-February 2023.
“Given the continuing decline in numbers since 2016, we need to closely monitor the population to help understand what may be driving the trend,” said Dr. David Weller, Director of the Marine Mammal and Turtle Division at the science center. “We’ve seen the population change over time, and we want to stay on top of it.”
An increase in gray whale strandings led NOAA Fisheries to declare an Unusual Mortality Event for the population in 2019, prompting an investigation into the likely causes. That ongoing investigation has identified several likely contributors. These include ecological changes in the Arctic affecting the seafloor and the amphipods and other invertebrates living in and above the sediment and in the water column that gray whales feed on each summer, according to new research published earlier this year.
Depending upon the age of the whales, this lower body condition may have led to delayed reproduction and lower calf counts, and/or reduced survival in thin whales.
Dr. Sue Ellen Moore
Some gray whales may have struggled to find food amid those shifts, said Dr. Sue Ellen Moore, a University of Washington researcher who leads the UME team assessing ecological influences. She noted that gray whales feed on a wide variety of prey over an enormous range, so there could be many variables affecting how, when, and where they find food.
While many of the roughly 600 dead whales recorded from 2019 to 2022 appeared malnourished, some did not. Some stranded whales had clearly died of other causes such as getting hit by ships or predation by killer whales. The number of strandings initially spiked in 2019 but then fell in subsequent years. That suggests that most of the gray whale population decline probably occurred in the years shortly after the UME was declared.
“There is no one thing that we can point to that explains all of the strandings,” said Deborah Fauquier, Veterinary Medical Officer in NOAA Fisheries’ Marine Mammal Health and Stranding Response Program, who coordinates the UME investigation. “There appears to be multiple factors that we are still working to understand.”
Population Reflects Changing Ocean Conditions
Gray whales are famous for their annual migration along the West Coast. The population has fluctuated greatly in the past, including a similar drop of roughly 40% from the late 1980s to the early 1990s. The population later recovered to a new high point. Gray whales in the eastern Pacific Ocean have fully recovered from commercial whaling and were removed from the endangered species list in 1994.
The majority of gray whales migrate between feeding grounds in the Arctic during the summer and lagoons in Baja Mexico during the winter, where they feed their newborn calves. This annual roundtrip of more than 10,000 miles exposes them to numerous stressors along the way. A small group of gray whales also spends the summer feeding along and around the Pacific Northwest Coast.
The population has likely always fluctuated in response to changes in its environment, without lasting effects, said biologist Dr. Tomo Eguchi, lead author of the new NOAA Fisheries reports on the whale population abundance and calf production. “The population has rebounded multiple times from low counts in the past,” he said. “We are cautiously optimistic that the same will happen this time. Continued monitoring will determine whether and when they rebound.”
Calf Numbers Also Decline
NOAA Fisheries researchers track the numbers of gray whales in the population by counting southbound whales heading for Mexico. They monitor calf production by counting mothers and calves migrating north each spring from lagoons in Baja California, where some whales give birth. The most recent count that concluded in May estimated the total calf production this year at about 217. This number was down from 383 calves last year and the lowest since the counts began in 1994.
The number of calves born each year has fluctuated along with the gray whale population as a whole. Low calf counts were observed for 3 to 4 years before rebounding. Two of the previous three periods of low calf production were associated with Unusual Mortality Events and population declines. The report on calf numbers concludes that the same factors that affect gray whale survival are likely to affect their reproduction as well.
Aerial photographs of gray whales in Mexican lagoons revealed declines in body condition in many adult whales, highlighting that link. “Depending upon the age of the whales, this lower body condition may have led to delayed reproduction and lower calf counts, and/or reduced survival in thin whales,” scientists reported.
