A gray whale mother-cub pair migrates along the central California coast from wintering grounds in Mexico to summer feeding grounds in the Arctic. Credit: NOAA Fisheries

The number of gray whales migrating along the west coast of North America has continued to decline over the past 2 years, according to a new estimate from NOAA Fisheries. The population is now down 38 percent from its peak in 2015 and 2016 as researchers work out the underlying causes.

The population also produced the fewest number of calves this year since counting began in 1994 accompanying reportt explains.

The 38 percent decline from a peak of about 27,000 whales in 2016 to 16,650 this year is reminiscent of past population fluctuations in the eastern North Pacific. Researchers at the Southwest Fisheries Science Center said this requires constant close monitoring. Census of population in the eastern part of the North Pacific Ocean gray whales usually conducted over a 2-year period. However, NOAA Fisheries will add to this study a third year of counting gray whales passing along the Central California coast from late December to mid-February 2023.

“Given the continued decline since 2016, we need to monitor the population closely to understand what may be driving this trend,” said Dr. David Weller, director of the science center’s marine mammal and turtle division. “We’ve seen population changes over time, and we want to be aware of that.”

An increase in gray whale strandings led NOAA Fisheries to declare the population an unusual die-off in 2019, prompting an investigation into the likely causes. The ongoing investigation has identified several likely participants. These include environmental changes in the Arctic that affect the seafloor, as well as amphipods and other invertebrates that live in and on top of the sediments and in the water column that gray whales feed on each summer, according to new research published earlier this year.

Some gray whales may have struggled to find food during these changes, said Dr. Sue Ellen Moore, a University of Washington research scientist who leads UME’s environmental impact assessment team. She noted that gray whales eat a lot of prey over a huge range, so there can be many variables that affect how, when and where they find food.

While many of the approximately 600 dead whales reported between 2019 and 2022 appeared to be malnourished, some were not. Some beached whales apparently died from other causes, such as ship strikes or predation killer whales. The number of exemptions initially rose sharply in 2019, but fell in subsequent years. This suggests that the greatest decline in gray whale populations occurred in the years shortly after the UME was declared.

“There’s nothing we can point to that explains all the strandings,” said Deborah Fauquier, a veterinarian with NOAA Fisheries’ Marine Mammal Health and Stranding Response Program, which is coordinating the UME investigation. “There seem to be a few factors that we’re still working to figure out.”

The population reflects changing ocean conditions

Gray whales are known for their visible migration along the West Coast each year. The population has fluctuated widely before, including a similar drop of about 40 percent from the late 1980s to the early 1990s. The population later recovered to a new high. Gray whales in the eastern Pacific Ocean have fully recovered since commercial whaling and were removed from the endangered species list in 1994.

A similar spike in strandings led to an earlier unusual mortality in 1999 and 2000, when the population declined by about 25 percent. It later returned to its peak in 2015-2016. (Although Table 1 in the report has a higher estimate for 2014-2015, that number was less accurate, so the researchers rely on the 2015-2016 estimate.)

Gray whale numbers continue to decline;  NOAA Fisheries will continue to monitor

Gray whale numbers and annual calf production. Credit: NOAA Fisheries

Most gray whales migrate between feeding grounds in the Arctic in the summer and lagoons in Lower Mexico in the winter, where they nurse their newborn calves. This annual round trip of more than 10,000 miles exposes them to many stressors along the way. A small group of gray whales also spend the summer feeding along and around the Pacific Northwest coast.

The population has probably always fluctuated in response to changes in the environment without lasting consequences, said biologist Dr. Tomo Eguchi, lead author of new NOAA Fisheries reports on whale populations and calf production. “The population has rebounded several times from past lows,” he said. “We are cautiously optimistic that the same will happen this time around. Continued monitoring will determine if and when they rebound.”

Calf numbers are also declining

NOAA Fisheries researchers track the number of gray whales in the population by counting whales that go south to Mexico. They monitor calf rearing by counting mothers and calves that migrate north each spring from the lagoons in Baja California, where some of the whales give birth. The the most recent count which ended in May, estimated total calf production this year at about 217. That number was down from 383 calves last year and the lowest since the count began in 1994.

Like the gray whale population as a whole, the number of calves born each year has also fluctuated. A low number of calves was recorded for periods of 3 to 4 years at a time before the rebound. Two of the previous three periods of low calf production coincided with unusual mortality and population declines. This suggests that the same factors that affect the survival of gray whales are likely also affecting their reproduction, the calf numbers report concludes.

Aerial photographs of gray whales in the lagoons of Mexico were shown deterioration of the body of many adult whales, emphasizing this connection. “Depending on the age of the whales, this lower body condition could have resulted in delayed reproduction and reduced calf numbers and/or reduced survival of lean whales,” the scientists said. informed.

In December, the teams will start another countdown with training binoculars whales migrating south past Granite Canyon, south of Monterey Bay in California. “What we hope to see in the next few years is that the numbers will stabilize and then start to show signs of increasing,” said Dr. Aimee Lang, co-author of the new reports. “We’ll be watching closely.”

The population of gray whales on the West Coast of the United States has decreased by a quarter

Additional information:
Abundance and migratory phenology of eastern North Pacific gray whales in 2021/2022. DOI: 10.25923/x88y-8p07

Courtesy of NOAA Fisheries West Coast Region

Citation: Gray whale numbers continue to decline; NOAA to Continue Monitoring Fisheries (2022, October 7) Retrieved October 7, 2022, from https://phys.org/news/2022-10-gray-whale-decline-noaa-fisheries.html

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