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Underlying Reasons for the Decline in Gray Whale Numbers

Gray whale migration along North America’s West Coast has been decreasing in recent years, according to a new NOAA Fisheries report. The population is currently down 38% from its peak in 2015 and 2016, and experts are currently investigating the causes.

Underlying Reasons for the Decline in Gray Whale Numbers.

A gray whale mother-calf pair migrating along the central California coast from the wintering grounds in Mexico to the summer feeding grounds in Alaska. Image Credit: NOAA Fisheries.

According to another report, the population also produced the fewest calves on record this year since counts began in 1994.

The 38% drop from a peak of over 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 experts believe it is important to keep an eye on it.

Population counts for gray whales in the eastern North Pacific are usually done every two years. However, NOAA Fisheries will add a third year to this survey, from late December to mid-February 2023, to count gray whales that transit along the Central California Coast.

Given the continuing decline in numbers since 2016, we need to be closely monitoring the population to help understand what may be driving the trend. We have observed the population changing over time, and we want to stay on top of that.

Dr. David Weller, Director, Marine Mammal and Turtle Division, Southwest Fisheries Science Center

In 2019, NOAA Fisheries declared an Unusual Mortality Event for the gray whale population, triggering an investigation into the possible reasons. Numerous potential contributors have been identified as part of the ongoing inquiry.

According to new studies released earlier this year, these include biological changes in the Arctic that impact the seafloor and the amphipods and other invertebrates dwelling in and above the sediment and in the water column on which gray whales graze each summer.

According to Dr. Sue Ellen Moore, a University of Washington researcher who is in charge of the UME team analyzing ecological factors, some gray whales may have had difficulty locating food during those changes. She pointed out that because gray whales consume a wide spectrum of animals, there may be a variety of factors influencing how, when, and where they find food.

Some of the nearly 600 dead whales that were discovered between 2019 and 2022 appear to be malnourished; however, many did not. It was evident that some stranded whales had passed away from other causes, such as being hit by ships or being eaten by killer whales.

After an initial peak in 2019, the number of strandings began to decline. This means that the years immediately following the UME’s declaration were probably when the largest decline in the gray whale population took place.

There is no one thing that we can point to that explains all of the strandings. There appears to be multiple factors that we are still working to understand.

Deborah Fauquier, Veterinary Medical Officer, Marine Mammal Health and Stranding Response Program, NOAA Fisheries

Deborah Fauquier coordinates the UME investigation.

Population Reflects Changing Ocean

Gray whales are well-known for their yearly migration along the West Coast. The population has varied significantly in the past, including a comparable decline of nearly 40% from the late 1980s to the early 1990s.

Later, the population recovered to a new high. Gray whales in the eastern Pacific Ocean have recovered completely from commercial whaling and were removed from the endangered species list in 1994.

A similar increase in strandings resulted in the declaration of an earlier Unusual Mortality Event in 1999 and 2000 when the population dropped by around 25%. It later returned to a peak in 2015–2016.

The majority of gray whales migrate between feeding grounds in the Arctic during the summer to lagoons in Baja, Mexico, during the winter to feed their newborn calves. This annual roundtrip of more than 10,000 miles subjects them to numerous stressors. A small pod of gray whales spends the summer feeding off the coast of the Pacific Northwest.

According to biologist Dr. Tomo Eguchi, lead author of the new NOAA Fisheries research on whale population abundance and calf production, the population has likely always varied in reaction to changes in its environment, with no lasting impacts.

The population has rebounded multiple times from low counts in the past. We are cautiously optimistic that the same will happen this time. Continued monitoring will determine whether and when they rebound.

Dr. Tomo Eguchi, Study Lead Author and Biologist, NOAA Fisheries

Calf Numbers Also Decline

NOAA Fisheries scientists estimated southbound whales headed to Mexico to determine the population size of gray whales. They tracked calf production by counting mothers and calves as they migrated north each spring from Baja California lagoons, where some whales give birth.

The most recent count, which ended in May, estimated overall calf birth this year to be around 217. This was a decrease from 383 calves the previous year and the lowest number since the census began in 1994.

The number of calves born every year has varied, as has the gray whale population as a whole. Before rebounding, low calf counts were observed for periods of three to four years.

Two of the previous three episodes of low calf production corresponded with Unusual Mortality Events and population decreases. The investigation on calf numbers has revealed that the same factors seem to influence gray whale survival and are also likely to impact their reproduction.

Aerial images of gray whales in Mexican lagoons revealed reductions in the bodily condition of several adult whales, highlighting that link.

The investigators stated, “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.”

Teams will start the next count in December by training binoculars on whales traveling south past Granite Canyon, just south of Monterey Bay in California.

Dr. Aimee Lang, Co-Author of the report, concludes, “What we hope to see in the next few years is that the abundance stabilizes and then starts to show signs of increase. We will be watching closely.”

Source: https://www.fisheries.noaa.gov

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