Pacific Coast Gray Whales Have Become 13% Smaller Over Past 20-30 Years, Study Finds

Gray whales that spend their summers feeding in shallow waters off the Pacific Northwest coast have suffered a significant decrease in body length since around 2000, according to a new study from the University of California. Oregon State.

According to the researchers, this smaller size could have major consequences on the health and reproductive success of affected whales, and raise the alarm about the state of the food web in which they coexist.

“This could be a warning sign that this population is starting to decline in abundance or is not healthy,” said KC Bierlich, study co-author and assistant professor at the Institute of Marine Mammals at OSU in Newport. “And whales are considered sentinels of the ecosystem, so if the whale population is not doing well, that could say a lot about the environment itself.”

The study, published in Biology of global change, examined the Pacific Coast Feeding Group (PCFG), a small subset of approximately 200 gray whales within the larger Northeast Pacific (ENP) population of approximately 14,500 individuals. This subgroup stays closer to the coast of Oregon, feeding in shallower, warmer waters than the Arctic seas where the bulk of the gray whale population spends most of the year.

Recent OSU studies have shown that whales in this subgroup are smaller and in overall worse physical condition than their ENP counterparts. The current study reveals that they have declined in recent decades.

The Marine Mammal Institute’s Geospatial Ecology of Marine Megafauna (GEMM) Lab has been studying this subgroup of gray whales since 2016, including flying drones above the whales to measure their size. Using images from 2016 to 2022 of 130 individual whales of known or estimated age, the researchers determined that an adult gray whale born in 2020 is expected to reach an adult body length of 1.65 meters (about 5 feet and 5 inches) shorter than that of a gray whale born before 2000. For PCFG gray whales that measure 38 to 41 feet long at full maturity, this represents a loss of more than 13% of their total length.

If the same trend were to occur in humans, it would be equivalent to the height of the average American woman increasing from 5 feet 4 inches to 4 feet 8 inches in 20 years.

“In general, size is critical for animals,” said Enrico Pirotta, lead author of the study and a researcher at the University of St. Andrews in Scotland. “It affects their behavior, their physiology, their life history, and it has cascading effects on the animals and on the community that they are a part of.”

Calves that are smaller at weaning age might be unable to cope with the uncertainty of becoming independent, which can affect survival rates, Pirotta said.

For adult gray whales, one of the biggest concerns is reproductive success.

“As they are smaller, one wonders how efficiently these PCFG gray whales can store and allocate energy to grow and maintain health. Importantly, are they able to devote enough energy to reproduction and maintain the growing population?” Bierlich said.

Scarring on PCFG whales from boat strikes and fishing gear entanglement also makes the team concerned that their smaller body sizes and lower energy reserves may make the whales less resilient to injury.

The study also examined patterns in the ocean environment that likely regulate food availability for these gray whales off the Pacific coast by following cycles of “upwelling” and “relaxation” in the ocean. . Upwelling transports nutrients from deeper to shallower regions, while periods of relaxation then allow these nutrients to remain in shallower areas where light allows the growth of plankton and other tiny organisms, including including the prey of gray whales.

“Without a balance between upwelling and relaxation, the ecosystem may not be able to produce enough prey to support the large size of these gray whales,” said co-author Leigh Torres, associate professor and director from the GEMM laboratory at OSU.

The data shows that whale sizes have declined alongside changes in the balance between upwelling and relaxation, Pirotta said.

“We have not looked specifically at how climate change affects these trends, but in general we know that climate change affects the oceanography of the Northeast Pacific through changes in wind patterns and sea temperature. water,” he said. “And these and other factors affect the dynamics of upwelling and relaxation in the region.”

Now that they know that the body size of PCFG gray whales is declining, researchers say they have many new questions about the downstream consequences of this decline and the factors that might be contributing to it.

“We are entering our ninth season in the field studying this PCFG subgroup,” Bierlich said. “This is a powerful dataset that allows us to detect changes in body condition each year, so we are now looking at the environmental drivers of these changes.”

Other co-authors on the paper were Lisa Hildebrand, Clara Bird and Alejandro Ajó of OSU and Leslie New of Ursinus College in Pennsylvania.