Research shows that gray whales on the Pacific coast have become 13% shorter in the past 20 to 30 years

Gray whales that spend their summers foraging in the shallow waters off the coast of the Pacific Northwest have undergone significant reductions in body size since about the year 2000, according to a new study from Oregon State University.

The smaller size could have major consequences for the health and reproductive success of the affected whales, and also raises alarm bells about the state of the food web in which they coexist, researchers say.

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

The study, published in Biology of global change, looked at the Pacific Coast Feeding Group (PCFG), a small subgroup of about 200 gray whales within the larger population of about 14,500 eastern North Pacific (ENP). This subgroup stays closer to shore along the Oregon coast and feeds in shallower, warmer waters than the Arctic seas, where most of the gray whale population spends most of the year.

Recent studies from OSU have shown that whales in this subgroup are smaller and generally in poorer body condition than their ENP counterparts. Current research shows that they have become smaller 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 over the whales to measure their size. Using images from 2016-2022 of 130 individual whales of known or estimated age, researchers determined that an adult gray whale born in 2020 is expected to reach an adult body length that is 1.65 meters (about 5 feet, 5 inches) shorter than a gray whale born before 2000. For PCFG gray whales that grow to 38-41 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 the equivalent of the average American woman’s height shrinking from 6 feet to 4 feet over the course of 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 for the animals and for the community they are part of.”

Whale calves that are smaller at weaning age may not be able to cope with the uncertainty that comes with being newly independent, which can affect survival rates, Pirotta said.

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

“Because they are smaller, there are questions about how effectively these PCFG gray whales can store and use energy for growth and maintenance of their health. Most importantly, are they able to expend sufficient energy on reproduction and population growth? to let grow?” said Bierlich.

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

The study also examined the patterns of the ocean environment that likely regulate food availability for these gray whales off the Pacific coast by tracking cycles of “upwelling” and “relaxation” in the ocean. Upwelling moves nutrients from deeper to shallower areas, while relaxation periods allow these nutrients to remain in shallower areas where light allows the growth of plankton and other small organisms, 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 of the GEMM Lab at OSU.

The data show that whale size decreased simultaneously with changes in the balance between upwelling and relaxation, Pirotta said.

“We haven’t looked specifically at how climate change is affecting these patterns, but in general we know that climate change is affecting the oceanography of the northeast Pacific Ocean through changes in wind patterns and water temperature,” he said. “And these and other factors influence the dynamics of upwelling and relaxation in the area.”

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

“We are entering our ninth field season studying this PCFG subgroup,” Bierlich said. “This is a powerful data set that allows us to detect changes in body condition every year, so now we are investigating the environmental drivers of those changes.”

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