Pacific coast gray whales have become 13% shorter in past 20-30 years, study finds

Gray whales that spend their summer feeding in shallow waters off the Pacific Northwest coast have suffered a significant decline in body length since 2000, a new Oregon State University study has found.

The smaller size can have major consequences for the health and reproductive success of affected whales, and also raises alarm bells about the state of the food web in which they coexist, the 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 an assistant professor at OSU’s Marine Mammal Institute in Newport. “And whales are considered guardians of the ecosystem, so if the whale population isn’t doing well, that can say a lot about the environment itself.”

The study, published in The Biology of Global Change, looked at the Pacific Coast Feeding Group (PCFG), a small subset of about 200 gray whales within the larger Eastern North Pacific (ENP) population of about 14,500. This subspecies stays closer to shore along the Oregon coast, feeding 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 subspecies are smaller and in overall worse body condition than their ENP counterparts. The current study finds that they have become smaller in recent decades.

The Marine Mammal Institute’s Geospatial Ecology of Marine Megafauna Laboratory (GEMM) has been studying this subpopulation 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, the researchers determined that an adult gray whale in 2020 is expected to reach an adult body length of 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 like the average American woman’s height shrinking from 5 feet, 4 inches to 4 feet, 8 inches tall over 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’re a part of.”

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

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

“With smaller ones, there are questions about how effectively these PCFG gray whales can store and allocate energy towards growth and maintaining their health. Importantly, are they able to put enough energy towards reproduction and keep the population growing?” Bierlich said.

Injuries to PCFG whales from boat strikes and fishing gear entanglement also lead the team to worry that smaller body size with lower energy reserves may make the whales less resilient to injury.

The study also looked at ocean environmental patterns that likely regulate food availability for these gray whales off the Pacific coast by following cycles of “rise” and “relaxation” in the ocean. Upwelling sweeps nutrients from deeper to shallower regions, while periods of relaxation then allow those nutrients to remain in shallower areas where light allows the growth of plankton and other small organisms, including prey. 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 the whale’s size decreased concurrently with changes in the balance between lifting and relaxing, 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 through changes in wind patterns and water temperature,” he said. “And these factors and others affect the dynamics of upwelling and relaxation in the area.”

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

“We are going into our ninth season in the field studying this PCFG subset,” Bierlich said. “This is a powerful data set 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 at OSU and Leslie New at Ursinus College in Pennsylvania.