Pacific coast gray whales have declined by 13% over the past 20-30 years, study finds

A new study from Oregon State University found that gray whales, which spend the summer feeding in shallow waters off the Pacific Northwest coast, have experienced a significant decline in body length since 2000.

The smaller size could have serious 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, scientists say.

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

The study, published in The biology of global changelooked at the Pacific Coast Feeding Group (PCFG), a small subgroup of about 200 gray whales within the larger Eastern North Pacific (ENP) population of about 14,500. This subgroup stays closer to shore along the Oregon coast and feeds in shallower, warmer waters than in 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 in overall poorer body condition than their ENP counterparts. The current study shows that they have been shrinking in recent decades.

The Marine Mammal Institute’s Geospatial Ecology of Marine Megafauna (GEMM) Laboratory 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 with known or estimated ages, the researchers found that an adult gray whale born in 2020 should reach an adult body length that is 1.65 meters shorter than a gray whale born before 2000. For PCFG gray whales, which grow to be 38-41 feet long at full maturity, this represents a loss of more than 13% of their total length.

If the same trend occurred in humans, it would be like the height of the average American woman shrinking from 5 feet, 4 inches to 4 feet, 8 inches in height 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 on the animals and on the community they’re part of.”

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

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

“Because they are smaller, questions arise about how efficiently these PCFG gray whales can store and allocate energy to grow and maintain their health. Importantly, are they able to put enough energy into reproduction and keep the population growing?” Bierlich said.

Scarring on PCFG whales from ship impacts and entanglement in team fishing gear also raises concerns that a smaller body size with lower energy reserves may make the whales less resistant 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 transports nutrients from deeper to shallower areas, while relaxation periods then allow those nutrients to remain in shallower areas where light allows plankton and other tiny organisms, including prey for gray whales, to grow.

“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 size of the whales declined concurrently 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 we know in general 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 the body size of PCFG gray whales is declining, scientists say they have a lot of new questions about the downstream consequences of this decline and the factors that could be contributing to it.

“We’re going into our ninth field season studying this subset of PCFG,” Bierlich said. “This is a powerful dataset that allows us to detect changes in body condition every year, so we are now investigating the environmental factors that influence 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.