Gray whales off the Pacific Northwest are shrinking

Gray whales that spend their summers feeding in the shallow waters off the Pacific Northwest coast have undergone a significant decline in body length since the beginning of the millennium.

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

“This could be an early warning sign that the abundance of this population is starting to decline, or is not healthy,” said K.C. Bierlich, co-author on the study published in Global Change Biology and an assistant professor at Oregon State University’s Marine Mammal Institute. “And whales are considered ecosystem sentinels, so if the whale population isn’t doing well, that might say a lot about the environment itself.”

The researchers looked at the Pacific Coast Feeding Group, a small subset of about 200 gray whales (Eschrichtius robustus) within the larger Eastern North Pacific (ENP) population of around 14,500. This subgroup stays closer to shore along the Oregon coast, feeding in shallower, warmer waters than the Arctic seas where the bulk of the gray whale population spends most of the year.

Recent studies have shown that whales in this subgroup are smaller and in overall worse body condition than their ENP counterparts. The current study reveals this trend has been going on for decades.

The Marine Mammal Institute’s Geospatial Ecology of Marine Megafauna 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 with known or estimated age, researchers determined that a full-grown gray whale born in 2020 is expected to reach an adult body length that is 1.65 meters shorter than a gray whale born prior to 2000. For these gray whales, which grow to be up to 12.5 meters, that accounts for a loss of more than 13% of their average total length.

“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 smaller at weaning age may be unable to cope with 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 them being smaller, there are questions of how effectively these PCFG gray whales can store and allocate energy toward growing and maintaining their health. Importantly, are they able to put enough energy toward reproduction and keep the population growing?” Bierlich said.

Scarring on PCFG whales from boat strikes and fishing gear entanglement makes the team concerned that smaller body size with lower energy reserves may make the whales less resilient to injuries.

The researchers 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 sweeps deep nutrients to shallower regions, while relaxation periods then allow those nutrients to remain in shallower areas. In those areas, light allows for growth of plankton and other tiny 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 declined concurrently with changes in the balance between upwelling and relaxation, Pirotta said.

“Climate change is affecting the oceanography of the Northeast Pacific through changes in wind patterns and water temperature,” he said. “These factors and others affect the dynamics of upwelling and relaxation in the area.”

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

“We’re heading into our ninth field season studying this PCFG subgroup,” Bierlich said. “This is a powerful dataset that allows us to detect changes in body condition each year, so now we’re examining the environmental drivers of those changes.”