Gray whales shrinking: A bleak warning from the Pacific coast

Gray whales, scientifically known as Eschrichtius robustus, found near the Pacific coast, have decreased by 13 percent over the last 20–30 years, reports "Global Change Biology".

Research conducted by scientists from Oregon State University found that since around 2000, the body length of gray whales foraging in the shallow waters off the northwest Pacific coast has shortened. According to the researchers, the size reduction could seriously impact these whales' health and reproductive capacity and indicate problems in their living environment.

– This may be an early warning signal that the population size of these whales is starting to decrease or that the population is not healthy – emphasized K.C. Bierlich, co-author of the study and assistant professor at the Marine Mammal Institute at OSU in Newport. – Whales are considered guardians of the ecosystem, so if the whale population is not doing well, it can say a lot about the environment itself – he added.

The research focused on the Pacific Coast Feeding Group (PCFG), a subgroup of about 200 gray whales in the larger Eastern North Pacific (ENP) population, numbering about 14,500 individuals. This subgroup stays off the coast of Oregon, foraging in shallower and warmer waters than the Arctic seas, where most of the gray whale population spends most of the year.

Recent studies conducted by OSU showed that whales in this subgroup are smaller and generally in poorer body condition than their ENP counterparts. The current study shows that in recent decades they have been getting progressively smaller.

The Geospatial Ecology of Marine Megafauna (GEMM) Lab at the Marine Mammal Institute has been studying this subgroup of gray whales since 2016 using, among other methods, drones flying over the whales to measure their size. Through photos of 130 individuals of known or estimated age from 2016–2022, researchers determined that an adult gray whale born in 2020 will reach an adult body length 1.5 meters shorter than a gray whale born before 2000. For PCFG gray whales, which reach maturity at lengths of 12 to 12.5 meters, this represents a loss of over 13 percent of their total body length.

If the same trend occurred in humans, the average height of a Canadian woman would decrease from 1.63 meters to 1.42 meters over 20 years.

Generally, size is critical for animals, emphasized Enrico Pirotta, the lead author of the study and a researcher at the University of St. Andrews in Scotland. It affects their behaviour, physiology, and life history and has cascading effects on the animals and the communities they are a part of.

As Pirotta pointed out, young whales (calves) that are smaller at the weaning age may not be able to cope with the uncertainty during the learning period of independence, which could affect survival rates. In the case of adult gray whales, one of the biggest concerns is reproductive success.

– Because they are smaller, questions arise about how effectively these PCFG whales can accumulate energy and allocate it to development and health maintenance. Importantly, can they allocate sufficient energy to reproduction and sustain population growth? – asked Bierlich.

The scars observed on PCFG whales, resulting from boat strikes and entanglements in fishing gear, are also concerns for the team, as smaller body size with lower energy reserves may make whales less resilient to injuries.

The study also analyzed ocean environmental patterns that likely regulate food availability for whales off the Pacific coast, tracking cycles of "upwelling" and "relaxation" in the ocean. Upwelling brings nutrients from deeper to shallower regions, while relaxation periods leave those nutrients in shallower areas where light allows plankton and other small organisms, including gray whale food, to thrive.

– Without a balance between upwelling and relaxation, the ecosystem may not be able to produce enough food to sustain the large sizes of these whales – said co-author Prof. Leigh Torres, the director of the GEMM lab at OSU.

As Pirotta indicated, data shows that whale size decreased simultaneously with changes in the balance between upwelling and relaxation.

– We did not specifically examine the impact of climate change on these patterns, but in general, we know that climate change affects the oceanography of the northeastern Pacific through changes in wind patterns and water temperature – said Pirotta.

Research continues to elucidate the consequences of shrinking whale bodies and the environmental factors contributing to it.