Pacific coast grey whales grow 13% shorter in the past 20-30 years

Aerial drone imagery of grey whales (Eschrichtius robustus) along the Pacific coast of North America reveals they have become significantly shorter in a little more than two decades.

Researchers say this finding adds to the mounting evidence that body size is shrinking in several marine populations, associated with climate change and other anthropogenic stressors.

“This could be an early warning sign that the abundance of this population is starting to decline, or is not healthy,” says KC Bierlich, an assistant professor at Oregan State University’s (OSU) Marine Mammal Institute in the US, and co-author of the study in the journal Global Change Biology.

“Whales are considered ecosystem sentinels, so if the whale population isn’t doing well, that might say a lot about the environment itself.”

The paper investigated a small subset of about 200 grey whales known as the Pacific Coast Feeding Group (PCFG). Unlike the majority of grey whale population, which spend most of the year in colder Arctic waters, the PCFG remains feeding along the shallower, warmer Oregon coast.

Researchers used images of 130 individual whales with known or estimated age, captured via drones from 2016-2022, to calculate the decline in length. They found that a grey whale born in 2020 is expected to reach a full-grown adult body length 1.65 meters shorter than a whale born in the 1980s.  

The researchers say that for PCFG grey whales that grow to be about 12-12.5 meters long at full maturity, that accounts for a loss of more than 13% of their total length.

Overhead photograph of a  grey whale close to the surface of the water. There is a larger yellow outline superimposed over the image.
This schematic shows the difference in length between a PCFG gray whale born in 2020 vs one born prior to the year 2000. Credit: Courtesy K.C. Bierlich, OSU Marine Mammal Institute

“In general, size is critical for animals,” says lead author Enrico Pirotta, a quantitative ecologist at the University of St Andrews in Scotland.

“It affects their behaviour, their physiology, their life history, and it has cascading effects for the animals and for the community they’re a part of.”

Pirotta says 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.

For adults, the findings raise questions about how effectively smaller whales can store and allocate energy toward growing and maintaining their health, and reproduction to keep the population growing.

Whale size declined alongside changes in the balance between cycles of “upwelling” and “relaxation” in the ocean. Upwelling events brings cold, nutrient-rich waters from deeper regions to the surface. These are followed by a period, which allows for the growth of plankton and the tiny crustaceans which grey whales eat.

“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,” says co-author Leigh Torres, a marine ecologist and director of the Geospatial Ecology of Marine Megafauna Laboratory at OSU.

“And these factors and others affect the dynamics of upwelling and relaxation in the area.”

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