Gray whales that forage on the Pacific Coast are smaller than expected

According to a new study led by Oregon State University, both male and female gray whales that spend their summers feeding off the coast of Oregon – known as the Pacific Coast Feeding Group – are smaller than their counterparts that travel north to the Arctic to forage (the Eastern North Pacific Group). 

The investigation revealed that females are three feet (about one meter) and the males 1.5 feet (half a meter) shorter on average than those in the Eastern North Pacific population. These findings raise new questions about the health, behavior, and management of whales in the Pacific Coast Feeding Group.

Additional size differences

“That is a significant difference in size. We also found that the Pacific Coast Feeding Group whales had slightly smaller skulls and flukes,” said study lead author K.C. Bierlich, a postdoctoral fellow in Geospatial Ecology at OSU. “It’s a surprising finding – we have not thought about these whales being different in this way before.”

Focus of the study

Most of the approximately 16,000 gray whales in the Eastern North Pacific population travel past Oregon’s coast as they migrate south to their winter breeding grounds in Mexico each year between October and December, as well as during their return in March to their feeding grounds in the Bering and Chukchi seas between Alaska and Russia. 

By contrast, whales in the Pacific Coast Feeding Group – which are also known as Oregon’s “southern resident” gray whales and number only 212 – spend their summer months feeding in the coastal waters of Oregon, northern California, Washington, and southern Canada. 

How the research was conducted

Due to their unique ecology and high exposure to boat traffic, noise, and pollution, the study authors have been investigating the health and behavior of these whales since 2015.

The team used photographs to identify individuals, nets to capture fecal samples, and drones to capture aerial images in order to measure the body size of individual whales. These noninvasive methods have already revealed a lot of information about the whales’ health and environment.

“What is really unique about our data on the Pacific Coast Feeding Group is that we know these whales really well. We see the same whales every year and can identify individuals based on unique markings, use sighting history from photo identification to estimate their age, collect fecal samples to determine their sex and use drone imagery to measure their length, skull, and fluke size,” Bierlich explained. 

“A big question our research group has been debating for a while is why these whales come here instead of going farther north like the larger group.”

To compare the Pacific Coast Feeding Group and the Eastern North Pacific one, the experts used a combination of historic whaling records, data from stranded animals, aerial observations, and data collected through state-of-the-art noninvasive techniques such as drone observations. 

What the researchers discovered 

The investigation revealed that individuals belonging to both populations grow at the same rate, but reach different final lengths.

“That raises some interesting questions: Is this size difference normal for this group of whales and they are a healthy population, but just differently shaped? Or is this difference a sign that they are stressed, unhealthy or not getting enough to eat?” Bierlich pondered.

Environmental differences 

Although reduced size and length are common adaptations for animals when they need to cope with limited resources, the difference between the two whale groups could also be attributed to the particularities of the environments in which they live and forage.

“These whales live in a very shallow environment, feeding in the kelp forest near shore, so the differences could be a reflection of their environment. Their smaller body size and shorter skulls and flukes could potentially help them feed more effectively in this habitat compared to the deeper waters where the Eastern North Pacific whales feed,” Bierlich said.

Study implications 

Although further research is needed to clarify these issues, these findings will also have crucial implications for future population management, since being smaller means that these whales have less energy storage available to support reproductive processes and responses to disturbances and injuries. 

While in the United States, the Pacific Coast Feeding Group is currently managed as part of the larger Eastern North Pacific Group, the differences in size identified by the scientists could raise questions about whether the groups should better be managed separately.

“With only 212 Pacific Coast Feeding Group whales, these whales might require different management strategies compared to the 16,000 whales in the Eastern North Pacific,” Bierlich concluded.

The study is published in the journal Biology Letters.

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By Andrei Ionescu, Earth.com Staff Writer

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