Gray whales face food shortages as the Arctic warms

Disappearing sea ice appears to be putting gray whales in the eastern North Pacific under pressure once again this year: many animals are emaciated, and the number of newborn calves is at a record low.

Every year, gray whales (Eschrichtius robustus) in the eastern North Pacific undertake one of the longest migrations in the animal kingdom. In spring, they travel north from the lagoons off Mexico’s Baja California Peninsula to their Arctic feeding grounds. In autumn, they return south to mate and give birth. Depending on how far north they forage, the whales cover more than 15,000 kilometers each year – an immense journey powered almost entirely by their fat reserves.

After being nearly wiped out by industrial whaling in the 20th century, the gray whale population recovered relatively quickly. By the early 1980s, numbers had already climbed back to around 25,000 individuals – possibly as many as before whaling began, or even more.

But since then, there have been two dramatic population declines. These so-called “Unusual Mortality Events” (UMEs) – in 1999/2000 and 2019–2023 – were triggered by unfavorable environmental conditions in the Arctic. As a result, significantly fewer prey animals were available to gray whales, as shown in a 2023 study published in the journal Science and led by Dr. Josh Stewart from the Marine Mammal Institute at Oregon State University.

The decisive factor is the sea ice situation: In years with significantly less ice cover, fewer sea ice algae reach the seafloor – a key food source for benthic crustaceans such as amphipods, which in turn are the preferred prey of gray whales.

Gray whales, which belong to the baleen whale group, typically stay in coastal waters and are known for their unique bottom-feeding behavior: they roll onto their sides to stir up sediments and filter out the prey living within.

The gray whale population in the eastern North Pacific, which dropped to fewer than 15,000 individuals following the multi-year UME and was estimated to have rebounded to around 19,000 by spring 2024, is divided into two groups: a small subgroup known as the Pacific Coast Feeding Group remains year-round along the west coast of the U.S. and Canada, foraging in coastal waters from California to British Columbia. All other gray whales belong to the Arctic Feeding Group, which migrates annually between Mexico and their Arctic feeding grounds in the Chukchi, Bering, and Beaufort Seas.

Signs of a dramatic new decline

The last unusual mass die-off from 2019 to 2023 is not so long ago. Yet there are growing signs that 2025 could once again become a critical year for gray whales. In April, the Lincoln Chronicle reported an unusually high number of emaciated animals along the U.S. West Coast.

“The numbers so far are the lowest ever and the whales we are seeing are extremely emaciated,” Alisa Schulman-Janiger, who leads a Pacific gray whale census initiative near Los Angeles, told the Lincoln Chronicle. “They have bulging ribs with shoulder blades and vertebrae visible even from shore. It’s really just horrific.”

High numbers of stranded whales have been reported from Mexico, as Dr. Stewart told polarjournal.net in an email – “as high as during the peak of the 2019 die-off”.

Contrary to Stewart’s expectations, strandings along the U.S. West Coast have fortunately not been unusually high so far. However, more gray whales than usual have been sighted in San Francisco Bay — some of which collided with ships — suggesting that the animals are in poor condition and searching for food.

Calf sightings are also at a historic low, according to Dr. Stewart: “The northbound counts are a good indicator of reproductive output. And yes, those numbers were the lowest on record, which suggests reproductive output has continued to be extremely depressed.” However, he notes that it is still too early to make a reliable statement about this year’s population size and mortality.

According to the researcher, the reason for the poor body condition of gray whales and the very low reproductive success lies in the low sea ice coverage in their Arctic feeding grounds — and the resulting lack of prey.

“Body condition is a very sensitive indicator of population status both in gray whales and other baleen whales. We know that during major mortality events, both birth rates and body condition decline precipitously. So tracking body condition gives us a pretty good idea of the population status. And poor condition is an indicator that the population remains food limited and is perhaps likely to continue declining,” explains Dr. Stewart.

When asked whether gray whales can adapt to changing environmental conditions, he replies: “This is the big question. We know that gray whales can feed on pelagic prey (e.g. krill, mysids, occasionally small fishes), but they’re not adapted to do this as well as engulfment feeders (e.g. humpback whales) or skim feeders (e.g. bowhead whales). They’re evolved to feed on benthic prey, so they’ve lost most of their throat pleats and have short baleen plates, which makes it harder to feed efficiently on pelagic prey.”

Indeed, during past mortality events, gray whales were more frequently observed foraging in unusual areas or feeding on pelagic prey such as krill and mysids—possibly indicating that alternative prey and habitats may offer at least short-term benefits. However, the Pacific Coast Feeding Group, which relies on exactly such alternative habitats, consists of only about 200 individuals and, according to Stewart, tends to be smaller and leaner than their Arctic-feeding counterparts—suggesting that these adaptive strategies may offer only limited success.

Stewart also considers a northward expansion of their range unlikely. The productive, shallow shelf regions end abruptly in the far north—beyond that, the seafloor drops off steeply, and the benthic feeding grounds typical for gray whales are absent. Simply “migrating further north” as the Arctic warms does not appear to be a viable solution.

Whether and how gray whales can adapt to the new environmental conditions remains uncertain—and likely depends largely on how rapidly Arctic warming continues.

The past has shown that the population was able to recover even after dramatic losses. One can only hope that gray whales will also find a way to withstand the impacts of climate change.