Does climate change kill gray whales in the Pacific?

Since the beginning of 2019 more and more gray whales along the west coast of North America from Mexico through Alaska have been getting stranded and dying. According to NOAA data, so far there have been 259 gray whale strandings in the United States and 251 occurred in Mexico. Many others likely died and sank to the ocean floor.

Two remaining populations

Once common throughout the Northern Hemisphere, gray whales are now only found in the North Pacific Ocean in two extant populations: one in the eastern and another in the western North Pacific. 

As a matter of fact, commercial whaling brought to near extinction both of these populations. To protect whales from over-exploitation international conservation measures were enacted in the 1930s and 1940s and in the mid-1980s the International Whaling Commission instituted a moratorium on commercial whaling. Currently, all gray whale stocks are protected under the Marine Mammal Protection Act. The western population is very small and listed as endangered under the ESA and depleted under the Marine Mammal Protection Act. The eastern population in its turn used to be listed as endangered too under the Endangered Species Act but in 1994 was delisted due to recovery. 

An epic journey

Each year, gray whales of the eastern North Pacific Ocean make an epic 16,000 to 23,000-kilometre migration between their Arctic summer feeding grounds in the Bering and Chukchi Seas and their winter calving grounds in the warm lagoons of Baja California in Mexico. 

Gray whales play an important role in the Arctic ecosystem due to their unique style of bottom-feeding. Thus they spend summers in the Arctic waters between Alaska and Siberia scooping up mouthfuls of mud from the ocean bottom and filtering out crustaceans and tubeworms with their baleen. The resulting gigantic mud plumes shuffle large volumes of nutrients, thus enriching life on the seafloor and bringing bottom-dwelling crustaceans to the surface for seabirds to feed on.

Unusual mortality event

Since the beginning of 2019, however, elevated gray whale strandings have occurred along gray whales’ migration route from Alaska to Mexico. Hence by the end of May 2019, NOAA Fisheries reported 70 dead gray whales, later revising these estimates to 149 strandings. That’s 11 times that of the 18-year average of roughly 15 strandings annually. Thus survey from 2016 estimated the population at 26,960. After two years of elevated strandings in 2021 scientists recorded 24 percent decline to 20,580.

This event has been declared an Unusual Mortality Event (UME). Under the Marine Mammal Protection Act, an unusual mortality event (UME) is defined as unexpected stranding that involves a significant die-off of any marine mammal population and demands immediate response.

Starvation hypothesis

Full or partial necropsy examinations were conducted on a subset of the whales. These findings have shown evidence of emaciation. Besides, researchers in Laguna San Ignacio (LSI) have observed a lot of skinny, undernourished whales since the start of the UME. The trend of declining body condition coincided with a drop in the number of mother-calf pairs, indicating a reduction in the reproductive rate of female gray whales. 

Even though studies suggest that the suppressed survival and reproductive rates of gray whales were caused by starvation, the underlying factors behind their poor body condition remain inconclusive. Since gray whales arrive on their Mexican breeding grounds already malnourished the decline must have occurred either during the previous feeding season or during the southbound migration. Hence the most probable explanation for the current UME lies in the availability of prey on whales’ main feeding grounds.

As a consequence, the starvation hypothesis has shifted scientists’ focus to Alaska’s Chukchi and Bering Seas, where whales spend summer and feed on bottom-dwelling shrimp-like amphipods, replenishing their stores of blubber for the migration back south.

Warming Arctic

Many ecologists point to the conditions in and around a rapidly changing Arctic Ocean. They are now wondering if a threshold has been passed for the Arctic, forever changing the prey and the predators that depend on them.

The Arctic is getting warmer. With solid ice cover, springtime brings algae blooms that feed the benthic organisms on the seafloor. Less sea ice means less growth of algae beneath it. Gray whales feed primarily on amphipods, these small bottom-dwelling organisms depending on the algae. Hence a reduction in algae can limit the amount of food available for the whales.

Another possibility is that grey whales might be at the height of the population that their habitat can sustain, known as the species’ carrying capacity. This would imply more competition for food. Paired with some other factors causing a decline in available prey – such as changes in the Arctic – the situation can result in whales’ starvation and death.

Robust species

It’s a troubling turn of events for gray whales, but gray whales aren’t called robustus (Eschrichtius robustus) for nothing. Fewer than 2,000 gray whales existed in the early 20th century and yet they rebounded after being hunted almost to extinction. International bans on commercial whaling helped the species recover. Furthermore, whales also suffered another die-off in 1999 and 2000. Then, just like today, gray whales stranded up and down the coast, and scientists reported a 23 percent drop – from 21,000 whales in 1997 to 16,000 in 2000.  Yet the whale population didn’t just recover after that event, it boomed, reaching 27,000 individuals in 2016.

In recent years, more whales have been taking a hundred miles detour off their migratory path to feed on alternative food source as ghost shrimp in the tidelands of Puget Sound. They’re known locally as “the Sounders” and most often seen near Whidbey Island.  A small tribe of whales seems to stop the southern migration altogether. It’s additional evidence of how adaptive gray whales are.