As long as yellow school bus and weighing as much as 20 cars, the eastern North Pacific gray whale is a gentle giant often seen breaching just off the California coast. As of 2016 the population consisted of 27,000 individuals, but around two years ago unusual numbers of whales started dying off, alarming scientists.
Since 2019, gray whales have been decimated by something called an unusual mortality event, or UME. A UME is an unexpected phenomenon during which a significant number of marine mammals die. So far, this UME has resulted in 378 confirmed gray whale deaths, and possibly many more that are unrecorded. This event continues.
Scientists are not exactly sure why the whales are dying, but in a newly released study, published in the journal Marine Ecology Progress Series, researchers conclude it is likely a result of starvation due lack of prey, perhaps caused by warming Arctic waters where they feed. If that's true, the concern is that mass die-offs like this may become more frequent in the future as waters continue to heat up due to human-caused climate change.
The eastern North Pacific gray whale travels over 4,000 miles a year each way up and down the west coast of North America between feeding grounds in the Bering Sea between Alaska and Russia in summer and breeding grounds along the Baja California Sur in winter.
Right now the whales are heading south towards their breeding lagoons like Laguna San Ignacio in Baja. It is still early, but the co-author of the study, Dr. Steven Swartz, co-director of the Laguna San Ignacio Science Program (LSIESP), told CBS News that the preliminary signs are "distressing." The COVID-19 pandemic has hampered monitoring, but from the limited observations so far, there are very few calves and some whales are emaciated.
Swartz says the majority of whales will arrive later in February and March in places like Laguna San Ignacio and La Paz, Mexico, where on-site researchers are waiting to evaluate the whales. That's when they will know if this mortality event continues to get worse.
UME's may be rare and unexpected, but they are not unheard of. In 1999-2000 a UME resulted in 651 gray whales recorded dead along the west coast of North America. But the total loss was even greater. From 1998 to 2002, the gray whale population declined 25%, from about 21,000 to about 16,000 — a loss of some 5,000 whales.
Understandably, researchers are concerned that the number of deaths in this current event is undercounted as well, especially given the limited ability to monitor the migration due to COVID.
When the team is able to do their work they use a high-tech method to monitor the whales. CBS News corresponded with the lead author, Dr. Fredrik Christiansen, of Aarhus University in Denmark, and he explained their process.
Christiansen says the researchers use what is called drone photogrammetry to measure the body condition of the whales by flying a drone equipped with a high resolution camera about 20 to 30 meters above the whale when they surface to breathe. When the whales are stretched out, measures can be taken of their length and width. From that, researchers are able to calculate the volume or fatness of the whale.
In addition, the team is able to identify individual whales based on unique color patterns on their backs. They can then compare the body condition of each whale from year to year to know if the whales have gained or lost body mass.
Their results show that many of these whales are getting skinnier. The image below shows a comparison of whales photographed in 2017, 2018 and 2019, showing their diminished body condition. It's clear the whale in the most recent photo especially is not getting the nutrition it needs.
Christiansen says it appears access to food in the Bering Sea is to blame. "The fact that gray whales already arrived in poorer condition (meaning that they were most likely thinner already when leaving their feeding grounds) in Mexico in 2018 and 2019, indicate that there is less prey on the feeding grounds, or less access to prey."
Swartz says there is some chance that the population of whales has reached a point where competition for food is impacting the whales, but the most obvious explanation is related to rapid changes in the North Pacific. "The rapid change in the Arctic is disrupting normal cycles," said Swartz.
In recent years theat three times the rate of the global average, a phenomenon known as Arctic amplification, and has reached near record lows during late summer and fall. This impacts weather patterns, upwelling and something called primary production in the North Pacific Ocean.
Primary productivity is a scientific term which is a measure of biological life in the ocean. Seasonally, as the weather warms and nutrients are transported to the surface from down below due to upwelling, parts of the ocean come alive with blooms of plankton. Like clockwork, these plankton blooms draw predators from near and far. But if these blooms decrease, it threatens the life that depends on them.
According to NOAA's 2020 Arctic Report Card, primary productivity in the Bering Sea did show lower-than-average values in 2020. Most of the investigated regions showed an uptick. Over the longer term, while most Arctic regions, like Greenland, have seen an increase in primary productivity, the Bering Sea has basically flatlined.
While climate change is clearly disrupting what used to be considered normal, the researchers did not look into that aspect of the UME and it is not clear if the changes to prey or access to prey is related to climate change or some other factor. However, researchers like Swartz and Christiansen are concerned that as the climate continues to warm, the challenges will continue to mount for gray whales and other marine mammals.
"Given the, I am indeed concerned that the prey availability for gray whales will be negatively affected, which no doubt would have strong effects on the population size of the gray whale," said Christiansen. "Since gray whales feed on a variety of prey in different feeding grounds, I think that some parts of the population will be more resilient to such changes in the Arctic, as long as their localized prey does not change. For the population as a whole though, I do fear that the population will decline to the new environmental carrying capacity."
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