Scientists have found that gray whales have died due to lack of nutrition for the third consecutive year. And this is due to a decrease in the number and biomass of amphipods, the main prey of gray whales.
As early as January, the first gray whales from the Pacific Ocean population began arriving at nesting lagoons in Baja California, Mexico. Since the beginning of their southward migration from their high-latitude feeding grounds (when the whale eats off after wintering), several cases of emaciated gray whales have been observed along their migration route.
This has raised scientists’ concern that an unusual mortality event (UME is an unexpected event during which a significant number of marine mammals die), which began in January 2019 and led to 378 confirmed gray whale deaths, and possibly many more unreported, has been going on for the third year.
Gray whales migrate annually between feeding grounds in the Bering, Chukchi, and Arctic Seas and breeding grounds from the Gulf of Southern California to the lagoons along the Pacific coast of Baja California, Mexico.
During the summer feeding season, from May to October, whales accumulate large reserves of energy, mainly in the form of fat, in order to maintain energy costs for migration and while living in breeding grounds. Adequate energy reserves are critical for the reproduction and survival of gray whales that do not feed during the migration and breeding season.
In 2017, LSIESP (Special Whale Assistance and Research Project) began to study the body condition of gray whales using drone photogrammetry. This method involves measuring the length and width of the animals from vertical photographs taken by drones, from which an estimate of the relative condition (or fatness) of individual specimens can be obtained.
As early as in the second year of sampling, researchers found a marked deterioration in the physical condition of young and adult gray whales visiting the San Ignacio Lagoon. The decline was also noticeable in 2019, at the start of the current UME. The deterioration in body condition also coincided with a decrease in the number of mother-calf pairs found in the San Ignacio Lagoon, indicating a decline in the fertility of female gray whales.
A similar UME was in 1999-2000 when 651 gray whales were found dead along the west coast of North America. During this two-year event, the gray whale population declined by about 25% from about 21,000 in 1998 to about 16,000 in 2002. It is not yet known what impact the current UME is having on the population of the eastern North Pacific.
Although the study suggests that the decline in survival and reproductive performance of gray whales during the current UME was caused by starvation, the main factors that caused this decline in body condition have not yet been identified. The fact that gray whales in 2018 and 2019 arrived at their breeding sites in Mexico already in significantly worse body condition indicates that this decline must have occurred either during the previous feeding season or during southward migration.
“It appears that large numbers of gray whales are leaving their feeding grounds already in poor condition, and by the time they complete their breeding season in Mexico, they are depleting their energy reserves and starving to death.”
Dr. Fredrik Christiansen
Thus, the most likely explanation for the current UME is the declining availability of prey in the main feeding grounds. Since the late 1980s, there has been a decline in the abundance and biomass of amphipods, the main prey for gray whales, in the central part of the Chirikov Basin, the main feeding area for gray whales in the Bering Sea. This, in turn, is believed to be caused by warming of Arctic waters as a result of natural and/or anthropogenic climate change. If so, such UMEs could become more frequent, which could lead to a decline in gray whales in the coming decades or their complete extinction.