It’s mid-January 2021, and the first gray whales (Eschrichtius robustus) from the eastern North Pacific population have started to arrive in the breeding lagoons in Baja California, Mexico. Since the start of their southbound migration from their high latitude feeding grounds, several sightings of emaciated gray whales have already been reported along their migration route. This has raised concern among scientists that the unusual mortality event (UME, an unexpected phenomenon during which a significant number of a marine mammal population dies), that started in January 2019, and which so far has resulted in 378 confirmed gray whale deaths (and possibly many more unrecorded), is entering its third year. Although the underlying cause of the current gray whale UME is still undetermined, a study published this week by an international research team (Aarhus University, Denmark, Universidad Autónoma de Baja California Sur, Mexico, Laguna San Ignacio Ecosystem Science Program, Mexico, Murdoch University, Australia and University of Hawai’i at Manoa, USA), in the journal Marine Ecology Progress Series, led by Aarhus University researcher Dr Fredrik Christiansen, suggest that starvation is contributing to these mortalities.
Gray whales undertake annual migrations between feeding grounds in the Bering, Chukchi, and Arctic Seas, and breeding grounds from the Southern California Bight to lagoons along the Pacific coast of Baja California, Mexico. During the summer feeding season, between May and October, the whales build up large amount of energy reserves, mainly in the form of blubber, to support the energetic costs of migration and while residing on the breeding grounds. Sufficient energy reserves is crucial for the reproduction and survival of gray whales, which do not feed during the migration and breeding season.
Mary Lou Jones and Steven Swartz (co-author on the current paper) conducted the first research and monitoring of the gray whales from 1977 to 1982 in Laguna San Ignacio (LSI) in Baja California Sur, Mexico. In 2006 with their colleague Dr Jorge Urban (co-author on the current paper) they initiated the Laguna San Ignacio Ecosystem Science Program (LSIESP), a project of the Ocean Foundation, which is a partnership with the Universidad Autónoma de Baja California Sur (UABCS) and international collaborators. In 2017, Dr Christiansen from Aarhus University, and Professor Lars Bejder from the Marine Mammal Research Program at the Hawai’i Institute of Marine Biology at the University of Hawai’i at Manoa, joined LSIESP to study the body condition of gray whales with the use of drone photogrammetry. The technique involves measuring the body length and width of gray whales from vertical photographs taken by drones above the whales, from which a measure of relative body condition (or fatness) of individual whales can be obtained.
Already in the second year of sampling, the researchers found a marked decline in the body condition of juvenile and adult gray whales visiting LSI. The decline was also visible in 2019, at the start of the current UME. The decline in body condition also coincided with a drop in the number of mother-calf pairs sighted in LSI, which indicated a reduction in the reproductive rate of female gray whales. A similar UME occurred in 1999-2000, when 651 gray whales were recorded dead along the west coast of North America. During that event, which only lasted two years, the gray whale population declined from ~21,000 animals in 1998 to ~16,000 in 2002, which is equivalent to a loss of nearly 25% of the population. It is yet unknown what effects the current UME is having on the eastern North Pacific population.
While the study by Dr Christiansen and colleagues suggests that the suppressed survival and reproductive rates of gray whale during the current UME was caused by starvation, the underlying factors that caused this reduction in body condition has not yet been determined. The fact that gray whales in 2018 and 2019 arrived on their Mexican breeding grounds already in significantly poorer body condition, indicates that this decline must have occurred either during the previous feeding season and/or during the southbound migration. A decline in prey availability on the main feeding grounds is hence the most probably explanation for the current UME. 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 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 human-induced climate change. If that is the case, UMEs like this one might become more frequent, which could result in a decline in gray whale numbers in coming decades.
As the world keeps struggling with the Covid-19 pandemic, LSIESP researchers are preparing for the 2021 field season in LSI to hopefully get one step closer to understanding the full extent of the current gray whale UME.
by Fredrik Christiansen, Fabian Rodríguez-González, Sergio Martínez-Aguilar, Jorge Urbán, Steve Swartz, Hunter Warick, Fabien Vivier and Lars Bejder.
Twitter: @FChristiansen83 , @AIAS_dk, @MMRP_UH, @fvivier89, @jurbanr, @FabianRguez94
Instagram: @fredrik_christiansen, @MMRP_UH