New Publication: Poor body condition associated with gray whale mortality event

A large number of gray whales are starving and dying in the eastern North Pacific

It’s mid-January 2021, and the first gray whales 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 in the journal Marine Ecology Progress Series, led by Aarhus University researcher Dr Fredrik Christiansen, suggest that starvation is contributing to these mortalities.

In 2017, Dr Christiansen from Aarhus University, and Professor Lars Bejder from the University of Hawai’i at Manoa, began a collaboration with LSIESP researchers 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.

In their second year of sampling, the researchers found a marked decline in the body condition of juvenile and adult gray whales visiting Laguna San Ignacio (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. 

In January 2021 Stewart and Weller (members of the NOAA Gray Whale UME Working Group) reported that the Eastern North Pacific gray whale population has declined 23.7% from an estimated 26,930 in 2015/2016 to 20,580 whales in 2019/2020. 

Three adult gray whales photographed between 2017-2019 in Laguna San Ignacio in Mexico, showing the poorer body condition of whales in 2018 and 2019. Photos: Fredrik Christiansen (left), Fabian Rodríguez-González (center) and Hunter Warick (right).

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.

Read the entire Christiansen et al. article Poor body condition associated with an unusual mortality event in gray whales’ in: Marine Ecology Progress Series 658: 237-252’ here…

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