Poor Body Condition Contributes to 2019 Gray Whale UME

PRELIMINARY FINDINGS SUGGEST POOR BODY CONDITION CONTRIBUTED TO 2019 UNUSUAL MORTALITY EVENT 

Laguna San Ignacio Ecosystem Science Program (LSIESP) collaborator Dr. Fredrik Christiansen and colleagues conducted a preliminary analysis of gray whale (Eschrichtius robustus) body condition that suggests the whales’ condition decreased from 2017 to 2019 in Laguna San Ignacio, Baja California Sur, Mexico. These findings are consistent with previous observations by LSIESP researchers of declining calf counts, increasing numbers of “skinny” (emaciated) whales, and delayed arrival and early departures from the lagoon during the same time period, and corroborate the suspicion that declining condition likely contributed to the Unusual Mortality Event (UME) of this past spring and summer along the Pacific coast of North America; as of September 30, 2019, 212 gray whales were found dead along the west coast of Mexico, the United States and Canada, representing a significant increase in the 18-yr average gray whale stranding rate along the whales’ North American migration route.
To determine if nutritional stress could be underlying the 2019 unusual mortality event, LSIESP researchers used unmanned aerial vehicles (UAV Drones) and photogrammetry methods to compare the body condition of Gray whales in Laguna San Ignacio during the 2017, 2018, and 2019 winter breeding seasons. To calculate the whales’ body condition, the body length and width of each whale was measured from the aerial photographs, and from these their volume was estimated for each reproductive class: lactating females, calves, juveniles, and adults in each year.
The body “condition value” of individual whales was then calculated from the relationship between body “volume” and body “length.”  The body “condition value” for any whale above the average values (red regression line) for the population was considered to be in a relatively good condition, while any values below the average values was considered to be in a relatively poor condition.

A total of 1,246 measurements were obtained, comprising 266 lactating females, 322 calves, 173 juveniles, and 485 adults. Of these, 75 were recorded in 2017, 531 in 2018 and 640 in 2019. While the body condition of Gray whales in Laguna San Ignacio varied between reproductive classes (calves, juveniles, adults and lactating females) and years, this preliminary analysis revealed  a significant difference (a decline) in the body condition among juveniles and adults between 2018 and 2019, with whales in 2019 being in relatively poorer condition.  The difference in body condition however, was only visible for juvenile and adult (non-lactating) whales, but not in lactating females or calves; this makes biological sense, since females that give birth and nurse their calves must already be in sufficiently good condition to complete gestation and the long migration to Laguna San Ignacio.
This is the first study to quantitatively describe the cross-sectional body shape of Gray whales in relation to estimating body condition. These preliminary findings suggest that Gray whales in Laguna San Ignacio were in poorer body condition in 2019 than in the previous two years, which coincides with the 2019 UME, and is consistent with previous observations of declining numbers of calves, increasing numbers of “skinny” whales, and late arrival and early departure of whales from the breeding/calving lagoons of Baja California.
In 2020, LSIESP researchers are planning to continue gathering photogrammetric data for gray whales from UAV Drones, along with boat based abundance and distribution counts, and photo-identification surveys to determine which individual whales return to Laguna San Ignacio, their length of stay in this lagoon, and whether known female whales are continuing to produce healthy calves.
The ongoing gray whale research in Laguna San Ignacio is supported by grants and donations from our supporters. If you haven’t already, please consider becoming a recurring contributor to our program, by making regular monthly donations to support the research, our student researchers, and to better understand and conserve gray whales. Please visit our website and donate your support today. Thank you!
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