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.