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MEPS prepress abstract   -  DOI:

Poor body condition associated with an unusual mortality event in gray whales

Fredrik Christiansen*, Fabian Rodríguez-González, Sergio Martínez-Aguilar, Jorge Urbán, Steven Swartz, Hunter Warick, Fabien Vivier, Lars Bejder

*Corresponding author:

ABSTRACT: The eastern North Pacific gray whale Eschrichtius robustus experienced an ‘unusual mortality event’ (UME) in 2019–2020, with 378 whales found dead along the Pacific coasts of Mexico, USA and Canada. A similar UME in 1999–2000 was speculated to have been caused by starvation, however body condition data were not available to test this. Between 2017 and 2019, we used unmanned aerial vehicles (drones) and photogrammetry methods to measure the body condition of gray whales in San Ignacio Lagoon, Mexico. The body condition was calculated from the residual of the relationship between body volume and length. The body condition of gray whales was significantly lower in 2018 (–11.1%, SE = 1.74, n = 531) and 2019 (–9.7%, SE = 1.76, n=628) compared to 2017 (n = 59) for all reproductive classes (calves, juveniles, adults and lactating females). Overall, lactating females were in good body condition. The reduction in body condition of whales in 2018–2019 is unlikely to have affected their survival, but could have reduced their reproductive rate by prolonging the post-weaning recovery time. This could explain the low number of mother–calf pairs observed in the San Ignacio Lagoon in 2018 and 2019. For juveniles and adults, who arrived in the lagoons with less energy reserves, their reduced body condition could be close to their survival threshold. This could explain the high proportion of juveniles and adults among the stranded dead whales in 2019–2020. Although the underlying cause of the reduction in gray whale body condition is unknown, starvation likely contributed to the 2019–2020 UME.