Gray whale stranding records in Mexico during the 2020 winter breeding season
Martinez-Aguilar, S, Casanovas-Gamba, P, Farriols-Garcia, M., et al
International Whaling Commission
While the occasional death and stranding of gray whales (Eschrichtius robustus) occur as part of their life cycle, sometimes unusual mortality events occur (UME) when; gray whale mortalities increase above an average annual rate. In the spring and summer of 2019, 215 gray whales stranded along the North Pacific Coast of North America, prompting the U.S. National Oceanic and Atmospheric Administration to declare a gray whale UME (NOAA 2020). Examination of some of the stranded whales suggested that a decline in body condition resulting from nutritional stress (lack of sufficient food) may have contributed to the increase in gray whale mortality by starvation. Gray whale stranding records collected in Mexico between December 2019 and April 2020 indicated that at least 87 gray whales stranded along the Pacific coast of Baja California and the Gulf of California, Mexico. The majority of the dead whales (79.3%) were encountered in Ojo de Liebre lagoon (LOL) and the surrounding areas. Twenty-three of these were female gray whales, 36 were males, and 18 were of undetermined sex. The age classes of the dead whales were: 28 adults, 27 subadults, 18 yearling whales, 13 calves, and 1 of unknown age. Compared to mortalities reported in Mexico in the previous year (2019), the number of stranded females decreased from 52 to 23, but the number of stranded male whales doubled from 18 to 36. The number of unidentified animals increased from 11 to 28 due to the advanced decomposition and position of the whales when they were discovered on the beach in 2020. The number of stranded calves increased from 2 in 2019 to 13 in 2020, while the number of stranded yearling whales decreased from 26 in 2019 to 18 in 2020. The number of stranded subadult and adult whales were 51 whales in 2019 and 55 in 2020. The gray whale strandings in 2020 appear to be related to a decline in body condition as numerous whales observed in the gray whales' winter aggregation areas and breeding lagoons appeared to be "emaciated" and "skinny".