Eastern North Pacific gray whale (Eschrichtius robustus) unusual mortality event, 1999-2000

Abstract

In 1999, the number of gray whale (Eschrichtius robustus) strandings documented along the west coast of North America increased to approximately seven times the annual mean of 41 animals reported between 1995 and 1998. The unusually high number (283) of stranded whales in 1999 prompted the National Marine Fisheries Service to consult the Working Group on Marine Mammal Unusual Mortality Events in July 1999. The Working Group then formally designated the strandings as an “unusual mortality event.” The number of stranded animals remained high in 2000, with 368 carcasses reported (a nine-fold increase over the 1995-98 average). In 2001 and 2002, however, total strandings decreased to 21 and 26 animals, respectively. Most of the strandings in 1999 and 2000 occurred in Mexican waters during the winter season. Increases in all regions except Oregon were significant. The greatest proportionate increases occurred in Alaska, resulting in part from an increase in survey effort. Only three (0.5%) of the 651 animals that stranded in 1999 and 2000 were examined thoroughly to determine cause of death. In 1999 and 2000, more adults and subadults stranded compared to 1996-98, when calf strandings were more common. Lipid content of blubber was low in stranded animals, but lipid composition was altered by degree of carcass decomposition. Several factors have been considered as possible causes for the high number of gray whale strandings reported in 1999 and 2000, including starvation, chemical contaminants, biotoxins, infectious diseases, parasites, fisheries interactions and ship strikes, variability in detection effort and reporting, and affects of winds and currents on carcass deposition. While the emaciated condition of many of the stranded whales supports the idea that starvation could be a significant contributing factor in these mortalities, the underlying cause of starvation during this event is unknown. As some animals were in good to fair nutritional condition, not all strandings can logically be linked to food resource limitation and starvation.


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