Abstract

In 1999 and 2000, the number of gray whale (Eschrichtius robustus) strandings from Mexico to Alaska increased to seven times the mean annual stranding between 1995 and 1998. Two hundred and eight-five whale carcasses were reported in 1999 and 377 in 2000. The previous high was 89 animals in the 1980s. By 2001 strandings decreased again to 20 whales in total. The majority of the dead whales occurred in the breeding lagoons in Baja California, Mexico. In the U.S., mortality occurred throughout the migration route, with a cluster of animals (about 30) found floating dead in San Francisco Bay in April and May 2000. The emaciated condition of many of the carcasses, coupled with the increase in the total gray whale population, lead to speculation that the whale population has reached carrying capacity and is starving². Although this explanation is possible, insufficient data exist to support or refute this contention.

Only limited data on stranded animals are available. Carcass examination often was limited because of inaccessibility or stage of decomposition of the carcasses. In 1999 and 2000, adult animals were the most common age class to strand, whereas in previous years, calves were the most common age class observed. Blubber thickness in animals examined ranged from 4.6 to 17 cm. Only three animals that stranded in the U.S. received complete post mortem examinations. These three animals stranded alive in California, and were euthanized due to poor prognosis based upon their emaciated condition and prolonged stranding. All three were young emaciated animals, one had severe intestinal parasitism with a granulomatous enteritis¹, one had histological changes in the cerebrum suggestive of viral encephalitis, the third had the biotoxin, domoic acid, in blood, urine and feces, suggesting intoxication, as well as transmural abscesses in the gut-associated lymphoid tissue. All three animals were emaciated, with blubber thickness over the sternum between 7 and 10 cm. Whales that were found dead in San Francisco Bay were mostly observed in or adjacent to the main shipping channels. One of these animals that beached had propeller wounds along its dorsum. A second whale was reported to have been hit by a tug-boat, but the carcass was not recovered.

It is likely that malnutrition was an important predisposing factor in the mortality of each of these cases. Malnutrition may cause immunosuppression or alter feeding behavior. The former may enhance susceptibility to infectious agents. The latter may result in whales feeding in locations or on prey not historically consumed, so that they could be in shipping channels and get struck by vessels, or ingest atypical parasites or toxins such as domoic acid. The ultimate cause of the malnutrition of gray whales is unexplained. Possible explanations are decreased availability of prey in the summer feeding grounds due to El Niño, decadal climatic changes, changes in seasonal ice cover³, global warming, overgrazing of the benthos due to increased gray whale numbers, or over-fishing by humans. Detailed examination of stranded animals, coupled with long term monitoring of gray whale populations and their prey base are needed to determine the ultimate cause of the increased strandings in 1999 and 2000.

Acknowledgements

We thank all the staff of the National Marine Fisheries Service and members of the National Stranding Network who collected data on stranded whales in the U.S., especially Joe Cordaro, Brent Norberg, Pat Gearin, Merril Gosho, John Calambokidis, Kate Wynne, Kaja Brix and Sue Moore; as well as Ed Lochbaum in Canada and Lorenzo Rojas and Jorge Urban and their staff in Mexico.

References

1.  Dailey MD, FMD Gulland, LJ Lowenstine, P Silvagni, D Howard, 2000. Prey, parasites and pathology associated with the mortality of a juvenile gray whale (Eschrichtius robustus) stranded along the northern California coast. Diseases of Aquatic Organisms, 42: 111-117.

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2.  Perryman WL, A Donahue, PC Perkins, SB Reilly, 2002. Gray whale calf production 1994-2000: are observed fluctuations related to changes in seasonal ice cover? Marne Mammal Science, 18: 121-144.