High gray whale mortality and low recruitment in 1999: potential causes and implications

Abstract

This paper documents the high incidence of gray whale deaths in 1999 and presents the hypothesis that some of the whales were starving. Predictions from this hypothesis are tested using data on the frequency and distribution of strandings, the sex and age composition of strandings, estimates of recruitment and physiological condition, and observations of foraging during migration. The data come from multiple sources. The mortality count of 274 gray whales in 1999 was twice as high as in any previous year dating back to 1985. Dead whales were widely distributed along the migratory route, with the majority observed on the northward migration from the Mexican breeding lagoons to Alaska. Most mortalities in Mexico and California were adults and immatures, rather than the usual calves and yearlings. The majority of dead adults in Mexico, California and the Oregon/Washington region were females. Many dead whales were emaciated. Calf production was lower than in the previous six years. Aberrancies in timing and location of migration, as well as foraging on pelagic prey, were observed. These results, and an even higher stranding rate in the Mexican breeding lagoons in 2000, are consistent with the hypothesis that the whales were undernourished. It is argued that the most likely cause of this condition was a decline in the biomass of their principal prey, the benthic amphipods in the Bering and Chukchi Seas over the last decade, due in part to the combined effects of increasing sea surface temperatures and increased predation from the growing population of gray whales themselves. A significant decline in amphipod density could have long-term effects on the future growth and stability of the gray whale population because amphipods recover slowly given their low fecundity and long generation times. Annual monitoring of the status of the amphipods in the Bering and Chukchi Seas, and the effect of inter alia sea surface temperature on their biomass, is vital for understanding fluctuations in gray whale mortality and numbers, and the extent to which they are dependent on this food resource.