Study on skinny western gray whales (Eshrichtius robustus) in relation to environmental changes in the North Pacific

Abstract

The population size of western gray whales (Eschrichtius robustus) in the western Pacific is estimated to be approximately 120 individuals and they are listed by The World Conservation Union as critically endangered. Most individuals of the western population are observed off Piltun Lagoon on the northeastern coast of Sakhalin Island, Russia, during the summer feeding season. Since 1995, a collaborative Russia-U.S. research program has been conducting individual monitoring of western gray whales summering off Piltun Lagoon by use of photo-identification methods. Body condition of individual whales was determined using a photo-based method that specifically examined the relative amount of subcutaneous fat in three body regions surrounding the head, shoulders and flanks. Loss of fat in these regions suggests some degree of abnormal nutritional stress. Since the body condition of western gray whales varied interannually, as apparent by the total number of individuals observed to be "skinny" in any given year, we hypothesized that this variability was likely to be linked with changes in the oceanic environment and climate of the North Pacific. To address this question, counts of skinny whales in their summer feeding area between 1997 and 2005 were compared to their environmental conditions such as sea surface temperature (SST) and amphipod biomass in the feeding ground, the Pacific Decadal Oscillation (PDO) Index, and Southern Oscillation Index (SOI). The results show that when the -6 months lagged PDO was in a positive phase the number of skinny whales observed was lower than in years when the index was in a negative phase during which time higher numbers of skinny whales were observed. In contrast, -1 year lagged SOI was positively correlated with percentage of skinny whales. It means, 1 year after El Nino, the number of skinny whales was increased.