Seasonal and annual variation in body condition of western gray whales off northeastern Sakhalin Island, Russia

The western gray whale population (Eschrichtius robustus) is critically endangered and its potential for recovery is uncertain. Along with other natural and anthropogenic threats, western gray whales are susceptible to nutritional stress, known from regular observations of individual whales in compromised body condition. Thus, the ability to visually quantify the relative body condition of free-ranging western gray whales and evaluate how this condition varies seasonally and annually is needed. A photoidentification study of western gray whales on their feeding ground off the northeastern coast of Sakhalin Island, Russia, produced a large dataset of digital, film, and video images of 150 identified individuals from 1994 to 2005. These images were utilized to visually assess the body condition (i.e., good, fair, poor) of western gray whales by evaluating the relative amount of subcutaneous fat in three body regions presumed to reflect reductions in body condition. Multinomial logistic regression for ordinal responses was used to evaluate the effects of year, month, whale class, and sex on the body condition of western gray whales. Although the correlation between observations of individual whales has not yet been accounted for, significant findings of the analysis indicate that: 1) the body condition of whales varied annually and seasonally; 2) the body condition of whales improved as each feeding season progressed; and 3) lactating females were in relatively poorer body condition nursing calves in comparatively better body condition. Additional work is needed to refine the statistical analysis. Investigating the causes and consequences of compromised body condition in western gray whales is important for understanding the health and viability of this population.