| Summary: | The western population of gray whales (Eschrichtius robustus) has not recovered from past exploitation and is listed as critically endangered. Current anthropogenic threats and environmental conditions may be limiting western gray whale recovery, warranting an examination of the magnitude and influence of these factors. Estimates of demographic parameters are also needed to characterize the population and quantify its status. Western gray whales have been monitored since 1997 on their primary feeding ground off the northeastern coast of Sakhalin Island, Russia. This effort resulted in a large dataset of photo-identification images of 169 individual whales that was used to quantify anthropogenic scarring, visually assess body condition, and estimate mark-recapture and other population parameters of western gray whales. In total, 24.3% (n=41) of identified individuals were determined to have been previously entangled in fishing gear at least once and 1.8% (n=3) were found to have survived at least one vessel collision. Body condition improved significantly as each feeding season progressed, but years of significantly better and worse body condition were identified. The body condition of lactating females was significantly worse than that of other whales at all times, but the body condition of their weaning calves exhibited no temporal variation and was consistently good. A significant male-bias in the calf sex ratio could not be linked to observations of maternal condition and other reproductive characteristics, but differential pre- or postnatal mortality of female calves could account for the sex ratio bias and lack of explanatory power. The first observed values of western gray whale age at first reproduction are seven and 11 years. Non-calf survival did not differ between males and females and was estimated as 0.973 (SE=0.007, 95% CI=0.954-0.984), with calf survival significantly lower at 0.717 (SE=0.063, 95% CI=0.579-0.824). Abundance estimates revealed that a maximum of 140 whales were associated with the Sakhalin feeding area by 2007. Recent movements of some Sakhalin individuals into the eastern Pacific suggest that this value may overestimate the number of gray whales that migrate and breed in Asian waters. That is, the critically endangered western gray whale population may be even smaller than presently recognized.
|
|---|