East or west: the energetic cost of being a gray whale and the consequence of losing energy to disturbance

Western gray whales (WGW) Eschrichtius robustus are considered one of the world's most endangered baleen whale populations. Development of oil and gas fields in northeastern Sakhalin, Russia, is a concern, because they overlap with WGW feeding grounds. Some WGW migrate similar to 10 000 km from feeding grounds around Sakhalin Island (Russia), to breeding grounds in Baja California (BajaC; Mexico) and possibly similar to 6000 km to the South China Sea (China). We developed a WGW female bioenergetics model to examine potential consequences of energy lost from foraging cessation caused by anthropogenic disturbance, and compared it to eastern gray whales (EGW). Energy loss was then linked to potential reductions in reproduction and survival. Mean total energy requirements were 11 and 15% greater for WGW breeding in BajaC and China, respectively, compared to EGW, due to longer migration distance (25%) to BajaC and higher metabolic rates at foraging grounds. However, this difference is minimal for EGW that use the northern extent of their foraging range. On average, WGW breeding in BajaC and China need 9 and 17% more energy for survival than EGW. Our model predicts that WGW mortality would likely occur at 38 to 40% annual energetic loss. Long-term yearly energy loss of <30% would re duce population growth due to lower reproductive rates. Ongoing yearly energy losses of >30% would result in adult female mortality the first year, followed by lower reproductive rates of survivors. Our model suggests that energy losses of >30% caused by disturbance should be considered a threshold for concern for this Critically Endangered population.