Nutritional Status of Dead Oiled Rhinoceros Auklets (Cerorhinca monocerata) in the Southern Japan Sea

We analysed the nutritional status of 44 oiled rhinoceros auklets (Cerorhinca monocerata) found dead along the Japan Sea coast. The species is a medium-sized Alcidae and one of the most vulnerable diving seabirds to oil spills in the northern North Pacific. We compared results with those obtained from 19 non-oiled auklets incidentally killed in fishing nets or predated at colonies as controls. All of the oiled birds were mal-nourished, having lost 60% of liver mass and 1/3rd each of their body, muscle and organ (e.g., uropygial gland) masses. They consumed 90% of their subcutaneous and abdominal fat and more than half of fats stored in muscles and other organs, thus having only 7 g total fat remaining. Bone marrow was almost entirely replaced with water. There were no stomach contents and livers were light, having lost nearly two thirds of mass. Low values for nutritional indices in the oiled group, compared with those of the non-oiled group, showed that oiled birds did not die instantly but died of nutritional exhaustion. A bird weighing 620 g on average was estimated to starve to death in one to two days after oiling, as oiled aquatic birds consume energy rapidly due to loss of thermoregulatory ability as a result of plumage oiling. As body mass is highly correlated both with organ mass and fat storage, as shown in detail in this study, body mass is effective as a simple and important index to evaluate nutritional recovery for use in determining the best time to return birds to sea after care in rehabilitation facilities. Based on our nutritional study, we recommend making every effort to rescue birds on the day they are oiled if rescue and rehabilitation are going to occur, as oiled birds quickly starve to death while at sea, especially during the cold winter.