Post-Release Survival of River Otters: Effects of Exposure to Crude Oil and Captivity

Abstract

Few data exist on post-release survival of rehabilitated oiled wildlife, fueling the controversy surrounding wildlife rehabilitation efforts following oil spills. In 1998, we initiated a captive study to assess the effects of exposure to crude oil on physiological and behavioral processes in coastal river otters (Lontra canadensis). This study provided the opportunity to explore the effects of oiling and rehabilitation separately from those of captivity by comparing post-release survival rates of control and oiled river otters held in captivity with those of wild free-ranging otters. Fifteen wild-caught male river otters were assigned to 3 groups, of which 2 were given weathered crude oil in food (i.e., control, low dose, high dose) under controlled conditions at the Alaska Sealife Center. At the end of the rehabilitation period, animals were surgically implanted with radiotransmitters and released at the original site of capture or at an adjacent site in Prince William Sound, Alaska, USA. Concurrently, survival of 55 coastal river otters radiotagged in the wild was monitored in the same geographical area. Our results indicated that the captive, newly released animals (i.e., experimental otters) had a significantly lower survival rate than wild animals. We found no effect from exposure to hydrocarbons once rehabilitation was accomplished, but noted that lower levels of hemoglobin (a likely condition of rehabilitated oiled wildlife) were negatively correlated with survival and likely resulted in death from starvation. Therefore, rehabilitation may be a viable option for animals that have the potential for full recovery. We detected no relationship between location of release or estimated age of the experimental animals and their subsequent survival, although these results may be an artifact of small sample sizes. We recommend that future studies evaluate the effects of the length of captivity on post-release survival to produce additional guidelines for release. Information on the potential for full recovery and the length of the captive period required for achieving this rehabilitation will provide professionals with tools necessary for deciding whether to rehabilitate or euthanize individual animals.