Factors Affecting Perinatal Survival In Captive Sea Otters During Oil Spill Rehabilitation
Pregnant sea otters and their pups (Enhydra lutris) presented special problems for Sea Otter Rehabilitation Programs in Alaska following the T/V Exxon Valdez oil spill. Sea otters in Prince William Sound tend to pup in the late spring and the March 24, 1989 incident impacted large numbers of pregnant animals.
Female otters made up the majority of all animals admitted; 53 (24%) of these females were diagnosed as pregnant at admission or at necropsy. Mortality in pregnant otters (41%) was higher than the overall mortality in captured otters (32%). Otters captured early in the spill period experienced the highest mortality and 17 (71%) of 24 pregnant females admitted during March and April died. Of the remaining seven, five delivered stillborn fetuses and two live born pups died within 48 hours after birth. 24 (81%) of 29 pregnant otters admitted in May and June survived but pup mortality remained high. Within this group, five females died, two were released prior to delivery, ten delivered stillborn pups and ten delivered live pups which subsequently died in nursery facilities. Only five females successfully delivered and reared captive born pups and all of these pups were born after June 25 (more than 60 days after the spill).
Multiple factors likely contributed to the high mortality. These include 1) stress associated with captivity, 2) possible maternal or fetal petroleum hydrocarbon toxicity, 3) age and experience of the female, 4) nutritional status of the female, and 5) complicating medical conditions.
Otters were ranked on initial examination by estimating the oil contamination of the pelage. Heavily oiled (>60% body coverage) otters (n = 11) experienced the poorest survival; 81% died in less than 3 days after admission. Necropsy revealed moderate to severe pulmonary emphysema or pneumonia in all of these animals and liver abnormalities were frequent findings. 24 of 31 surviving females from the pregnant population were rated as light (>30% body coverage or sheen), unoiled or unknown; the animals which were released with surviving pups were rated as moderate (n = 1), unoiled (n = 2) or unknown (n = 2).
Uterine torsion was diagnosed post mortem in three females with very large full term fetuses (2.0, 2.3 and 2.5 kg). Gross necropsy of fetal (n = 10) and stillborn (n = 10) otter pups showed no consistent abnormalities. Stillborn pups varied from 1.2 to 2.04 kg in body weight; five of these pups showed lesions suggesting death in utero before the onset of labor and one large pup sustained cranial and visceral fractures as a result of dystocia. Neonatal death (less than 3 days) occurred in ten of the live born pups; 8 of these weighed less than 1.4 kg and may have been born prematurely. One pup was born with a large umbilical defect with evisceration and a second underwent successful surgical repair of a large umbilical hernia at 2 days of age.
Initially, dexamethasone was given to all otters at admission to the centers to combat shock and stress. Although dexamethasone is effective in treating these conditions, glucocorticoids such as dexamethasone are known to induce abortion in cattle and some other species of mammals. Because of the occurrence of stillbirths and low birth weights in live born pups, this treatment was limited to male or non pregnant otters after mid May. The incidence of these problems did not appear to be affected.
Records indicated that labor and delivery occurred both while otters were swimming and on haul outs (dry platforms). Only two females appeared to experience difficult or delayed labor. One of these was treated with injections of oxytocin and calcium preparations with little apparent benefit. Both animals eventually delivered stillborn pups and were subsequently release at the end of the rehabilitation effort.
|Figure 1. Does not include pups born in captivity.|
|Figure 2. Pregnant female and perinatal survival in the three centers.|
Females with live pups were usually kept in separate pools for several weeks before introducing them to group situations. Two females were housed in a single large pool for several days prior to delivery and were not moved after their pups were born two days apart. After 8 days, one of the females began attempts to steal the others' pup and eventually that pup had to be taken to the nursery facility due to poor grooming by its mother. No problems were noted when older pups were moved with their mothers into small female groups.
Contingency planning for possible future sea otter rehabilitation efforts will need to include provisions for care of pregnant females and pups. Our experience indicates that oil contaminated otters will have high mortality and poor pup survival rates but that some animals can successfully rear captive born pups with proper supportive care.