Sea Otters and Oil: Avenues of Exposure and Health Effects
Terrie M. Williams also contributed to this piece.
Sea otters are more vulnerable to the effects of an oil spill than any other marine mammal. This results from the small size, high metabolic rate, complete reliance on fur for thermo-insulation and prolonged periods of time floating or swimming and the sea surface where oil concentrates. During the 1989 Exxon Valdez Oil Spill (EVOS), 357 sea otters were treated at rehabilitation centers and over 100 were necropsied. In addition, extensive data were collected on hematology, serum chemistry, toxicology and histology. The result was the largest data set ever collected on the physiological and behavioral responses of a marine mammal to oil contamination (Williams and Davis, 1995). We now have a better understanding of the routes of exposure and the effects on the animals’ health and well-being.
Sea otters can be exposed to petroleum hydrocarbons in crude oil: 1) ingestion, 2) inhalation, and 3) dermal absorption. Since sea otters spend considerable time on the water surface, the first contact with an oil slick generally results in external contamination of the fur and inhalation of volatile petroleum hydrocarbons. External contamination leads to transdermal absorption of hydrocarbons which is accompanied by ingestion of oil during grooming and feeding. These quickly lead to wide-spread internal contamination which will impact many organ systems, especially those involved in detoxification and excretion.
In many ways, the physical effects of oil contamination appear to be more damaging to sea otters than the toxicological effects. Many petroleum products, including fresh crude oil, may contain chemicals that are irritating to the interdigital webbing of the hind flippers and sensitive membranes around the eyes, nose, mouth, and urogenital tissues of the otters. In addition, sea otters actively spread even small amounts of oil across their entire body during grooming, thereby exacerbating the problem.
Heavily and moderately oiled sea otters often become hypothermic following contamination of their pelage. Crude oil rapidly penetrates the fur and eliminates the air layer that is trapped next to the skin and which provides 70% of the insulating properties of the fur (Costa and Kooyman, 1982, Williams et al., 1988; Davis et al., 1988). The loss of thermal insulation initiates a suite of physiological, biochemical, and behavioral changes associated with hypothermia. Aside from the mortality directly associated with a decrease in core body temperature, a hypothermic event may also lead to long-term organ damage and dysfunction due to vascular system collapse and congestion.
Oiled sea otters display a suite of medical disorders related to the age and toxicity of the oil encountered. As might be expected, heavily oiled otters contaminated early in a spill show the severest medical problems and consequently, the highest rate of mortality. As the oil weathers and the level of aromatic petroleum hydrocarbons decline with weathering, the severity of disorders diminishes. Four commonly diagnosed disorders in oiled otters requiring immediate veterinary intervention are: 1) hypothermia and hyperthermia as discussed above, 2) hypoglycemia, 3) subcutaneous and interstitial emphysema, and 4) gastrointestinal injury.
The liver and kidneys have a high capacity for binding toxicants and are considered major sites of toxicant concentration and accumulation. Enzymes for the biotransformation of toxicants are located in these organs as well as the lungs and intestine. Petroleum hydrocarbons may be excreted through feces, urine, expired air, and, to a lesser extent, other secretions. Alternatively, petroleum hydrocarbons may be stored, and hence accumulate, in lipid rich tissues such as fat or blubber.
The EVOS demonstrated the value of performing macroscopic and microscopic examinations on oiled sea otters that died. It also indicated the importance of baseline measurements for wildlife before an oil spill. Detailed postmortem examinations, histopathologic assessment, and blood chemistries provided the most valuable information for veterinarians caring for sea otters in the rehabilitation centers. Conversely, toxicological tests are comparatively expensive, time consuming, and often yield ambiguous results because petroleum hydrocarbons are so rapidly cleared by the liver and kidneys even though tissue and organ damage may persist for the remainder of the animal’s life. The most important factors contributing to the mortality of oiled sea otters appear to be: 1) hypothermia, 2) shock and secondary organ dysfunction, 3) interstitial emphysema, 4) gastrointestinal ulceration, and 5) stress during captivity. Direct oil toxicosis may be a contributing factor, but is difficult to verify in otters contaminated during an oil spill.