A large concern with an oil spill is the health of marine species. The effect of oil on Louisiana wildlife is dependent on many factors including the type of oil which is spilled, the location of the oil, the weather during the spill, and the specific species and life stage. The oil from Deepwater Horizon has been classified as mostly light crude according to U.S. Fish and Wildlife Service. Light crude is relatively volatile, and many of the polycyclic aromatic hydrocarbons (PAHs), a main source of toxicity, evaporate within 24 to 48 hours. However, even after this time, mortality and long-term effects still occur from weathered oil.
Oil exposure can occur in several ways and can vary by species. Direct exposure occurs through inhalation, absorption and ingestion. Benthic organism like oysters, clams and mussels can be exposed through oil directly covering their habitat or through filter feeding. As dispersants and natural means break down the slicks of oil into droplets, filter feeders can ingest oil droplets. Crabs, shrimp and other crustaceans can also have direct exposure to the oil, or they can ingest it from contaminated plant and animal material they consume. The oil contaminants, such as PAHs, can therefore, work up the food chain to organisms that never came into direct contact with the oil. The U.S. Food and Drug Administration (FDA) is monitoring PAH levels in seafood to check for this contamination and to ensure seafood safety.
For marine species oil effects and mortality can result one of two ways: acute or chronic. Acute toxicity of the oil is very quick. Acute mortality usually results from an individual animal being smothered by the oil when the gills or lungs are covered in oil, preventing the animal from getting oxygen. Smothering can occur across all groups of animals: crustaceans, fish, birds and marine mammals and affects all life stages. Marine and marsh plants can also be smothered from the oil. Weathered oil gets increasingly sticky resulting in a higher risk. Fish, crabs, oysters and shrimp can be smothered when they swim through an oiled area, or when the oil washes over their habitat. Birds can be smothered when they dive down through oil to hunt or when nesting in a region covered in oil. Acute toxicity can also occur if the animal is exposed to oil with PAHs or breathing in the volatiles coming off. Younger life stages are often more susceptible to acute toxicity. It is spawning season for many fish and invertebrates in the Gulf of Mexico including Bluefin tuna, snapper, grouper, spiny lobsters, blue crabs, brown and white shrimp, and many more. As the eggs and juveniles drift in plankton communities, they are at a very high risk of direct oil exposure resulting in smothering or acute toxicity.
However, mortality and toxicity effects are not always immediate. Chronic effects can also be seen across all groups of wildlife. External effects of oil exposure can be skin and eye irritation. Internal effects can be damage to respiratory systems, ulcers, bleeding, and damage to liver, kidney and reproductive systems. This damage may not be fatal to an adult, but is often fatal in a developing juvenile. These chronic effects often result in mortality of adult species through resulting infection, but problems remain even if the animal recovers. Long-term chronic effects are often decreased survival but also lowered reproductive success. Oil contaminants that do not result in immediate death may be passed along to offspring resulting in defects of future generations or increased juvenile mortality. Therefore, oil spill effects may not even be seen for several years, past the immediate deaths seen from acute exposure. However, long-term research on the effects of oil on marine organism is difficult, leaving many unknowns in the duration of effects.
Sources: NOAA and FWS fact sheet