USE OF DISPERSANTS - U.S. National Response Team (NRT)

seabird species that assemble regularly or seasonally such as roseate terns, which form large nesting and stagingaggregations. Some species, such as raptors, can be impacted indirectly if their primary food sources are affected.Dispersant use can reduce the risk of these impacts by reducing the amount of oil washing ashore and remainingafloat at sea with potential to contaminate seabirds.Analysis of Biological Effects of Proposed ActionA primary objective of an oil spill response is to quickly remove as much oil as possible from the surface of thewater, thereby minimizing direct contact with wildlife and preventing movement of the oil into nearshore andshoreline areas where removal is more difficult and environmental impacts severe. Dispersants, applied underappropriate conditions, may offer the best response option to help achieve this objective. Dispersion of oil at sea,before a slick washes ashore, reduces the overall and particularly the chronic impacts of oil on sensitive inshorehabitats including salt marshes, coral reefs, sea grasses, and mangroves. Dispersed oil is less likely than a surfaceslick to reach shoreline areas. Any dispersed oil that does move inshore is less likely to stick to shorelines andvegetation because dispersants alter the adhering property of oil droplets. Consequently, habitats recover faster ifthe oil is dispersed before it reaches them (NRC, 1989). By protecting nearshore and shoreline habitats fromcontamination, dispersant use benefits listed species and other wildlife that rely on them for nesting, feeding, orresting. This would include the listed species that nest along Caribbean shorelines such as the roseate tern, brownpelican, leatherback sea turtle, and Atlantic green sea turtles.Some of the listed species in the Caribbean Region, such as most sea turtle species and brown pelicans, occur or feedpredominantly in shallower, nearshore waters rather than in the area where dispersant use will be pre-authorized bythe Letters of Agreement and so are not likely to be adversely affected. Dispersant application would benefit theseand other listed species by preventing contamination of shoreline and nearshore habitat and, concomitantly, theimpacts associated with shoreline cleanup activity. Brown pelicans, for example, are known to be highly sensitive tohuman disturbance, especially when nesting. The primary human-related cause of mortality to manatees is collisionwith watercraft. Such potential nearshore impacts from cleanup activities would be minimized by preventing oilfrom stranding ashore.Potential effects of dispersant use on listed species that may occur more frequently in the open waters pre-authorizedfor dispersant use, are considered below. In some cases, the species are present in the pre-authorized area seasonallyrather than year-round, reducing the risk they would be affected.Direct Contact and IngestionBy removing the surface oil slick, dispersants reduce the risk of direct contact with wildlife that dwell at or passthrough the water surface to feed or breath such as sea birds, sea turtles, sirenians, and cetaceans. Diving sea birdssuch as the brown pelican and roseate tern are particularly vulnerable to surface slicks. Chemically dispersed oildroplets are less sticky and therefore less likely to adhere to feathers, skin, or other body surfaces than undispersedor naturally dispersed oil (Neff, 1990). Juvenile sea turtles, which often are found with drifting sargassum mats inconvergence areas further from shore, would particularly benefit from reduced surface exposure in the area underconsideration. Exposure of sea turtles to tar balls, which they are known to ingest and which also may adhere tojuveniles, would be reduced because dispersants help prevent tarball formation. Sea turtles and manatees inoffshore areas may temporarily experience exposure to higher concentrations of oil droplets in the water column,primarily in the upper few meters, following dispersion. In open waters with continuous mixing and dilutioncapabilities, however, dispersed oil is rapidly diluted. Considering that concentrations fall to background levelswithin the first few hours following dispersion, exposure will be short-term and concentrations low.Prey ContaminationIf zooplankton, fish, and other water column or benthic organisms become oiled or accumulate oil in their tissues,they could ultimately expose species that prey upon them. Diving seabirds and several sea turtle species that occurin the area under consideration prey on fish and aquatic invertebrates. Prey species that occur in open waters furtherfrom shore where dispersant use will be pre-authorized are the primary concern. Prey species that occur in nearshoreareas where dispersant use will not be pre-authorized by the Letters of Agreement are unlikely to be impacted.