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A demographic model suggested that the differences in over-winter survival between oiled and unoiled areas was sufficient to account for continued declines in the populations of Harlequin Ducks in the oiled areas. These effects reflect loss of individuals from habitually used wintering or foraging sites. Wiens et al. However, all species recovered at the same rate, so community composition was affected over time, though the consequences of these effects are unknown.

It is also less than clear that the immense discharges of petroleum into the marine environment during the Persian Gulf War in had a lasting effect on the populations of seabirds breeding in the northern Arabian Gulf Case History For example, during the war, an estimated 8, to 10, Socotra Cormorants Phalacrocorax nigrogularis were killed, approximately 50 percent of the Saudi Arabian population Symens and Werner, As of , the population had rebounded to 30, pairs, suggesting that the losses to oil during the war had little population-level effect, except possibly in slowing the rate of post-war increase.

In contrast, four species of terns nesting on the offshore islands of the northern Gulf of Arabia showed little evidence of oiling during Although about 1 percent of the total adult tern population was moderately to heavily. Cormorants, black-legged kittiwakes, murres, pigeon guillemot, mergansers, and loon.

Oiling apparently occurred when terns encountered tar balls while plunge diving in pursuit of small fish. Small spots of oil transferred from adults to eggs caused no decline in hatching success Symens and Alsuhaibany, There was evidence that the oil spilled during the Persian Gulf War had an indirect effect on tern reproductive success.

The clutch sizes of the White-cheeked Tern Sterna repressa were reduced in and , and the breeding success chicks per pair of Lesser Crested Terns Sterna bengalensis , White-cheeked Terns and Bridled Terns Sterna anaethetus were less in and than those in either or This decline in and was apparently caused by a lack of small fish on which to forage. Exposure to the massive spills during the Persian Gulf War significantly reduced the abundance of fish eggs and larvae McCain and Hassan, ; Symens and Alsuhaibany suggest that this mortality of forage fishes resulted in a scarcity of fish prey for the terns in and In those two years, the diets of the terns shifted, and one of the larger species, the Swift Tern Sterna bergii , resorted to eating the chicks of the smaller White-cheeked Terns, and stealing food from Lesser Crested Terns returning to their colonies.

Although this example shows effects of an oil spill on the reproductive ecology of marine birds up to two years after the spill, Symens and Alsuhaibany suggested that this two-year interruption would have a negligible effect on the population biology of these long-lived seabirds. Among marine mammals, river otters Lutra lutra in the British Isles and Alaska, and sea otters Enhydra lutris and harbor seals Phoca vitulina in Prince William Sound, Alaska, all showed short-term population declines after oiling of their inshore marine habitats Baker et al.

For some species, these effects may have persisted over ten years e.

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However, in the case of the Exxon Valdez oil spill in Prince William Sound, Alaska, considerable controversy remains concerning the magnitude of the initial losses and the duration of population-level effects e. These uncertainties stem from the lack of sufficient pre-spill data to characterize the population status of these species and difficulties in obtaining adequate post-spill data to distinguish between local movements of animals and area-wide population effects.

Chronic or delayed responses of marine bird and mammal populations to petroleum hydrocarbons in the sea can occur because of continued ingestion of oil via contaminated prey, or because of failure of prey populations to recover subsequent to injury. In the 10 years since the Exxon Valdez oil spill, several species of birds and marine mammals have demonstrated indirect or delayed responses to the spill. These responses were found in sea ducks and shorebirds, species.


PHOTO 23 Spills from coastal facilities such as marine terminals and tank farms make up nearly one quarter of the spills by volume associated with the transportation of petroleum. Photo courtesy of Environmental Research Consulting. Seabird responses were of three types: reduced use of oiled habitats as compared to use of unoiled habitats for up to nine years post-spill, reduced numbers post-spill as compared to pre-spill, and lower growth and delayed fledging in a species that fed contaminated mussels to its young.

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Marine mammal populations that may have exhibited prolonged effects subsequent to an oil spill include sea otter and harbor seal populations in Prince William Sound Garshelis and Johnson, ; Hoover-Miller et al. In some regions of the Prince William Sound, sea otter abundance had not recovered as of Dean et al.

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Other results indicating damage to otters that persisted for more than several years include the finding that the over-wintering mortality of juveniles was higher at oiled as compared to unoiled sites in the winters of and Monson et al. The presumed cause of the failure of the sea otter population to recover is continued contamination via their prey. Some of the measured increases or decreases in sea otter populations may have resulted from local movements of otters or other behavioral or demographic phenomena, and assessment of long-term population effects of the oil spill to sea otters remains difficult and controversial.

In the case of harbor seals, there is some controversy as to whether they have declined in Prince William Sound Peterson, , or whether the apparent declines are the result of movement of seals that were avoiding or moving away from oil contaminated haulout Hoover-Miller et al. Harbor seals were declining before the spill, and if there has been a continued decline, it may be a continuation of the past decline, or it may be the result of a decrease in the abundance of near-shore fish prey, but the available evidence is inconclusive Peterson What is important here is that sub-lethal effects can be identified in marine birds and mammals for several years after the acute effects of a spill have passed.

In summary, it has proven difficult, except in a few notable exceptions, to demonstrate population-level effects of oil spills for either marine birds or marine mammals based on censuses.

Reviews of Environmental Contamination and Toxicology 170

Although many individuals may be killed, it is frequently difficult to demonstrate commensurate declines in local or regional populations, or to show significant demographic effects, because the power of present techniques to detect change is weak. Without more complete knowledge of the structure of populations of marine birds and mammals and their demography, it may remain beyond our reach to assign damage or recovery except in cases where ongoing monitoring provides an adequate basis for comparative studies. The temporal and spatial variability found in ecosystems makes even the most sophisticated statistical approaches open to individual interpretation and controversy Wiens and Parker, ; Day et al.

As both Nisbet and Piatt and Anderson point out, even though we often cannot demonstrate statistically that oil pollution has caused population-level effects in marine birds, given what we know of their life history patterns, including long life spans, low adult mortality, and low rates of reproduction, it is risky to assume that increased rates of mortality are without population-level effect. Total population size, including breeders and non-breeders, has not been determined for any seabird species, and thus it is impossible to determine directly whether pollution is affecting global populations Nisbet, Only if the effects of oil pollution are compensatory and not additive to other, natural, causes of mortality can we hope that large removals of individuals are without population-level consequences.

The same arguments should hold true for marine mammals. Modeling has been used in many ways to assess the impacts of oil spills on living resources and habitats:. To evaluate the impacts of an oil spill using a model, the fate of the oil must first be quantified. Surface trajectory models are used to calculate the intersection of the trajectory path with maps of resources of concern including biota and habitats e.

This approach is appropriate for quantification of impacts to birds, mammals, and shoreline habitats. Bird and mammal impacts have also been modeled by backtracking from locations where oiled animals have stranded on beaches, accounting for losses at sea Seip et al. To evaluate the effects of subsurface oil, subsurface oil must be explicitly tracked French et al. A prime example is the North Cape oil spill of January that occurred during a severe winter storm French, a,b,c Case History, To evaluate impacts on aquatic biota, oil entrainment and dissolution into the water must be simulated.

The relevant concentrations are of those components that might have an impact on aquatic organisms and habitats. The concentrations of main concern are the soluble and volatile lower- and intermediate-molecular-weight compounds that are acutely toxic to biota, primarily monoaromatic hydrocarbons MAHs and polycyclic aromatic hydrocarbons PAH French, ; French McCay, Other compounds in oil may also contribute to toxicity. Submerged oil and oil smothering on shorelines are also important exposure pathways. Thus, the model must consider the entire fate of the oil and all its components over time, both on and in the water, and in sediments.

Oil trajectory models have been used to determine where oil will intersect the shoreline and impacts are presumed if oil reaches a location. The problem with this approach is that impacts are related to the amount and weathering state of the oil. Thus, this simply identifies areas that might be exposed to some amount of oil, but does not quantify an impact. Computer models of the physical fates, biological effects, and economic damages resulting from releases of oil and.

The NRDA models simulate spreading and shoreline stranding of oil. The amount of oil remaining on the shoreline is a function of oil viscosity and shoreline type. Stranded oil is assumed to be removed by waves and other physical processes at a constant rate.

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The holding capacities and removal rates are based on data collected after spills. Impacts on intertidal habitats, such as salt marshes, mangroves, macroalgal beds and coral reefs, are assumed to be a percent loss if a threshold thickness dose is exceeded for any interval of time. The threshold is based on observational data for salt marsh impacts French et al.

Wildlife birds, mammals, and reptiles are primarily impacted by direct exposure to floating oil, ingestion of contaminated prey or depletion of food resources. Models used to assess impacts of oil on wildlife populations are summarized in Table In evaluating the wildlife impacts of the Exxon Valdez , Ford et al. Oiled and dead birds are scavenged and may sink at sea.

The percent stranded is related to the trajectory of the carcasses. Ford et al. Wildlife are assumed to move randomly within the habitats they normally use for foraging.

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The dose is estimated from the oil thickness, path length through the oil, and the width of a swimming bird. A portion of wildlife in the area swept by the slick is assumed to die based on the probability of encounter with the slick, dosage, and mortality once oiled. Estimates for these probabilities are derived from information on behavior and field observations of mortality after oil spills.

French and Rines performed hind-casts on 27 oil spills to validate the wildlife impact model. The results showed that the model is capable of hind-casting the oil trajectory and shoreline oiling, given 1 accurate observed wind data following the spill, and 2 a reasonable depiction of surface currents. Since winds and currents are the primary forcing variables on oil fate, obtaining accurate data on these is very important to the accuracy of any simulation.

The accuracy of the impact model is primarily dependent on the accuracy of the wildlife abundance data for the time and location of the event.

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In the validation study, regional mean abundances from literature sources were assumed. In nearly all cases, impact information for a spill consists primarily of counts of rescued or dead wildlife. Model validation is necessary to illustrate where the model predicts reasonable estimates of impacts on wildlife. Modeling results show that the wildlife impact algorithm in the model is valid when input data on abundance are accurate French and Rines, In a few cases, the model estimated more birds killed than were observed. These cases were for species impacts not normally assessed or reported.

Even in cases where large efforts were made to recover oiled wildlife, such as following the Exxon Valdez , it is well recognized that many oiled animals are lost at sea or scavenged and not counted directly as oiled. Small and less visible species and. Sea birds and marine mammals—Oil slick encounter and subsequent mortality.

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Fur seal model—Simulated population processes and mortality due to oiling. Sea birds—Estimate numbers oiled from strandings of oiled animals on beaches. Exxon Valdez —Experimental bird drift and loss rates to estimate the percent of oiled animals that would reach a beach and be stranded.

Thus, it is not possible to verify some of the model estimates of impacts. The model results point to where additional observations are needed to evaluate impacts to these less obvious species French and Rines, Oil toxicity models have been developed to estimate water column toxicity after an oil spill French, ; French McCay, As discussed above oil toxicity may be attributed to many different compounds. Exposure concentrations of each compound in the mixture, as well as their toxicities, must be estimated to quantify the toxicity of oil to water column organisms French et al.

Typically, for surface releases of fuel and crude oils, only the PAH are dissolved in sufficient quantity and remain in the water long enough for their toxic effects to be significant. The more turbulent the release i. For a subsurface release deep in the water column or for a gasoline or other product spill where the MAHs and lower molecular weight aliphatics are significant fractions of the oil, all of these compounds may cause significant acute toxic effects French, ; French McCay, Other, less familiar models may address the challenges of modeling oil spill fates and effects at least as well.

Fish and their eggs and larvae are affected by dissolved contaminant concentration in the water or sediment. Mortality is calculated using LC 50 , corrected for temperature and duration of exposure, and assuming a log-normal relationship between percent mortality and dissolved concentration. Movements of biota, either active or by current transport, are accounted for in determining concentration and duration of exposure. Organisms killed are integrated over space and time by habitat type to calculate a total kill. Lost production of plants and animals at the base of the food chain is also computed.

Lost production of fish, shellfish, birds, and mammals due to reduction or contamination of food supply is estimated using a simple food web model French et al. In addition to the direct kill and food-web losses of eggs and larvae, young-of-the-year fish may be lost via habitat disruption. This is included in the model for wetland and other nursery habitats destroyed by lethal concentrations or oiling.

Losses are related to the habitat loss. Thus, recovery of spawning and nursery habitat in wetlands follows recovery of plant biomass and production French et al. Success of a model simulation is dependent on both the algorithms and the accuracy of the input data.