oiled, less than 25 percent of the average of the <10 adult terns found dead each year between 1991 and 1994 were oiled (Symens and Alsuhaibany, 1996). 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, 1996). 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 1992 and 1993, and the breeding success (chicks per pair) of Lesser Crested Terns (Sterna bengalensis), White-cheeked Terns and Bridled Terns (Sterna anaethetus) were less in 1992 and 1993 than those in either 1991 or 1994. This decline in 1992 and 1993 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, 1993); Symens and Alsuhaibany (1996) suggest that this mortality of forage fishes resulted in a scarcity of fish prey for the terns in 1992 and 1993. 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 (1996) 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., 1981; Spraker et al., 1994; Monson et al., 2000; Peterson, 2001). For some species, these effects may have persisted over ten years (e.g., sea otters, Monson et al., 2000). 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.g., Garshelis and Johnson, 1995; Hoover-Miller et al., 2001). 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
that forage primarily on intertidal and shallow subtidal invertebrates, as well as in several species that forage on small fish caught in inshore waters (Peterson, 2001). 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. Species of ducks with populations that continued to decline post-spill (Harlequin Ducks and Barrow’s Goldeneye) both feed on shallow-water invertebrates, including mussels, and both showed elevated levels of the enzyme cytochrome P450 1A, indicating continued ingestion of petroleum hydrocarbons (Trust et al., 2000).
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, 1995; Hoover-Miller et al., 2001; Peterson, 2001). In some regions of the Prince William Sound, sea otter abundance had not recovered as of 1998 (Dean et al., 2000), whereas in other areas, sea otter numbers were as high or