erable controversy concerning changes in avian use of nearshore marine habitats within Prince William Sound that might indicate long-term depression of bird populations using these waters (Day et al., 1995, 1997; Wiens, 1995; Wiens et al., 1996; Irons et al., 2000; Lance et al., 2001). Some studies found that, within two years of the spill 23 of 42 species showed no evidence of negative impacts from the spill (e.g., Day et al., 1995), whereas other investigators (Irons et al., 2000) found negative effects in 6 of 14 taxa up to nine years after the spill (Table 5-1). Some of these differences reflect methodologies used, whereas others appear to be matters of interpretation (Day et al., 1997; Murphy et al., 1997; Peterson, 2001). Effects differed between avian species that were apparently chronically exposed to oil residues through their epibenthic prey. For example, in surveys of habitat use by Barrow’s Goldeneye (Bucephala islandica) in 1995, 1996 and 1997, Esler et al. (2000b) were unable to detect a significant effect of oiling history on habitat use, even though concurrent studies (Trust et al., 2000) found elevated levels of the enzyme cytochrome P450 1A in these birds, thus indicating on-going ingestion of oil-contaminated prey. Trust et al. (2000) also found elevated levels of cytochrome P450 1A in Harlequin Ducks (Histrionicus histrionicus) in oiled areas of Prince William Sound. In contrast to Barrow’s Goldeneye, between 1995 and 1998, Harlequin Ducks within oiled areas of the Sound had lower densities (Esler et al., 2000a) and lower over-winter survival than did individuals over-wintering in non-oiled areas of the Sound. 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. Since it is unclear how these local “subpopulations” relate to biologically defined populations (stocks in fisheries parlance), it remains difficult to assess the “population” effects of this damage.

Wiens et al. (2001), using canonical correspondence analyses, found that although there was a clear effect of the spill, in years subsequent to the spill there was increasing occupancy of previously oiled sites by all species that had exhibited initial spill impacts. 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 1991 had a lasting effect on the populations of seabirds breeding in the northern Arabian Gulf (Case History 5-3). For example, during the war, an estimated 8,000 to 10,000 Socotra Cormorants (Phalacrocorax nigrogularis) were killed, approximately 50 percent of the Saudi Arabian population (Symens and Werner, 1996). As of 1995, the population had rebounded to 30,000 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 1991. Although about 1 percent of the total adult tern population was moderately to heavily

TABLE 5-1 Indirect, chronic, or delayed responses of birds to the Exxon Valdez oil spill (after Peterson, 2001)

Species

Foraging Ecology

Type of Response

Period/Duration

References

Black Oystercatcher

Intertidal invertebrates

Numbers declined post 1989

1990, with recovery by 1993

Klosiewski and Laing, 1994

Chicks fed oiled mussels required more food for less growth and fledged later

1990 only

Andres, 1996, 1997

Laid fewer eggs on renesting

1990 only

Andres, 1996, 1997

Nesting disrupted on oiled island as compared to unoiled island

Recovery by 1993

Sharp et al., 1996

Harlequin Duck

Shallow sub-tidal invertebrates

Lack of recovery in numbers present in oiled areas

Not until at least 1991

Klosiewski and Laing, 1994; Day et al., 1995, 1997; Irons et al., 2000

Decline in winter counts in western (oiled) vs. eastern (unoiled) Sound

Through 1997-1998

Rosenberg and Petrula, 1998; Rosenberg, 1999

P450 1A induction

Tested for in 1998

 

Barrow’s Goldeneye

Shallow sub-tidal invertebrates, mussels

Declining numbers in oiled vs. unoiled areas

Through 1998

Holland-Bartels et al., 1999; Irons et al., 2000

Elevated P450 1A levels

1996-1997

Trust et al., 2000

Cormorants, black-legged kittiwakes, murres, pigeon guillemot, mergansers, and loon

Shallow subtidal fishes

Continued depression in census counts along oiled shores vs. expectation

Through 1998 (except for 1993 for loons)

Irons et al., 2000



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