weak log-log correlation between the number of birds killed and the volume of oil spilled when he analyzed 45 spills. In another analysis of 98 spills, there was no correlation (Dagmar Etkin, Environmental Research Consulting, personal communication). Large spills that occur over the deep ocean in open water that has little bird life will have a lesser effect on seabirds than a small spill in a critical habitat where high numbers of birds are aggregated on the water. The season in which a spill occurs is also critical (Hunt, 1987). If the spill occurs when birds are aggregated during breeding or migration, the impact will be much greater than if they are widely dispersed at sea. It is likely that the cumulative effect of numerous “small” spills and chronic pollution has had a greater effect on seabird populations than the rarer large spills. Recent evidence, however, suggests that the incidence of seabird mortality from small spills may be declining in the North Sea region (Camphuysen, 1998). This shift may reflect the reduction of small discharges of oil noted earlier in Chapter 3.
There has been a “general-rule-of-thumb” that the body count of birds recovered after a spill represents about 10 percent of the birds killed in a spill (Tanis and Morzer Bruijns, 1969; NRC, 1985). Burger (1993), however, found that there was no justification for this assumption; the mean estimate for 21 spills for which the number of birds found dead and that the overall mortality were determined to be between four and five times the number of birds actually counted. The relationship among the number of carcasses recovered and the total mortality will vary between species, being dependent among other factors on body size, average distance to shore carcass buoyancy, and the prevailing winds during and after the spill.
Few studies of oil spills have provided the information necessary to assess delayed or long-term effects on community-level processes in the affected communities or ecosystems. Where keystone predators are removed, competitive dominants at the next lower trophic level can increase and change the structure of the community. For example, in areas of Prince William Sound where sea otters were removed by oiling, their preferred prey, sea urchins, have increased in some locations (Peterson, 2001). Elsewhere in Alaska where sea urchin populations have been able to increase in the absence of sea otters, severe damage to kelp beds have resulted (Estes, 1995; Estes and Duggins, 1995).
In addition to the strong evidence for the impact of massive contamination associated with an oil spill, there is increasing evidence that chronic, low-level exposures to hydrocarbons in the sea can have a significant effect on the survival and reproductive performance of seabirds and some marine mammals. Sublethal effects of oil on seabirds include reduced reproductive success, and physiological impairment, including increased vulnerability to stress (reviewed in Fry and Addiego, 1987,1988; Hunt, 1987; Briggs et al., 1996). In contrast, in marine mammals, sublethal exposure to petroleum hydrocarbons has been shown to cause minimal damage to pinnipeds and cetaceans (e.g., Geraci, 1990; St. Aubin, 1990b), although sea otters appear to be more sensitive (Geraci and Williams, 1990; Monson et al., 2000). Because both marine birds and marine mammals have the enzymes necessary for the detoxification and elimination of petroleum hydrocarbons, parent compounds of petroleum hydrocarbons are not accumulated and sequestered in tissues as chlorinated hydrocarbons. Toxic metabolites produced by metabolism of polycyclic aromatic hydrocarbons, however, may accumulate and induce toxic effects (Brunstrom et al., 1991; Melancon, 1996, 1995). Chronic pathologies would not be expected once oil ceased to be ingested.
There is mixed evidence that oil pollution can have demonstrable effects on the population trajectories of marine birds and mammals. Spilled oil has had and still poses a potentially devastating effect on African Penguins (Spheniscus demersus) in southern Africa (Westphal and Rowan, 1970; Vermeer and Vermeer, 1975; Clark, 1984; Dagmar Etkin, Environmental Research Consulting, personal communication). At Les Sept Iles, France, declines in the numbers of Common Murres (Urea aalge) attending colonies have been attributed to the effects of oil spilled in the Torrey Canyon and Amoco Cadiz accidents (Nisbet, 1994). In northern Europe between 1915 and 1988, 60-90 percent of beached alcids were oiled (Camphuysen, 1989), and Hudson (1985) found that oiling accounted for between 18 and 28 percent of mortality of banded alcids. In addition, there is conflicting sentiment that populations of sea ducks in the Baltic declined as a result of oil pollution (Lemmetyinen, 1966; Vermeer and Vermeer, 1975; Clark, 1984; vs. Joensen and Hansen, 1977). In the cases of two of the largest recent spills, the Exxon Valdez spill in Prince William Sound, Alaska and the Persian Gulf War (Box 5-4) release of oil in the northern Arabian Sea, the population-level impacts on seabirds are not clear.
There has been considerable variability in the estimates of the number of seabirds killed in the 1989 Exxon Valdez oil spill, which has led to much contention (e.g., Piatt and Lensink, 1989; Parrish and Boersma, 1995a,b; Piatt, 1995; Wiens et al., 1995, 1996; Ford et al., 1996, Piatt and Ford, 1996; Wiens et al., 1996; Day et al., 1997; Murphy et al., 1997; Lance et al., 2001). Piatt et al. (1990) provided an initial estimate that the number of seabirds killed in the spill was on the order of 100,000 to 300,000 birds (Piatt et al., 1990), and later, Piatt and Ford (1996) provided a best estimate of 250,000 birds killed. Even more difficult has been the determination of population-level impacts. Irons (1996) obtained evidence of lower rates of production of young in the surface-foraging black-legged kittiwake (Rissa tridactyla), but this did not translate into a decrease in the size of colonies in Prince William Sound, or even in the oiled portion of the Sound. The Common Murre (Urea aalge) was the species that sustained the highest mortality (Piatt and Anderson, 1996; Piatt and Ford, 1996), and it might have been expected that a population-level effect of this mortality