Why aren't pigeon guillemots in Prince William Sound, Alaska recovering from the Exxon Valdez oil spill? Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rb68xg25k

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  • The Pigeon Guillemot (Cepphus columba) is now the only species of marine bird in Prince William Sound (PWS), Alaska that is listed as "not recovering" on the Exxon Valdez Oil Spill (EVOS) Trustee Council's Injured Resources List and has shown no sign of population recovery. During the 20 years since EVOS, the guillemot population in PWS has gradually declined by nearly 50% following the initial mortality event caused by direct contact with spilled oil. This decline has continued even though there is no longer evidence that guillemots are negatively affected by residual oil from EVOS. My objectives in this study were to (1) identify the primary factor now limiting Pigeon Guillemot population recovery at the Naked Island group, the most important historical breeding area for guillemots in PWS, and (2) determine whether guillemot population trends across PWS are consistent with my understanding of the primary limiting factor. I investigated two competing hypotheses for the lack of guillemot recovery at the Naked Island group: availability of high quality prey (i.e., schooling forage fish) and nest predation. The prevalence of schooling forage fish in the diet of Pigeon Guillemots at the Naked Island group has not recovered to pre- EVOS levels. However, data from both aerial surveys and beach seines provided evidence of an increase in abundance of schooling forage fish near the Naked Island group since the late 1990s. Yet between 1990 and 2008, there was a precipitous 12% per annum decline in the guillemot population at the Naked Island group, where mink are present, while at the nearby mink-free Smith Island group guillemot numbers were stable. The mortality rate of guillemot eggs and chicks at the Naked Island group was high during the late 1990s, largely attributable to predation by mink. The weight of evidence indicates that predation by mink is now the primary factor limiting the reproductive success and population recovery of Pigeon Guillemots at the Naked Island group. Differences in guillemot population trends between the Naked Island group and the remainder of PWS are also consistent with the mink predation hypothesis. The median decline in density of Pigeon Guillemots along transects at the Naked Island group was much greater (> 7 times) than the decline along transects throughout the remainder of PWS. The proportion of all guillemots in isolated pairs (as opposed to multi-pair groups) increased substantially only at the Naked Island group. This is consistent with the hypothesis that mink predation negatively affected guillemot colonies more than isolated nesting pairs; perhaps because guillemot nests in colonies were more apparent or more accessible to mink. At other high-density guillemot nesting areas in PWS, average group size of guillemots declined from 12 to 8 individuals suggesting that other factors may play a role in constraining of guillemots on a region-wide scale, perhaps availability of schooling forage fishes. Nevertheless, I conclude that the key to restoring the injured guillemot population at the Naked Island group is to eliminate mink predation pressure on guillemot eggs, nestlings, and attending adults.
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