|Abstract or Summary
- The pigeon guillemot (Cepphus columba) population in Prince William
Sound has failed to recover from declines that occurred both before and after the
Exxon Valdez Oil Spill (EVOS). Post-spill studies of pigeon guillemot breeding
biology have identified three potential factors limiting recovery: (1) predation on
eggs and nestlings; (2) declines in the proportion of high-lipid, schooling forage
fish (sand lance [Ammodytes hexapterus], herring [Clupea pallasi], and capelin
[Mallotus villosus]) in the diet; and (3) continued exposure to residual oil from the
spill. This laboratory study with captive-reared pigeon guillemots at the Alaska
SeaLife Center investigated two aspects of the species' biology that are relevant to
restoration in the aftermath of EVOS. First, we investigated the role of dietary
factors (prey type, quantity of food consumed, dietary fat content, and energy
intake rate) in limiting the growth, development, survival, and fledging condition of
nestling pigeon guillemots. The objective was to understand how changes in prey
availability and prey quality might affect pigeon guillemot productivity. Second,
we fed nestlings sublethal doses of weathered Prudhoe Bay crude oil (PBCO) and
then measured several potential biomarkers of effects from this pollutant. These
dose-response experiments were designed to (1) better understand the impact on
nestling guillemots of petroleum hydrocarbons in food, (2) calibrate existing and
potential biomarkers of exposure to PBCO in pigeon guillemots in a controlled,
laboratory setting, and (3) develop better nondestructive biomarkers of exposure to
PBCO in pigeon guillemots in particular, and seabirds in general.
Results of feeding experiments indicated that most variation in nestling
growth rates could be explained by variation in daily energy intake. The type of
forage fish consumed, the lipid or protein content of the forage fish, and even the
quantity of food consumed daily did not have as strong an effect on nestling
guillemot growth as did daily energy intake. The metabolic efficiency and growth
performance of nestling guillemots was not enhanced on high-lipid diets, contrary
to results with nestlings of some other seabird species. Instead, structural growth
(wing length) in nestling guillemots was somewhat stunted on high-lipid diets.
These attributes of guillemot nutritional requirements are associated with the
guillemots' nearshore foraging niche and high food provisioning rates to nestlings.
The average lipid content of sand lance, juvenile herring, and capelin may represent
the optimal dietary lipid content for nestling pigeon guillemots. This study supports
the hypothesis that guillemot productivity is limited by the availability of these forage fishes through effects on energy provisioning rates to nestling guillemots.
Consequently, recovery of pigeon guillemot populations injured by EVOS is likely
linked to recovery of these key forage fish stocks.
Results of the oil-dosing experiments indicated that nestling guillemots are
resistant to small doses of weathered PBCO in their food. No nestlings died or
suffered noticeable health effects following dosing. The high dose in this study (0.5
ml kg⁻¹ day⁻¹) was sufficient to induce hepatic cytochrome P450A1 (a liver enzyme
indicative of contaminant exposure), but growth rate, fledging mass, and blood
chemistry were largely unaffected. None of the 12 plasma or hematological
markers examined responded in a dose-dependent manner to ingestion of weathered
PBCO, except lactate dehydrogenase (LDH). Although baseline stress hormone
(corticosterone) levels were not different between oil-dosed and control nestlings, a
standardized acute stress protocol revealed that corticosterone was more elevated
during stress for oiled nestlings compared to controls. Although we were not
successful in identifying a noninvasive biomarker (e.g., growth) or a blood
biomarker (e.g., haptoglobin) of crude oil exposure in nestlings, we were able to
confirm that levels of hepatic cytochrome P4SO1AI and corticosterone during
stress were elevated by the sublethal doses administered during our experiments.
Based on this and other studies, it is unlikely that the failure of pigeon guillemots to
recover from EVOS is due to effects on nestling health of residual oil in food.