EVOS Alaska Predator Ecosystem Experiment
Seabird Reproductive Energetics and the Fat Content of Fish: Revisiting the Junk Food Hypothesis
Summary
This research addresses whether shifts in diet quality may constrain reproduction of Pigeon Guillemots, Black-legged Kittiwakes, and other piscivorous seabirds nesting within the Exxon Valdez oil spill (EVOS) area. We measured lipid content of forage fishes because it is the primary factor determining energy density. Lipid content of seabird prey ranged from 2% to 61% of dry mass, resulting in a five-fold difference in energy density (2.0 to 10.8 kJ/g wet mass). Most of this variation was due to among-species differences, but intraspecific variation in lipid content was related to age, sex, location, and reproductive status of fish. Of the main fishes consumed by seabirds, juvenile herring, pre-spawning capelin, and sand lance had the highest energy densities. Kittiwake diets were dominated by these high-lipid forage fishes at all study sites, and the rates of energy provisioning to nests were correlated with nestling growth and survival. The trend established in the earlier part of this decade of higher kittiwake productivity associated with increasing availability of sand lance, capelin, and herring was broken in 1997, a poor year at most kittiwake study colonies. For guillemots, the proportion of high-lipid schooling fish (sand lance, herring) in the diet was associated with higher growth rates, fledging weights, and chick survival, compared with those consuming mostly nearshore demersal fishes or gadids. Higher productivity was more a reflection of higher prey delivery rates than higher lipid content of prey per se. Our results support the hypothesis that productivity of kittiwakes and guillemots in the EVOS area is strongly linked to the availability of three species of forage fishes: Pacific sand lance, Pacific herring, and capelin. These three species form schools nearshore and have high energy densities compared with most other forage fishes. Productivity of seabird populations will depend on recovery of these key fish stocks.
Principal Investigators
Daniel D. Roby, Oregon State University, Oregon Cooperative Fish & Wildlife
Research Unit, Corvallis, OR
Kathy R. Turco, University of Alaska Fairbanks, Institute of Marine Science, Fairbanks, AK
Jill A. Anthony, Oregon State University, Department of Fisheries and Wildlife, Corvallis,
OR