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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 |