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The Effects of Climate on the Reproductive Success of
Red-Legged Kittiwakes on St. George Island, Alaska

by S. Dean Kildaw, Department of Biology and Wildlife, Institute of Arctic Biology

The red-legged kittiwake is a rare species of gull that is en- demic to the Bering Sea where it breeds at only four known locations. Approximately 80% of a global population of more than 230,000 red-legged kittiwakes breeds on St. George Island in the southeastern Bering Sea. For unknown reasons, red-legged kittiwakes on St. George Island have experienced generally poor reproductive performance over the past 20 years and a 50% decline in population between 1976 and the mid-1980s. Although it is unclear what factors are responsible for the troubled status of the red-legged kittiwake, a "regime shift" towards warmer climatic conditions in the Bering Sea in the late 1970s was coincident with, and may be causally linked to, the onset of reproductive failures and population declines of kittiwakes on St. George Island.

The objective of this study was to develop multiple regression models that describe the effect of climate on several measures of reproductive performance of red-legged kittiwakes on St. George Island and to use these models for two purposes: 1) to evaluate the contribution of climate to the poor reproductive performance exhibited by kittiwakes over the past 20 years, and 2) to predict how global warming might affect the reproductive performance of kittiwakes in the future. I constructed multiple regression models using 20 years of reproductive data for red-legged kittiwakes on St. George Island (hatching date, laying success, hatching suc-cess, fledging success, and productivity) and climatic data for the same time period (1975-1995) that was obtained from the National Weather Service (air temperature, precipitation, and wind speed), from the Scripps Institute of Ocean-ography (sea-surface temperature and barometric pressure), and from J. Neibauer (sea ice cover) at the Institute of Marine Science of the University of Alaska in Fairbanks.

I found that all measures of red-legged kittiwake repro-duction were best related to a climatic variable that was a composite of air temperature and sea ice conditions during the winter months (January, February, and March). Kittiwakes bred earlier and had higher reproductive success in years with cold winter weather and heavy sea ice conditions in the Bering Sea, and also did better in years with dryer, calmer springs. Overall, multiple regression models explained roughly 50% of the variability in the reproductive performance of red-legged kittiwakes. The relationship between winter weather and the timing of reproduction was curvilinear (quadratic) and had an r2 of 0.68: red-legged kittiwakes bred earliest in years with moderately cold winters and later in years with both warm and extremely cold winters. In contrast, the relation between winter weather and all other measures of kittiwake reproduction was negative and monotonic.

The linkage between winter weather and kittiwake productivity during the following summer is most likely mediated by the effects of melting sea ice on the extent of the "cold pool," (bottom water < 2°C) in the "middle domain" (waters 50-100 m deep) of the continental shelf of the Bering Sea. In cold winters with heavy sea ice conditions, the large volume of melting sea ice cools and "freshens" the water mass overlying the continental shelf and, during the following summer, extends the cold pool of the middle domain eastward and well beyond St. George Island. The cold pool is known to affect oceanographic conditions and the distribution of fishes in the southeastern Bering Sea and, thus, may affect the reproductive performance of kittiwakes on St. George Island by affecting the abundance or distribution of their food.

Lastly, I used regression models to evaluate the potential effects of global warming on kittiwake reproduction on St. George Island and found that a 5°C increase in average winter temperature in the Bering Sea would, on average, result in an 8.4-day delay in the timing of reproduction of red-legged kittiwakes and a 15% reduction in overall reproductive performance

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