Physical–biological interactions in a subarctic estuary: How do environmental and physical factors impact the movement and survival of beluga whales in Cook Inlet, Alaska?

Abstract

The annual abundance population estimates of the endangered beluga whales in Cook Inlet (CI), Alaska, have indicated a decline in the population over the past decades and no convincing explanation has been offered so far for this trend. Satellite tracking data of about 20 Cook Inlet beluga whales (CIBW) over some 5-year period, as well as annual population counts and stranding reports were compared with environmental observations. The seasonal movements of the belugas within CI appear to be affected by variations in water temperature, ice coverage and most significantly, by river discharge. Statistically significant correlations were found between the seasonal movements of the CIBW and the timing of peak flows at different rivers. Interannual variations of the CIBW movements were found to relate to year-to-year variations in water temperature and the time of ice melt/freezing; thus, in warmer years the CIBW will stay in the upper inlet longer, before dispersing to the mid and lower CI in winter. Both, the population of the beluga and stranding numbers show a 3–4-year cycle of interannual variations resembling the Pacific Decadal Oscillation (PDO) index, though the exact mechanism in which large-scale climate variations impact the CIBW habitat is not clear yet. The analysis suggests that the method of estimating the CIBW population from annual aerial surveys needs further evaluation, since interannual changes in the spatial distribution of CIBW preferred locations may have a confounding effect on estimates of population abundance.