Tidal Influences on Behavior and Dispersal of Humpback Whales in Glacier Bay and Icy Strait, Alaska

Abstract

Humpback whales typically spend the summer foraging at high latitudes where prey is abundant, and they migrate in the winter to breed and mate in the tropics where prey is scarce. Humpback whales must feed enough in the summer to support themselves throughout the year. To maximize the efficiency of their foraging,they rely on dense aggregations of their prey: in this case, small schooling fish.Since Southeast Alaska has extreme tides that likely affect the density of their prey, we expect that their behavior has adapted to exploit the changing oceanic conditions associated with the tide cycle. We tested this hypothesis using 10 summers of whale-survey data which included notes on behavior and location of humpback whales in Glacier Bay National Park and adjacent Icy Strait. These data were analyzed alongside three tide effects: water level,current strength and direction of water flow, all of which had been divided into discrete variables. We found that frequency of feeding behavior was significantly affected by water level, and frequency of traveling behavior was significantly related to the direction of water flow. Surface activity and resting/sleeping were not significantly related to any of the tide effects. We also found that the abundance of humpback whales in different local areas was significantly related to tide effects in 24 of 30 tested places. The nature of this relationship varied widely among places suggesting local conditions such as bathymetry play an important role in mediating this effect.