Using dive behavior and active acoustics to assess prey use and partitioning by fin and humpback whales near Kodiak Island, Alaska

Abstract

Near the Kodiak Archipelago, fin (Balaenoptera physalus) and humpback (Megaptera novaeangliae) whales frequently overlap spatially and temporally. The Gulf Apex Predator-prey study (GAP) investigated the prey use and potential prey partitioning between these sympatric species by combining concurrent analysis of vertical whale distribution with acoustic assessment of pelagic prey. Acoustic backscatter was classified as consistent with either fish or zooplankton. Whale dive depths were determined through suction cup tags. Tagged humpback whales (n = 10) were most often associated with distribution of fish, except when zooplankton density was very high. Associations between the dive depths of tagged fin whales (n = 4) and the vertical distribution of either prey type were less conclusive. However, prey assessment methods did not adequately describe the distribution of copepods, a potentially significant resource for fin whales. Mean dive parameters showed no significant difference between species when compared across all surveys. However, fin whales spent a greater proportion of dive time in the foraging phase than humpbacks, suggesting a possible difference in foraging efficiency between the two. These results suggest that humpback and fin whales may target different prey, with the greatest potential for diet overlap occurring when the density of zooplankton is very high.