Prey density and depth affect the fine-scale foraging behavior of humpback whales Megaptera novaeangliae in Sitka Sound, Alaska, USA

Humpback whales Megaptera novaeangliae are filter feeders that use discrete lunges to effectively capture densely aggregated prey. The objective of this research was to examine how foraging humpback whales in Southeast Alaska responded to varying prey patch densities and depths. Digital acoustic recording tags (DTAGs; n = 6) were deployed and focal follows were conducted on foraging whales in Sitka Sound, Alaska in September 2012. Prey density was recorded around tagged whales using a Simrad EK60 scientific echosounder and ground-truthed with net tows. Lunges were identified from peaks in jerk in the accelerometer signal, and krill were identified from echosounder data using decibel differencing. Lunge depth was 111 +/- 9 m (mean +/- SD) for the shallowest diving whale (foraging past sunset) and 144 +/- 7 (mean +/- SD) m for the deepest diving whale (foraging diurnally). Ninety-five percent of lunges occurred within a 300 m and 30 min spatio-temporal buffer of krill, indicating that tagged whales fed on krill. Generalized additive mixed model (GAMM) results for spatio-temporally integrated prey and lunge data indicated that mean volume backscattering strength, a proxy for krill density, and krill depth significantly affected the occurrence of a lunge (density: p = 0.006, depth: p < 0.001). Whales fed in the densest region of the krill layer, where mean volume backscatter was -57 dB (range: -50 to -81 dB re 1 m(-1) at 120 kHz). By targeting the densest prey layer, whales maximized their energetic gain by capturing the most prey with each lunge.