Diel patterns of biosonar-based foraging effort in endangered southern resident killer whales Orcinus orca Endangered Species Research

Diel periodicity is a common feature of animal activities and is often shaped by factors related to optimizing resources over daily timescales. Additionally, many terrestrial mammals utilize nighttime periods to avoid diurnal human disturbance; however, examples in marine mammals are limited. Southern resident killer whales (SRKWs) Orcinus orca extensively echolocate to hunt their fish prey. While investigations have documented vessel and noise impacts on SRKWs foraging, with greater effects in females, findings are based on daytime observations. It remains unknown how individuals partition their foraging between day and night periods and whether individuals exploit nighttime periods that coincide with reduced disturbance. Here, we used sound and movement tags to investigate diel periodicity of biosonar-based foraging by classifying SRKW echolocation bouts into 3 foraging phases: (1) search (regular/slow clicking: inter-click interval, ICI > 100 ms), (2) pursuit (fast clicking: 10 ms < ICI ≤ 100 ms), and (3) capture attempts (buzzing: ICI ≤ 10 ms). We often detected crunching sounds following pursuit and/or capture attempts, indicating successful prey capture. SRKWs echolocated at shallower depths at night, reflecting the behavior of their prey. We found no diel effect on the probability of searching. However, females had a lower probability than males of prey pursuit and capture attempts, and the lowest probability of capture attempts at night, implying reduced foraging efficiency even during periods with reduced disturbance. These results, although sample-size limited, advance our understanding of temporal patterns of foraging to better predict consequences of disturbance in coastal cetaceans that rely on sound for energy acquisition.