Abstract: While many baleen whale species exhibit temporal patterns in their diving behavior, our understanding of these patterns have often been drawn from short duration tags and a small number of individuals. Herein, we describe extended patterns of diving behavior and vertical habitat use of 10 fin whales in the Southern California Bight (SCB) that were tagged with dive-and-location reporting satellite transmitters (total data collection 245.4 d; mean data per whale 24.5 d). Dive behavior was strongly diel and consisted of two primary modes: (1) prolonged use of the upper water column (< 20 m) at night and (2) daytime dives of variable depth. The crepuscular periods exhibited gradual transition between these modes: at dawn, dives became progressively deeper; and at dusk, they were progressively shallower. Although the median percentage of time spent at or immediately below the surface (< 5 m) was roughly equal between day and night, the percentage spent above 20 m depth increased from 42% during the day to 58% at night (p < 0.001), and whales spent virtually all nighttime hours above 40 m. Diving behavior also appeared to vary seasonally. These whales spent the greatest proportion of time in the upper water column during spring and winter nights. Data from two tagged animals that left the SCB suggest these patterns may extend beyond the region. These findings suggest that exposure to surface-oriented risk—in particular, vessel collision—also varies temporally and underscore the importance of developing mitigation measures that are robust to nighttime conditions. Fin whales are the most frequently ship-struck species in the region, and their tendency to use surface waters most when they are least detectable may be a contributing factor.
Key Words: diel dive behavior, fin whale, Balaenoptera physalus, Argos, Southern California Bight, dive strategy, ship strike
Document: Article
DOI: https://doi.org/10.1578/AM.45.2.2019.233
Page Numbers: 233-243

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