Abstract
The behavior, distribution, and life history of sei whales ( Balaenoptera borealis ) are largely unknown in the North Pacific. We analyzed data from three bottom‐mounted acoustic recorders deployed 25 (shelf; depth = 300 m), 45 (slope; depth = 630 m), and 65 (abyssal; depth = 2860 m) nautical miles off of Newport, Oregon, USA that recorded continuously from October 2021 to December 2022. Acoustic data were visually and aurally reviewed in search of sei whale calls; putative calls were annotated and assigned confidence levels based on existing descriptions in the literature. High confidence sei whale calls were characterized as broadband downsweeps with an average frequency range of 103–30 Hz and a duration of 2.73 s. Calls occurred in groups ranging from 1 to 32 calls with an average of 9 calls per bout. We document a strong peak in putative sei whale calls in October and November and a significant diel pattern (Hermans‐Rasson Test, p = 0.001) with decreasing effect size at more offshore locations. Our findings indicate that sei whales may be present in this region more frequently than previously thought, providing key information on sei whale occurrence in the North Pacific to guide future conservation management efforts.