Characterizing sei whale (Balaenoptera borealis) movement patterns in the Gulf of Maine

Sei whales are difficult to visually monitor due to their cryptic presence and lack of breaching; therefore, passive acoustic monitoring is now being used to expand this effort. The fine-scale temporal and spatial movements of sei whales in the Northwest Atlantic are unknown. The present study characterizes sei whale movement patterns related to migration dynamics in the Gulf of Maine. Five bottom-mounted Acoustic Long-Term Observatories landers, each equipped with a tetrahedral array of omni-directional hydrophones, were deployed throughout the Gulf of Maine. A combination of manual analysis and automated detections was used to assess sei whale daily presence and seasonality. Horizontal bearing estimation of detected vocalizations was used to examine sei whale directional movement patterns among sites. The absolute minimum number of animals present was calculated by totaling the number of directions that a species’ calls arrive over a short period of time (15 min). Daily abundance was defined by the highest absolute minimum number of any 15-min time period throughout the course of the day. Presence/absence data suggests that sei whales are present in spring and fall from 2021–2024 at varying sites. This effort will contribute to the improvement of regional soundscape modeling, automated detector accuracy, and density estimation methods.