Living Near the Surface: Dive Behavior of Sei Whales ( <i>Balaenoptera borealis</i> ) in the Southwestern Atlantic Ocean

ABSTRACT Understanding the diving behavior of baleen whales in tropical breeding areas is essential for evaluating both ecological functions and anthropogenic threats. In this study, we combined satellite transmitters and generalized additive mixed models (GAMMs) to investigate the dive behavior of sei whales ( Balaenoptera borealis ) in Santos Basin (SB), off southeastern Brazil. A total of 9963 dives were analyzed, revealing a dominance of shallow dives, with 93% of dives occurring at depths ≤ 20 m. Environmental variables such as sea surface temperature, local bathymetry, and chlorophyll concentration significantly influenced dive depth and duration. Cooler waters and deeper offshore areas were associated with longer and deeper dives, while higher surface productivity (indicated by increased chlorophyll a levels) shortened dive times. Behavioral and temporal covariates, including diel period, lunar phase, and dive shape, further modulated diving patterns. The prevalence of shallow diving behavior within the depth range of vessel drafts (≤ 20 m) raises significant conservation concerns, particularly regarding the risk of ship strikes in high‐traffic areas like the Santos Basin. Our findings highlight the behavioral plasticity of sei whales in tropical low‐latitude habitats and emphasize the importance of integrating dive behavior data into marine spatial planning and mitigation strategies. Reducing vessel speed and rerouting traffic in high‐density whale habitats should be considered in the future with further studies to reduce the risk of anthropogenic impacts on this threatened species.