Thirty years of herd focal follows reveal the functions of important habitats identified for the endangered St. Lawrence Estuary beluga

Identifying habitat functions is crucial for understanding how endangered populations such as the St. Lawrence Estuary (SLE) beluga Delphinapterus leucas use their environment, and for guiding effective conservation measures. Often, the only way to infer the behavior of cetacean herds is by observing their surface activity and configuration. A previous study developed a classification connecting SLE beluga herd surface characteristics to their underwater diving behavior, enabling the inference of behavior from surface cues alone. Using this method, we analyzed surface activity descriptors collected every 30 min during multi-hour focal follows of 1496 beluga herds tracked in 1991-2020 to classify their behavior into 6 categories: (1) benthic feeding/resting/caring for young, (2) pelagic feeding, (3) pelagic exploration, (4) socialization/benthic feeding, (5) transit, and (6) mixed activities. The relative frequency of these behaviors within each of 27 previously identified key beluga areas formed the basis for statistically grouping areas by their dominant function. Two groups characterized by high directional movement closely matched (≥90%) previously hypothesized transit corridors. The other 2 groups predominantly exhibited foraging and social behaviors, and matched mainly (≥70%) areas with high beluga residency whose specific function was previously unclear. Although habitat groups varied in environmental characteristics, no clear separation was observed, suggesting other factors may shape habitat use at multiple scales. This study highlighted that beluga have limited alternative habitats to support vital functions should local stressors compromise habitat quality or accessibility. These findings are critical because segments of the population show strong fidelity to specific summer sites.