Seasonal and Diurnal Patterns of Harbor Porpoise ( <scp> <i>Phocoena phocoena</i> </scp> ) Behavior and the Disruptive Effects of Vessel Presence in a High‐Traffic Coastal Habitat

ABSTRACT Chronic anthropogenic activity is an ongoing threat to the marine environment and presents unique challenges in shallow, coastal habitats where harbor porpoises reside. Harbor porpoises have high metabolic needs and are highly susceptible to negative environmental factors; however, no long‐term studies have been conducted on wild populations to quantify the negative impacts of vessel presence on their behavior. To evaluate this impact on harbor porpoises in the Little Belt, Denmark, data from a static hydrophone array were used to distinguish porpoise click train events and quantify buzzes (vocalizations with an inter‐click interval &lt; 10 ms), used as an indication of foraging and social behavior, and Automated Identification System data were collected to identify vessels within a 3000 m radius of the study site. Using generalized additive models, we showed that harbor porpoises were most active during the night in summer months but most often overlapped with vessels during the day in the summer. Generalized linear models showed that harbor porpoise buzz rate was significantly reduced in the presence of vessels, particularly during daylight hours in the summer when vessel traffic peaked. These results suggest that porpoise foraging and socializing are negatively affected by vessel presence. Furthermore, high porpoise density, which is likely driven by prey availability as found in other studies, may prevent animals from moving to less trafficked areas. Mitigation measures to reduce impacts should be considered to help reduce the ongoing decline in the studied population.