The Effects of Vessel Traffic on the Behavior Patterns of Common Dolphins in the Tagus Estuary (Portugal)

The impact of vessels on dolphin populations has been extensively studied worldwide. The common dolphin, Delphinus delphis, has been observed in the Tagus estuary for the past two centuries, and during the last several years, these sightings seem to have increased. This area has high levels of maritime traffic throughout the year, both commercial and recreational. To understand the possible effects of vessel traffic on dolphins' behavior, land-based observations were carried out from March 2022 to March 2023. For a total of 67 events (48.9 h of dolphin sightings), differences in behavioral budgets were noted. Although "neutral reaction" was the most observed response when vessels were in the vicinity of dolphins, "negative reaction" was also common and five times more abundant than "positive reaction". The GEE model showed statistical differences between these reaction types (positive, neutral, and negative). Markov chains' analysis revealed distinct patterns in the behavioral transition probabilities, as dolphins were more likely to switch to a traveling state when vessels were nearby. This study is the first step towards understanding a potential impact source in the area since it is expected that tourism companies expand due to the increase in dolphin sightings in the estuary.