Location-specific factors affecting dolphin group size dynamics in southern US coastal waters

Group size dynamics represent a complex aspect of the ecology of social predators such as the common bottlenose dolphin. Group sizes of dolphins are often strongly influenced by the local environment, but the location-specific effects of these factors on group sizes are poorly understood. Here, we compared dolphin occurrence and group size variation between two adjacent but geomorphologically distinct estuaries along the southern U.S. coast. The larger Mobile Bay (MOB) estuary had larger dolphin groups, especially in summer compared to the smaller enclosed estuary of Perdido Bay (PER), where group sizes remained similar between seasons. In both areas, groups with young dolphins had more adults and occurred in higher-salinity waters. Water temperature and the presence of young dolphins were common ecological drivers influencing group size in both embayments. Additionally, in MOB proximity to Mobile Pass (connecting Mobile Bay to open water) and distance to gillnet exclusion zones were key factors driving group sizes, whereas in PER distance to shrimping regulation zones affected dolphin group size. These findings suggest dolphin group sizes are shaped by a combination of local population demographics and environmental attributes including geomorphology, salinity, temperature, and fishery activities, potentially reflecting behavioral strategies adapted to each location.