Abundance and Residency Patterns of Common Bottlenose Dolphins (Tursiops truncatus) in Freshwater-Influenced Estuaries of the Northern Gulf of Mexico

Abstract

Multiple stressors are affecting common bottlenose dolphins in the northern Gulf of Mexico, including two embayments in Alabama, Mobile Bay (MOB) and Perdido Bay (PER), where no comprehensive abundance estimates and residency data are currently available. This study provides the first seasonal abundance estimates and residency patterns for bottlenose dolphins in MOB and PER and discusses the effects of seasonal entry of large volumes of freshwater on dolphin abundance. In MOB, abundance estimates were larger in summer, with the highest abundance recorded in summer 2022 (1712 dolphins, 95% CI: 1520–1928) and the lowest in winter 2019–2020 (518 dolphins, 95% CI: 260–1032). The opposite pattern was found in PER, where abundances were larger in winter, with the highest abundance in winter 2021–2022 (191 dolphins, 95% CI: 157–232) and the lowest in summer 2020 (100 dolphins, 95% CI: 81–122). Stronger residency patterns were found in PER, with 28% (n = 52) encountered in more than three seasons compared to MOB where only 9% (n = 57) were encountered in more than three seasons, and considered resident dolphins. The two studied embayments support a larger number of dolphins than previously documented and likely provide seasonally different resources, indicating high and potentially complex use of these estuaries. Combined with other data concurrently collected in this area, this study will inform conservation management and strategies for these highly impacted dolphin stocks.