Past to Present: Multidisciplinary Insights into Tamanend’s Bottlenose Dolphins (Tursiops erebennus) in the Potomac River and Chesapeake Bay, U.S.

Abstract

The Chesapeake Bay is the largest estuary in the U.S. and one of its best studied estuarine ecosystems. However, beyond the Bay’s mouth, little is known about one of its most visible and charismatic predators, the Tamanend’s bottlenose dolphin (Tursiops erebennus), posing challenges to conservation efforts. Here, I use a multidisciplinary approach to gain insights into the population dynamics, behavior, and ecology of Tamanend’s bottlenose dolphins that occupy the Potomac River and middle Chesapeake. My research aims to inform current management and conservation efforts, as well as generate questions and hypotheses for future study. First, I evaluate the existing narrative that dolphins were extirpated from the Potomac River and have only recently returned. In the absence of long-term ecological monitoring, I reconstructed the historical occurrence of dolphins in the tidal Potomac through synthesis of local ecological knowledge and historical texts. To do this, I conducted semi-structured interviews with highly knowledgeable, long-time watermen and charter fishermen and systematically searched through newspapers published between 1787 and 2023. I found that dolphins were present in the Potomac every decade from the 1830s to the 2020s. However, a lack of both newspaper records and watermen’s and charter fishermen’s sightings suggest that dolphins were extirpated from the upper portion of the Potomac (114 to 191 km upriver) since the 1930s. Both sources of information indicated that dolphin occurrence throughout the river peaks during the summer and is related to temperature and the presence of prey, specifically spot (Leiostomus xanthurus), Atlantic croaker (Micropogonias undulatus), white perch (Morone americana), and menhaden (Brevoortia tyrannus). Interviews revealed that sightings were sporadic in nature and group sizes were highly variable. Dolphins were observed in more months and seasons, as well as in larger groups further downriver. Further upriver, observations were less frequent within a year and years would pass between sightings. Salinity was mentioned by watermen and charter fishermen as an important driver of the presence of dolphins further upriver. That is, upriver sightings increased during periods with higher salinity. Next, I established a baseline of the current occurrence, behavior, and ecology of Tamanend’s bottlenose dolphins at the mouth of the Potomac River by assessing individual and population-level metrics. I used behavioral survey and focal follow data collected by the Potomac-Chesapeake Dolphin Project between 2015 and 2022. From a subset of those years 2015 to 2019, I used photo-identification methods to identify 2,013 unique individuals, of which 1,141 met restrictive criteria of dorsal fin distinctiveness and quality. Dolphins were observed traveling most often (69% of behavioral surveys) in groups ranging from 1 to 260 and in depths ranging from 0.6 m to 43.5 m. Dolphins were observed foraging (18% of behavioral surveys) on at least four different prey species, including spot, weakfish (Cynoscion regalis), striped bass (Morone saxatilis), and a flounder species within the suborder Pleuronectoidei. They also exhibited a novel foraging behavior named silt circle foraging, similar to mud ring foraging. Dolphin presence, corrected for field effort, peaked from June to August and varied across months and was positively correlated with water temperature. The percentage of dolphins seen on more than one day, month, and year was 45.1%, 36.6%, and 27.5%, respectively. Repeat sightings of the same individuals from spring through fall across years suggested that the area is important dolphin habitat for foraging and reproduction, given that 87 neonates and 242 mother-calf pairs were observed. The number of unique neonates, corrected for field effort, peaked in August and varied significantly across months; most estimated birth dates were in May and June. In addition, three incidents of calf-directed aggression are consistent with previous findings of infanticide adjacent to the study area. The rate of first-year mortality was estimated to be 22% and calves are dependent on maternal care for 1.47 to 3.41 years. The presence of the ectoparasite Xenobalanus globicipitis in 13.7% of surveys and 4.8% of individuals, suggests that coastal dolphin stocks use the area.Lastly, I identified seasonal clusters among dolphins in the Potomac River, assessed whether these clusters differed in ecological and behavioral traits, and compared sighting histories of individuals from these clusters along the mid-Atlantic to current definitions of stock structure. I then generated hypotheses about the identity of possible stocks of the dolphins that inhabit the area. Specifically, I used monthly sighting records of individuals observed by the PCDP (2015 to 2019) to determine three clusters. All three clusters differed significantly in habitat use (depth and distance to shore), sociality (group size and social module membership), the presence of ectoparasites, and site fidelity to the study area (number of years). I determined the sighting history patterns of individuals matched to other sites along the mid-Atlantic using the Mid-Atlantic Bottlenose Dolphin Catalog (MABDC) and compared them to current stock definitions used in management. Based on these findings, I hypothesized that a previously undescribed stock exists in the Chesapeake Bay. I predicted that this proposed stock occupies the Chesapeake Bay and/or coastal Virginia waters year-round. Based on seasonal cluster assignment of individuals with this sighting history pattern, I predicted that defining characteristics of this stock are use of shallow waters close to shore, moderately large group sizes, very low presence of Xenobalanus globicipitis, high cohesion, and high resight rates within and across years. Analysis of sighting histories also supported use of the area by the Northern North Carolina Estuarine Stock (NNCES) and Northern Migratory Coastal Stock (NMCS). I hypothesized that the NNCES uses the Chesapeake for a greater extent of the year (late spring through summer) than currently recognized and that the NMCS moves between the Chesapeake and New Jersey during the spring and summer. Together, these findings highlight the value of taking a multidisciplinary approach to understand the ecology and behavior of an important, but understudied, predator and generating data critical for conservation. Tamandend’s bottlenose dolphins have used the Potomac River and middle Chesapeake Bay since at least the early 1800s. Today, well over a thousand dolphins, including mothers and calves, use the area each year from spring through fall and are important upper-trophic level predators in this iconic ecosystem.