Very little is known about the Tamanend's bottlenose dolphin (Tursiops erebennus) that inhabit the Chesapeake Bay and Potomac River. While the PCDP research is broadly concerned with better understanding all aspects of these animals’ biology, we are focusing our initial efforts on a few key areas.
Our current research focuses on the dolphins’ abundance and distribution, population structure, and the intersection of their social behavior and disease risk.
Abundance and Distribution
Two fundamental pieces of information crucial for understanding wild animal populations: population size and geographical distribution.
For migratory animals, obtaining answers to these questions poses significant challenges.
In 2015, the PCDP initiated data collection on dolphins near the mouth of the Potomac River.
Initial findings, gathered in just 14 days and ~73 hours of observation, revealed 193 unique individuals.
Identified individuals included 27 mother-calf pairs, 2 suspected adult females, 53 suspected adult males, and several juvenile dolphins of unknown sex.
High sighting rate suggests a dense dolphin population in the Potomac River and Chesapeake Bay.
In comparison, Shark Bay, Australia, with one of the world's highest bottlenose dolphin densities, would expect less than half as many dolphins (~80) with similar research effort.
By 2023, almost 2000 individuals documented in the Potomac River, with some dolphins returning annually.
Our Research
Understanding if dolphins are adapting their habitat preferences due to sea-level rise, which in the Chesapeake Bay is twice the global average.
Figure 1. Map of dolphin sightings from 2015 (top; blue dolphin symbols represent a dolphin sighting) and shipping traffic from 2013–2014 (bottom; colors reflect a heat map of shipping traffic). Shipping map taken from www.marinetraffic.com
Population Structure
Dolphin Populations in Chesapeake Bay
The National Marine Fisheries Service (NMFS) identifies at least two, possibly three populations (stocks) of dolphins that may visit Chesapeake Bay and its tributaries.
Northern Migratory Stock
Southern Migratory Stock
Northern North Carolina Estuarine Stock
Photo ID for Population Determination
Utilizing Photo ID is a key method to identify the population of dolphins in the Chesapeake Bay.
High-quality photographs of individual dolphin dorsal fins are compared to the Mid-Atlantic Bottlenose Dolphin Catalog.
This matching process aids in understanding the specific population(s) of dolphins in the Chesapeake Bay region.
Our Research
Understanding the dolphin populations helps determine their winter migration patterns, as bottlenose dolphins are typically present in Chesapeake Bay during warmer months.
The research contributes to a broader understanding of how these dolphins fit into the larger structure of Atlantic bottlenose dolphin populations.
Figure 2. Map showing both coastal and BSE (bay, sound, and estuary) bottlenose dolphin stocks from Balmer et al. (2018).
Social Behavior & Disease Risk
Background
Interest stems from the 2013-2015 Unusual Mortality Event.
4-fold increase in dolphin strandings on the Mid-Atlantic coast, with the largest increase (> 400 animals) in the Chesapeake Bay region.
Suspected cause is morbillivirus, but our understanding of this virus is hindered by the lack of knowledge on how it spreads.
Melissa Collier photographs as two pairs of dolphins surface and breathe synchronously, a behavior that facilitates both social bonding and disease transmission. Video courtesy of Milan Dolezal. Taken under NOAA NMFS Permit No. 23782
Transmission Mechanisms
Synchronous breathing: Two or more dolphins surface and breathe at the same time, in close proximity (Video).
Dolphins may exchange infectious respiratory droplets.
If a dolphin engages in this behavior more often, it might be at higher risk of contracting and spreading disease.
Direct contact: Also possible that dolphins contract the virus through direct contact with an infected individual.
Our Research
Data collection: We have gathered information on synchronous breathing interactions on over 100 dolphins in the Potomac-Chesapeake.
Network modeling
We represent our data with contact networks to model transmission interactions and simulate the spread of disease.
We can then compare these simulated outbreaks to actual stranding data to uncover the link between behavior and disease risk.
Our research has found that juvenile (4-10 years of age) and adult male dolphins have more synchronous breathing contact than adult females and calves.
This suggests that they are at higher risk for contracting and spreading morbillivirus.
These findings reflect the National Marine Fisheries Service stranding data (Collier et al., 2023).
Figure 3. Collier et al., 2023.
Top Left: The average number of synchronous breathing contacts for adult male, adult female and juvenile dolphins based on our data collection.
Bottom Left: A mixing matrix that shows the proportion of synchronized breaths that we observed between each demographic group. We found that dolphins tend to "sync" most with dolphins of their own age/sex class.
Right: A contact network that uses these findings to demonstrate how dolphins are connected based on their synchronized breathing patterns. Circles represent dolphins and lines between them represent a synchronized breathing interaction. We show an example of an individual's (large circles) synchrony network for each demographic group (isolated to the right of the network) to highlight the differences in contact among age and sex. We can use this network to simulate how disease can spread in PCDP dolphins.