Very little is known about the common bottlenose dolphins (Tursiops trunctaus) that inhabit the Chesapeake Bay and Potomac River. While the PCDP research is broadly concerned with better understanding all aspects of these animal's biology, we are focusing our initial efforts on a few key areas.

Abundance and distribution

Two of the most basic pieces of information needed about any wild animal population is: how many of them are there and where are they? While these seems like fairly straightforward questions, for migratory animals, coming up with the answers is quite difficult. In 2015 (July – October) the PCDP began collecting initial data on the location and number of dolphins in an ~37 square km area near the mouth of the Potomac River. What we found was quite surprising! In just 14 days on the water and ~73 hours of observation we identified 193 unique individuals, including 27 mother-calf pairs, 2 additional suspected adult females, 53 suspected adult males, and several juvenile dolphins of unknown sex. This is an extremely high sighting rate, indicating a high density of dolphins the Potomac River and Chesapeake Bay. For comparison, in Shark Bay, Australia, which is thought to have one of the highest densities of bottlenose dolphins in the world, with similar research effort one would expect to see less than half as many dolphins (~80). What's more, almost all (86%) of our dolphin sightings occurred in shallow water (< 3 meters). While using these shallow waters may keep dolphins out of heavily used shipping channels (Fig. 1), it does place them in closer proximity to recreational boaters and crab pot and pound net fisheries. It also means they are experiencing a rapid change in their preferred habitat due to sea-level rise, which in the Chesapeake Bay is twice the global average.

Figure 1. Map of dolphin sightings from 2015 (left, blue dolphin symbols represent a dolphin sightings) and shipping traffic from 2013–2014 (right, colors reflect a heat map, from green to red, of shipping traffic). Shipping map taken from www.marinetraffic.com

Figure 1. Map of dolphin sightings from 2015 (left, blue dolphin symbols represent a dolphin sightings) and shipping traffic from 2013–2014 (right, colors reflect a heat map, from green to red, of shipping traffic). Shipping map taken from www.marinetraffic.com

Population Structure

Figure 2. Coastal bottlenose dolphins populations (stocks) according to the National Marine Fisheries Service (NMFS).

Figure 2. Coastal bottlenose dolphins populations (stocks) according to the National Marine Fisheries Service (NMFS).

Another major aim of the PCDP is to understand the population structure of the Potomac-Chesapeake bottlenose dolphins. As defined by the National Marine Fisheries Service (NMFS), there are at least two, maybe three populations (stocks) of dolphins that might visit the Chesapeake Bay and its tributaries: the Northern Migratory Stock, the Southern Migratory Stock, and possibly the South Carolina-Georgia Coastal Stock. One way to help determine which population the PC dolphins belong to is through what is called Photo ID. Using high quality photographs of individual dolphin dorsal fins (see a sample of our catalog here) we can match our PC dolphins to photographs in the Mid-Atlantic Bottlenose Dolphin Catalog. Doing so will help us better understand what population(s) of dolphins are visiting the Chesapeake Bay region, where they go during the winter (bottlenose dolphins are usually only found in the Chesapeake Bay area during warmer months), and how they fit into the greater Atlantic bottlenose dolphins population structures.

Figure 3. Two bottlenose dolphins in the Potomac River, surfacing in almost perfect synchrony.

Figure 3. Two bottlenose dolphins in the Potomac River, surfacing in almost perfect synchrony.

Social Behavior and Disease Risk

Figure 4. Association (A & C) and synchrony (B & D) networks for two bottlenose dolphins (data taken from the Shark Bay Dolphin Research Project for illustrative purposes). Circles represent individual dolphins with the colored circles representing the two individuals (blue, red) for which networks were constructed. Lines indicate connections based on either associations or synchronous breathing and the thickness of the lines indicates the strength of the connections.

Figure 4. Association (A & C) and synchrony (B & D) networks for two bottlenose dolphins (data taken from the Shark Bay Dolphin Research Project for illustrative purposes). Circles represent individual dolphins with the colored circles representing the two individuals (blue, red) for which networks were constructed. Lines indicate connections based on either associations or synchronous breathing and the thickness of the lines indicates the strength of the connections.

A third major aim of our initial work with the PCDP is to better understand how variation in social behavior affects an individual's likelihood of spreading and contracting disease. Our interest in this work stems from the recent 2013–2015 Unusual Mortality Event, in which there was a near 4-fold increase in dolphin strandings up and down the mid-Atlantic coast, with the largest increase (>400 animals) occurring in the Chesapeake Bay region. The suspected cause of this mortality event is a type of morbillivirus, a genus of viruses that affects a variety of animals worldwide. In fact, humans are affected by a type of morbillivirus, better known as the measles. Our understanding of this recent morbillivirus outbreak is somewhat hindered by our understanding of how the disease spreads in wild animals. One possible way is through what is called synchronous breathing (Fig. 3), when two or more dolphins surface and breathe at the same time in close proximity to each other. It is thought that during such events dolphins may exchange infectious respiratory droplets. Thus, if a dolphin is particularly synchronous, it may be at greater risk of both spreading and contracting a morbillivirus-like disease (e.g., the dolphin in Fig. 4D shows much greater synchrony than the one represented in 4B). However, it is also possible that dolphins contract the virus through direct contact with an infected individual, which is likely to occur simply by being in close proximity, regardless of breathing synchrony (Fig. 4A & 4C). To help us better understand morbillivirus transmission, the PCDP is collecting detailed data on social behavior (associations, synchronous breathing, direct contact, etc.) on bottlenose dolphins in the Potomac-Chesapeake region. Using network modeling techniques, we plan to investigate both transmission mechanism and compare simulated outbreaks to actual stranding data in order to elucidate the most likely mechanism of transmission.