Why do dolphins use the Potomac River?
/Baby dolphins may be key to answering this question
A calf swims in infant position alongside its mother. IMAGE COURTESY OF ANN-MARIE JACOBY, THE POTOMAC-CHESAPEAKE DOLPHIN PROJECT. TAKEN UNDER NMFS PERMIT NO. 19403
Guest Author
Ann-Marie Jacoby
Associate Director, Potomac-Chesapeake Dolphin Project
Potomac Conservancy is pleased to bring you another blog in our series on the dolphins of the Potomac River 🐬, from Potomac-Chesapeake Dolphin Project (PCDP) Associate Director Ann-Marie Jacoby.
Research led by Dr. Susan Barco done in the 1990s off of Virginia Beach, Virginia found a higher ratio of baby dolphins, called calves, in the lower Chesapeake Bay than in the adjacent coastal waters of the Atlantic Ocean. This finding stressed the need to study the area further to determine the Bay’s importance to calf growth and survival.
Other than being cute, calves and their mothers, or reproductive females, are the linchpins of a healthy population.
A newborn calf swims in echelon position alongside its mother.. Image courtesy of Ann-Marie Jacoby, the Potomac-Chesapeake Dolphin Project. Taken under NOAA NMFS Permit No. 23782.
Understanding how many dolphins in the Potomac-Chesapeake area are reproductive females, the number of calves they’re having, and how many of those calves survive reveals much about the current state of the population and its trajectory.
The Potomac-Chesapeake Dolphin Project (PCDP), a non-profit research organization based out of Georgetown University and led by Professor Janet Mann, has been studying dolphins in the Potomac River since 2015. One of the PCDP’s goals is to learn more about the importance of the Potomac River to mothers, their calves, and in turn the health of the mid-Atlantic dolphin populations that use the area.
Here is what we know so far about mothers and calves in the Potomac River:
#1 An abundance of food supply in the Chesapeake Bay is thought to keep mothers and their calves well-fed
An adult dolphin tosses and stuns a summer flounder before catching it.. Image courtesy of Ann-Marie Jacoby, the Potomac-Chesapeake Dolphin Project. Taken Under NMFS Permit No. 19403.
The Chesapeake Bay is the largest estuary in the U.S. and highly productive. In fact, it once had the highest biomass or living material per area of any estuary in the U.S.
This outstanding ecological feat can be attributed to several physical and biological factors, including the transport of nutrients by thousands of tributaries, shallow sun penetrating depths, nutrient storing and filtering ecosystems, and seasonally variable conditions in salinity and temperature which mix and replenish nutrients, minerals, and oxygen in its waters.
The Bay’s physical and biological characteristics allow for high primary production, reduced competition for food, and shelter from predators in the Bay’s array of complex ecosystems like wetlands, seagrasses, and oyster reefs.
When there are high levels of primary production, there is ample food in the form of algae, phytoplankton, and underwater plants for consumer species at lower levels of the food web, like small fish. Smaller fish subsequently provide food for higher-level consumers, like larger fish, which in turn provide food for tertiary and top predators, like dolphins and sharks.
The Bay’s abundance of food and shelter has made it home to over 3,000 migratory and residential species, with many of its fish and invertebrate species using it as a nursery ground.
Given its temperate climate, the Chesapeake Bay is most productive during the warmer months of the year and biomass peaks during the summertime bringing fish species, such as Altantic croaker, spot, and summer flounder, that dolphins feed on into the area.
The productivity of the Chesapeake, including the Potomac, provides food necessary for mothers to sustain themselves and their calves.
Dolphins are marine mammals and therefore nurse their young like any other mammal. Calves are dependent on their mothers anywhere between two to nine years. The demands it takes for mothers to nurse their calves are extremely taxing. Mothers must eat about 50% more while lactating because they are not only feeding themselves but also their calf. Dolphin milk is high in fat (>25%), making it energetically expensive.
#2 Fewer predators in the Bay’s waters are thought to keep mothers and their calves safe
A healed shark bite wound is visible on the side of this mother. Image courtesy of Ann-Marie Jacoby, Potomac-Chesapeake Dolphin Project. Taken under NOAA NMFS Permit No. 19403.
Many shark species cannot tolerate brackish waters, since they have adapted to live in highly saline environments. Species that are commonly preyed upon by sharks can better avoid them by inhabiting the Chesapeake’s brackish waters, pending they too can tolerate low salinity conditions.
The primary shark species that use waters as far north in the Chesapeake as the Potomac are the sandbar, sand tiger, smooth dogfish, and bull shark.
"bull shark 1" by AlKok is licensed under CC BY-NC-SA 2.0
The sandbar and sand tiger sharks are bottom feeders, and the smooth dogfish feeds on crustaceans. None of these are known to prey on dolphins, but bull sharks, which can tolerate low salinity conditions well, occasionally attack and can kill dolphins, making it the dolphins’ only potential predator (aside from humans) in the Potomac-Chesapeake area.
Although bull sharks reside in the Potomac-Chesapeake, the Potomac River is a relatively safe place for mothers and calves compared to ocean waters.
We, the PCDP, have observed few fresh shark bite wounds and shark bite scars on dolphins, which provides evidence that sharks are not a major threat to dolphins in the Potomac.
#3 Hundreds of mother-calf pairs use the Potomac River but there may be too few mothers to sustain a healthy population
A newborn calf with fetal folds swims in echelon position alongisde its mother. Fetal folds are only evident in very young calves. Image courtesy of Melissa Collier, the Potomac-Chesapeake Dolphin Project. Taken Under NMFS Permit No. 23782
Between July 2015 and October 2018, the Potomac-Chesapeake Dolphin Project identified over 250 mothers and nearly 300 calves. The number of mothers and calves is not the same because some mothers have had more than one calf (but never at the same time).
We identify mother-calf pairs by close association, swimming position, and physical features.
A newborn calf with faint fetal lines swims in infant position alongside its mother. Image courtesy of Ann-Marie Jacoby, the Potomac-Chesapeake Dolphin Project. Taken Under NMFS Permit No. 19403
Calves swim and surface alongside their mothers in what is called infant position. When in infant position, the calf swims directly under the mother’s peduncle or tail stock where the calf is carried in her slipstream. In infant position, calves expend less energy, and they are close to the mammary glands for nursing.
When calves are first born, they are a little over 3 feet in length, lumpy with fetal lines, with poor motor control causing them to move rapidly and break the surface in a rather uncoordinated manner. All of these features help us identify calves, their mothers, and estimate their ages and birth dates.
Within the Potomac River, newborn calves are typically seen between mid-June to mid-August, when water temperatures are also the warmest. While we’ve observed several hundred mothers and calves, the overall percentage of lactating females in the Potomac is 17% of the total number of dolphins identified. According to other studies on bottlenose dolphin populations, this percentage is on the lower side and may signify a declining population.
#4 There are several threats to calves in the Potomac, some of which may shock you…
A dolphin is entangled in fishing gear around its upper body and dorsal fin. This dolphin has not been seen since it was entangled. Image courtesy of Dr. Vivienne Foroughirad, the Potomac-Chesapeake Dolphin Project. Taken Under NMFS Permit No. 19403
Natural threats to calves in the Potomac River do not just include bull sharks but include other bottlenose dolphins.
Dolphins in the Potomac River and other parts of the world have been documented attacking and/or killing calves, a phenomenon called infanticide.
Infanticide is documented in species where males might benefit by killing offspring that are not theirs so they can replace it with their own much sooner. Infanticide has been documented in primates, felines, bears, and other mammals. Although rare, it occurs when males encounter a very young calf with a female they probably have never mated with.
Multiple populations of dolphins use the Potomac, and we suspect there are also multiple communities within these unique populations. With so many different groups of dolphins in the same place during the same season, behaviors such as infanticide may have a higher chance of occurring. These populations and communities overlap in the Potomac during the summer months which is the breeding and birthing season.
Professor Janet Mann observes dolphins at the mouth of the Potomac, many of which are mother-calf pairs including the calf Gwendolyn Mink that the PCDP witnessed being born in August 2019. Image courtesy of Ann-Marie Jacoby, the Potomac-Chesapeake Dolphin Project. Taken Under NMFS Permit No. 19403
Sadly, mothers too can unintentionally harm their calves by feeding them a chemical concoction.
The fish that dolphins feed on have bioaccumulated higher levels of chemicals like mercury and PCBs, which enter the water through improper handling of industrial wastes and common consumer products. These toxins also accumulate in dolphin tissue, such as blubber, over their lifetimes. Mothers offload these toxins through their milk to their calves. This is particularly problematic for a female’s first calf.
Research done on wild bottlenose dolphins in Sarasota, Florida found that first-born calves have a significantly lower chance of survival than subsequent calves due to chemical offloading. Males, on the other hand, end up with much higher levels of toxins in their blubber, as they have no means of getting rid of it.
A young calf sports fresh wounds that are likely from a propeller strike. Image courtesy of Ann-Marie Jacoby, the Potomac-Chesapeake Dolphin Project. Taken Under NMFS Permit No. 19403.
Calves face both indirect and direct threats from humans.
Indirect threats from humans include overfishing and habitat degradation from poor waste management and development. Direct impacts include boat injury from engine propeller strikes and entanglement in fishing gear. Calves do not yet know how to swim around boats to avoid injury. Motorboats can also be very loud, making it difficult for mothers and calves to communicate and keep track of one another.
Fishing line that is left unattended is the greatest direct human threat to dolphins regardless of age. Calves can become entangled and ingest fishing line impairing their ability to move, feed, and grow. Entanglement often leads to death. Unfortunately, we have observed both propeller strikes and fishing line entanglement in calves in the Potomac more than once.
Help mothers and calves in the Potomac River - Here’s how
Two calves, one older and one newborn, swim in infant position alongside their mothers. Image courtesy of Meliss Collier, Potomac-Chesapeake Dolphin Project. Taken under NOAA NMFS Permit No. 23782
Drive safely around dolphins. When you see dolphins, enjoy watching them from a safe distance. It’s federal law to remain 50 yards or half a football field away from bottlenose dolphins (NOAA guidelines: https://www.fisheries.noaa.gov/topic/marine-life-viewing-guidelines#guidelines-&-distances). Calves, especially newborns, are not familiar with boats and can get easily disoriented by engine noise and injured by engine propellers.
Do not leave any unattended fishing line. Dolphins can get entangled and ingest fishing line causing death. Unattended fishing lines are the leading human cause of death in dolphins.
Think twice about your run-off and properly dispose of trash. Pollution causes both habitat degradation and toxins can accumulate in dolphins and poison calves through milk. If you see any plastic trash on or near the water, please pick it up!
Please support Potomac-Chesapeake dolphin research! Research to study mothers and their calves is both labor and time intensive. All funds go directly to research (no overhead!) Please donate to our research and follow our research via our website at www.pcdolphinproject.org, Instagram, and Facebook pages.
About the Expert
Ann-Marie Jacoby is a PhD Candidate in Marine Science and Conservation at the Duke University Marine Lab, as well as the Associate Director of the Potomac-Chesapeake Dolphin Project. Her doctoral research focuses on the historic and present-day occurrence of bottlenose dolphins (Tursiops truncatus) in the Potomac River and middle Chesapeake Bay to improve the conservation of mid-Atlantic bottlenose dolphin populations and the Potomac River-Chesapeake Bay ecosystems they are a part of.
