FAQ: Bottlenose Dolphins
by Jessica Weiss
: Range/Migration and Conservation
Where are bottlenose dolphins found?
Bottlenose dolphins are considered a cosmopolitan species, as they are found in many temperate oceans of the world. Along the U.S. Atlantic coast, bottlenose dolphins range from New Jersey south to Florida and throughout the Gulf of Mexico. Coastal bottlenose dolphins are typically found in bays, sounds, and estuaries as well as along the oceanfront. The ranges of offshore bottlenose dolphins overlap with coastal dolphins in the ocean up to areas approximately 25 meters in depth.
How many bottlenose dolphins are there?
The numbers of bottlenose dolphins in the United States are compiled based upon where they live. In July 2000, approximately 1,033 dolphins were identified in coastal North Carolina waters.
Although we have identified approximately 350 bottlenose dolphins in the Outer Banks, we estimate that about 400-500 use the sound waters during the summer (May through September). Estimates during the winter season (October through April) are yet to be determined.
Are they migratory?
Bottlenose dolphins exhibit many types of residency patterns. Depending upon area, individuals may be year-round residents, seasonal residents, or transients. Along the U.S. Atlantic coast, it is believed that all dolphins north of North Carolina are migratory and migrate south for the winter. Migrations are triggered by water temperature and fish movements. Many dolphins migrating south likely overwinter just offshore of Cape Hatteras, North Carolina in the warm waters of the Gulf Stream.
Many bottlenose dolphins in the Outer Banks are also seasonally resident in the area. In the fall, a group of dolphins migrate south to Beaufort, North Carolina to overwinter and return to the Outer Banks in the spring.
Year-round resident bottlenose dolphin communities are also found along the U.S. Atlantic coast from North Carolina south to Florida and throughout the bays along the Gulf of Mexico. These dolphins are not migratory and exhibit long-term residency patterns. Perhaps the most well-studied bottlenose dolphin community in the world is located in Sarasota, Florida, the site of a year-round resident community.
What are the threats to bottlenose dolphins?
The greatest threats to bottlenose dolphins along the U.S. Atlantic coast are fishery interactions and pollution. Many times, dolphins become entangled in recreational and commercial fishing gear, such as pound nets, monofilament gill nets, and crab pots. Dolphins in some areas have also become accustomed to begging for food from boats and stealing fish from nets, which puts them at greater risk of becoming entangled. In order to reduce fishery interactions, there is much collaboration between marine mammal researchers and fishermen on gear modifications, fishery closures, and ways to deter marine mammals from approaching fishing nets.
Chemicals in the marine environment are also problematic to bottlenose dolphins. Persistent organic pollutants (POPs) include insecticides and flame retardants, many of which are toxic and are no longer produced by industry. However, these materials had previouslyrun off into the water and persisted in the marine environment over time. These toxins enter into the dolphins’ bodies through the fish they eat; dolphins store these toxins in their blubber. Throughout their lives, dolphins accumulate more and more of these toxins. Acquiring enough of these toxins in the body may eventually result in reproductive failure and other adverse health effects. Females have also been found to pass toxins to their calves via milk while the calves are nursing, therefore negatively affecting the younger population as well.
How are bottlenose dolphins protected?
In the United States, bottlenose dolphins are protected under the Marine Mammal Protection Act of 1972. This act states that it is illegal to harm a dolphin or change its behavior. Changing a dolphin’s behavior may be caused by approaching wild dolphins, feeding them, trying to swim with them, or pursuing them. The National Marine Fisheries Service is responsible for completing a report every year stating the status of the species. This is known as a Stock Assessment Report. This report contains valuable information about bottlenose dolphins, such as the species range, current population size, and potential threats in the environment.
Bottlenose dolphins along the U.S. Atlantic coast are also considered to be a depleted population, due to a major die-off in 1987. Although several theories exist for the reasons for the die-off, the actual cause has never been determined. However, in six months, approximately 740 dolphins stranded on the beaches from New Jersey south to Florida. It is unknown what percentage of the actual population stranded, so they were considered depleted along the east coast. Photo-identification studies are conducted by various research organizations to better determine population sizes and movement patterns of bottlenose dolphins along the U.S. Atlantic coast.
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About the author:
Jessica Weiss is the Scientific Advisor for the Outer Banks Center for
Dolphin Research (OBXCDR).
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