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New Jersey Endangered and Threatened Species Field Guide
Species Group: Mammal
State: Special Concern
The bottlenose dolphin is a large, sleek, gray-colored dolphin. Compared to most other dolphins, it has a stubby beak, long flippers, and a moderately tall dorsal fin. There is a crease between their forehead and beak. Their coloration is gray with countershading (darker gray on their back, with lighter gray to white on their underside).
Male bottlenose dolphins grow to a size of 8 to 12 ½ feet and 1100 pounds. The slightly smaller females grow to 7 ½ to 12 feet in length and 570 pounds. Newborn calves are 3 to 4 ½ feet in length and weigh between 31-44 pounds.
DISTRIBUTION AND HABITAT
The bottlenose dolphin can be found within temperate and tropical waters around the world. They are found in the Atlantic, Indian, and Pacific Oceans.
There are two different forms or “stock” of bottlenose dolphins, a coastal form and an offshore form. Whether these two forms are different species is unclear at this time. What is known is that they are genetically distinct from each other, prefer different habitat, look a little different (the coastal form is lighter and smaller than the offshore form), and have different ranges, behavior, and prey. It also appears that the two forms do not associate with one another.
New Jersey’s coastal waters are home to the coastal form of bottlenose dolphin. More specifically, it is referred to as the “Western North Atlantic northern migratory coastal stock”. These dolphins prefer marine waters relatively close to shore and over the continental shelf. They will also occasionally enter bays and estuaries in search of prey. Besides the Atlantic Ocean, they have been observed in Delaware and Raritan Bays as well as semi-enclosed water bodies such as Barnegat and Great Bays and the Navesink and Shrewsbury Rivers.
The coastal form of bottlenose dolphin is typically found in New Jersey waters between spring and fall, but may rarely be observed during winter as well. They are migratory and spend their winters as far south as North Carolina. New Jersey and New York waters represent the northern extent of the range for the coastal form along the US Atlantic coast.
The offshore form of bottlenose dolphin can be found at the eastern edge of the continental shelf and the continental slope. They range as far north as Canada and as far south as Florida. Additional stocks of offshore bottlenose dolphins can be found within the tropics and within the South Atlantic Ocean as far south as Brazil.
Bottlenose dolphins feed on schooling fish and bottom-dwelling invertebrates. Typical prey consists of croakers, weakfish, mackerel, and mullet. The offshore form will also feed on squid and may dive as deep as 1600 for their prey.
Bottlenose dolphins will feed individually or cooperately with other dolphins within their pod.
The bottlenose dolphin is an intelligent, active and agile species. It is generally a social animal. They usually travel in groups (pods) of 2-15 individuals or sometimes more. The offshore form has been known to travel in groups numbering in the hundreds.
Males and females with young generally travel in separate groups. A group of related females may stay together for many years. The males will travel within their own groups and will visit the females only occasionally and briefly in order to mate.
Most bottlenose dolphins in New Jersey are born during the summer. Gestation lasts for about a year. Calves will feed on their mother’s milk for about a year but may remain with their mother for several more years after that. Females will typically give birth every three years. Males appear to play no role in raising the calf.
Bottlenose dolphins in New Jersey waters are migratory. They spend their winters as far south as North Carolina and then return to New Jersey in May and remain here until September or October before travelling south again. There have been several instances where individuals or groups remain in New Jersey into the winter, especially when prey is plentiful.
Female bottlenose dolphins may live to an age of over 50 years while males typically live 40-45 years. They have few predators other than man, but large sharks or killer whales will prey on them. Their great intelligence, size, and their strength of numbers are usually enough to discourage most predators from attacking.
CURRENT STATUS, THREATS, AND CONSERVATION
The bottlenose dolphin remains widespread and abundant throughout much of the world. However, some regional and local populations are at risk of disappearing because of habitat degradation, fishery conflicts (such as entanglement in fishing gear), pollution, or disease.
Negative impacts to dolphin habitat may take the form of offshore development, pollution, noise, overfishing, and climate change. Offshore energy development may destroy dolphin habitat or displace dolphins which would normally use the area. Oil spills and other chemical pollutants are also a threat to dolphins and the prey which they feed on.
Another form of pollution is noise pollution. Dolphins’ primary means of communication, navigation, locating food, locating mates, and avoiding predators and other threats is through their sense of hearing, which is much more highly developed than that of humans. Noise pollution created by ship traffic or offshore construction may negatively impact dolphins by disrupting otherwise normal behaviors associated with migration, feeding, alluding predators, rest, breeding, etc. Any changes to these behaviors may decrease survival, simply by increasing efforts directed at avoidance of the noise and the perceived threat.
Active sonar, such as that used by the Navy, also threatens marine mammals by disrupting navigation, foraging and communication abilities. There have been instances of stranding and death caused by acoustic trauma. This may be due to a fatal injury within the structure of the ear, or may result from the distressed animal surfacing too rapidly and developing nitrogen bubbles within their blood (decompression sickness). In addition to the direct threat posed by active sonar, it may indirectly harm marine species by causing changes in behavior.
Overfishing of the fish and squid which dolphins feed upon is another potential cause for concern as well as the potential impacts of global climate change. This issue may be the greatest long-term threat to the marine habitat and its species. Climate change may significantly alter the chemical balance of the seas, off-shore currents, and plankton distribution and abundance, thereby affecting migration routes of marine species and impacting the entire food web.
Disease has been a cause for concern with some bottlenose dolphin populations. Major die-offs have occurred along the US Atlantic and Gulf of Mexico coasts. These die-offs have been linked to disease and exposure to toxins in the water. When large amounts of pollutants get into the dolphin’s body, it weakens their resistance to disease.
Dolphins around the world are also intentionally killed by humans. Sometimes, this happens because fishermen believe that the dolphins are competing with them for fish. Other times, it may be to use the dolphin meat as bait or even as human food.
All too often along the New Jersey shore and elsewhere, another problem occurs when people attempt to feed dolphins or get too close to them. Dolphins are naturally curious animals and will approach boats. But feeding them can create a dangerous situation whereby dolphins learn to associate humans with food. This can cause harm to both dolphins and humans and it is illegal. A distance of 50 yards must always be kept between humans and dolphins or any other marine mammals.
The bottlenose dolphin, like all marine mammals has been provided with protection with the Marine Mammal Protection Act of 1972. Although they are not considered endangered or threatened within the United States, it is illegal to harm them in any way.
The US National Marine Fisheries Service has designated the coastal migratory stock as depleted under the Marine Mammal Protection Act. A 2002 estimate of their population size indicated that 5,582-7,489 individuals were within the northern migratory stock of bottlenose dolphins.
A status review completed in 2009 by the New Jersey Endangered and Nongame Species Program found that the bottlenose dolphin should be listed as a species of special concern. This listing is due to recent die-offs and diseases which have struck the species in New Jersey waters as well as due to the critical role New Jersey waters play in the survival of the coastal migratory population as a calving and nursery area. Further survey effort targeted at this species will be necessary in order to determine distribution and abundance within New Jersey waters.
HOW TO HELP
The New Jersey Endangered and Nongame Species Program would like for individuals to report their sightings of bottlenose dolphins, or any other marine mammals. Record the date, time, location, and condition of the animal and submit the information by submitting a Sighting Report Form. The information will be entered into the state’s natural heritage program, commonly referred to as Biotics. Biologists map the sighting and the resulting maps allow state, county, municipal, and private agencies to identify important wildlife habitats and protect them in a variety of ways. This information assists in preserving wildlife habitat remaining in New Jersey.
Text written by Michael J. Davenport in 2011.
Species: T. truncatus
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