Backgrounder on Bottlenose Dolphins in the Shrewsbury River (November 8, 2000) Bottlenose Dolphin Backgrounder

George Liles
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Teri Frady
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NMFS Northeast Region

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The Shrewsbury River flows into a large basin at its mouth, connected by a channel to Sandy Hook Bay and the Atlantic Ocean. Bottlenose dolphins often follow migrating forage fish (mostly herring and menhaden) into the river, then return to the open ocean as forage fish leave the area and water conditions become colder and less saline. Sometimes dolphins fail to take nature's hint and may perish in the river as conditions become less favorable for them. That's when NOAA Fisheries Marine Mammal Health and Stranding Program leaders have to make decisions about intervening to move animals into better habitat.


What do they look like?: The Atlantic bottlenose dolphin has a medium sized, robust body, a dorsal fin and dark color, ranging from light gray to black on the top and sides, with a light belly. The flippers have pointed tips, and the flukes (tail fins) are curved along the rear margin and notched in the center.

How big?: Adult lengths range from 6.5 to 13.1 feet. That's larger than Pacific bottlenose dolphin, and smaller than those in the Mediterranean. Males usually get a little longer and heavier than females, but females grow much faster than males as juveniles. The offshore animals tend to have larger bodies. At 8 to 9 feet, weights are between 400 and nearly 600 pounds.

How often do they strand?: They are the most commonly stranded dolphin in the United States. The network reported 750 in 1996, 564 in 1997, and 593 in 1998.

Where do they live?: There are believed to be two populations of Western Atlantic bottlenose dolphin, one that stays offshore in deep water and one that stays in coastal waters. Limits to the coastal stock's range appear to be directly related to temperature, and/or distribution of prey. They also tend to stay in waters with surface temperatures ranging from about 10C to 32C, migrating seasonally, with a more southerly distribution in the winter.

How old? Females take 7 to 10 years to reach the full adult size, and males take about 12 years. Females can begin to have young at approximately age 5 to 12, and males can father young at age 10 to 13. Life spans of more than 40 years for males, and more than 50 years for females have been documented.

Baby bottlenose: Calves may be born at any time during the year, but are primarily born in the spring or summer. They are carried by their mothers for about one year, and newborns average 3.5 feet long and a little more than 40 pounds. Females can have a calf every two years, but average three years between births.

What do babies eat?: The mothers nurses young with a rich milk helps the baby rapidly develop a thick, insulating blubber layer. Calves can nurse for up to 18 months, and begin to switch to fish after their teeth have begun to grow in, but stay with their mothers for several years.

What do adults eat?: Fish, squids, small crustaceans such as shrimp. They eat their food whole (no chewing!), and often help one another hunt. They consume about 4 to 5 % of their body weight in food per day, with nursing females consuming about twice that amount.

How many are there?: The exact number is unknown. Based on surveys and analyses of sighting data, the offshore population appears to be larger than the coastal population. There are indications that the inshore population may be declining. Are western Atlantic bottlenose dolphin endangered?: No. They are protected under the federal Marine Mammal Protection Act and classified as depleted, and are therefore a population receiving increased evaluation for additional protection.

Major threats to bottlenose dolphins: During 1987-88 a massive die-off occurred in the mid-Atlantic coastal migratory bottlenose dolphin population, an epidemic that lasted 11 months that is believed to have killed more than half the population. Possible causes include toxins produced by red tide organisms, environmental contaminants, or natural diseases. Bottlenose dolphins are also taken in coastal gillnet fisheries throughout the mid-Atlantic region. The magnitude of this take is not yet quantified. Finally, these dolphins are increasingly sharing their habitat with humans, and this may result in interrupting feeding, mating, nursing, and tending young. Remember, it is unlawful to approach or feed dolphins. Be a responsible watcher!

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