In scientific studies, survivorship has been examined either by determining the annual survival rate of a population (excluding calves under one year of age), from which median life span can be calculated, or by constructing life tables from which median life span can be read directly. These studies have shown that:
- Survival rates have been increasing in marine mammal parks and aquariums, with the most recent study showing an annual survival rate of 0.97 (which means 97% of the population is expected to survive from one year to the next) for dolphins in U.S. facilities. This corresponds to a median life span of 22.8 years.
- Survival rates reported for wild populations have varied widely, from .902 to .961, with median life spans between 8.3 and 17.4 years. These differences may be due to differences in study methodology (i.e., tracking a live population vs determining the age of dead animals that stranded), or may reflect true lifespan variations in different wild populations.
- Currently, there are dolphins in Alliance of Marine Mammal Parks & Aquariums (AMMPA) member facilities that are in their 40s and 50s.
- The longest lived U.S. dolphin in human care was born February 27, 1953, at Marineland of Florida (now the Dolphin Conservation Center at Marineland, St. Augustine, Florida). Nellie the dolphin passed away at Marineland on April 30, 2014. She was 61 years old.
- The Sarasota Dolphin Research Program has studied dolphins in Sarasota Bay, Florida since 1970 and is the longest running study of a wild dolphin population in the world. Based on this study, the oldest bottlenose dolphin is Nicklo, who was 67 years old when she was last sighted in 2017 in the Sarasota Bay population. Researchers extracted a tooth from her in 1984 to determine her age.
Advances in medical and husbandry knowledge have contributed to improved longevity of dolphins in human care, to the point where today, survivorship measures for bottlenose dolphins in facilities accredited by the Alliance of Marine Mammal Parks and Aquariums (AMMPA), are similar to or better than those reported for wild populations.
- Dolphins and other marine mammals in zoos, aquariums and marine life parks are well loved and well cared for.
- They enjoy extensive social enrichment programs and high quality food.
- The animals also receive the finest veterinary care and benefit from state-of-the-art medical technologies developed for human health care.
- They breed successfully and form complex social groups. Because the animals are content and in excellent physical health, the animals reproduce well and live long lives.
Marine mammals, like all animals in the wild, experience more mortalities during the first 2 years of their lives — a time when they are particularly susceptible to disease, predators, and adverse climate.
- Studies of beached, dead dolphins in the Indian/Banana River system of Florida indicate that 38% died before they reached 2 years of age.
- The mortality rate of dolphins in marine parks is well below the mortality rate of dolphins in the wild.
- Wild bottlenose dolphin calf first year survivorship in the Sarasota Bay study was 76 to 77.5% in different populations.
- Bottlenose dolphin calf first year survivorship in AMMPA facilities is 78 to 86.3% in calves in different subgroups of U.S. facilities.
Studies also indicate that survival rates continue to improve as the marine mammal community’s knowledge of the species’ biology and husbandry advances.
As a dolphin ages, it periodically produces growth layer groups (GLGs) of dental material. Age can be estimated by examining a sliced section of a tooth and counting these layers.
Researchers studying bottlenose dolphin survivorship in the Indian River Lagoon System on Florida's east shore used teeth from stranded animals for age estimation. The researchers decalcified and stained 2-mm (0.8 in.) tooth sections then examined GLGs, which represent annual layers. They placed dolphins into age classes by rounding to the last fully formed GLG.
Age determination can be estimated by examining a sliced section of a tooth and counting these layers.
Disease & Parasitism
As in any animal population, a variety of diseases and parasites can be responsible for dolphin deaths.
- Dolphins may suffer from viral, bacterial, and fungal infections. In addition, they may develop stomach ulcers, skin diseases, tumors, heart disease, urogenital disorders, and respiratory disorders.
- Parasites that typically affect dolphins include tapeworms, flukes, and roundworms.
In 1987 and 1988, more than 740 dead bottlenose dolphins washed ashore on the east coast of the United States. Scientists originally believed that the dolphin deaths were triggered by a naturally occurring "red tide" toxin (originating in small marine organisms called dinoflagellates) combined with bacterial and viral infections. Further analysis concluded that, while exposure to brevetoxin may have made the dolphins more vulnerable to infection, morbillivirus caused the deaths. Morbillivirus is a deadly disease that rapidly affects many animals in the same area at the same time.
An Unusual Mortality Event, or UME, is declared when NOAA Fisheries Office of Protected Resources finds an unexpected, significant stranding and die off of a marine mammal population. In addition to mobilizing government wildlife officials, at times part of the response to an UME involves assistance from rescue and rehabilitation facilities like those found at SeaWorld. Since the program was established in 1991, 62 UME have been declared — unfortunately the last 14 since 2010. Two UMEs were declared in 2013 for bottlenose dolphins.
- 2013 Bottlenose Dolphin Unusual Mortality Event in the Mid-Atlantic
- An elevated number of bottlenose dolphins (more than 900 from January 1, 2013 through October 23, 2013) of all ages have been found dead in the Mid-Atlantic region (New York, New Jersey, Delaware, Maryland, Virginia, North Carolina, and South Carolina). The suspected cause is cetacean morbillivirus (Brucella sp.), affecting the joints, brain, and reproductive organs of infected dolphins.
- 2013 Bottlenose Dolphin Unusual Mortality Event in Florida
- In January of 2013, the Hubbs-SeaWorld Research Institute started to notice an increase in bottlenose dolphin strandings in the Indian River Lagoon (IRL) area. This UME was issued in response to an elevated amount of bottlenose dolphin strandings in the northern and central IRL system in Brevard County.
- From January through December 2013, 78 bottlenose dolphin strandings had been documented. Necropsies showed the most significant factor in their deaths were starvation/emaciation.
- The 600 to 800 bottlenose dolphins that live in the IRL area do not extend outside their territory to feed. Prey normally found in the IRL may have shifted away or had been depleted due to large losses of sea grasses in the area.
Natural predators include certain large shark species such as tiger sharks (Galeocerdo cuvier), dusky sharks (Carcharhinus obscurus), bull sharks (Carcharhinus leucas), and great white sharks (Carcharhinus carcharias). In Sarasota Bay, Florida, about 31% of dolphins have shark bite scars.
Killer whales (Orcinus orca) may occasionally prey on bottlenose dolphins, but documented cases are rare.
While stingrays are not dolphin predators, some dolphin deaths have been attributed to trauma, infection, and poisoning from stingray wounds..
Dolphins, particularly coastal animals, are affected by heavy boat traffic, habitat loss and pollution. Industrial and agricultural pollutants (including pesticides, herbicides and fire retardants) often enter coastal habitats through runoff,
resulting in high levels of toxins both in the water and in dolphin tissues. Accumulation of toxins in body tissues, even at low levels, may lead to reproductive failure, make dolphins more vulnerable to disease, or even lead to death.
In the past, bottlenose dolphins have been taken directly for meat, leather, oil, and meal (for fertilizer and animal feed). Hunting still occurs in various parts of the world including Peru, Japan, the Solomon Islands, and the Faro Islands.
- In the course of fishing operations, gear and nets can accidentally entangle and injure or drown dolphins. Dolphins that are injured or killed and discarded in the course of fishing operations are called bycatch. Experts have concluded that it's likely hundreds of thousands of marine mammals are killed as bycatch each year. The incidental capture of whales and dolphins in fishing gear is one of the most significant threats to some bottlenose dolphin populations. In the United States, when data indicate that the bycatch of a species exceeds its sustainable removal threshold, experts must develop and implement a plan for reducing bycatch. As a result, the bycatch of whales has declined since 1995.
- Dolphins have been observed following recreational vessels and “depredating” lines (removing the fish and eating it), sometimes resulting in entanglement/ingestion-related mortality. Dr. Randall Wells, head of the Sarasota Dolphin Research Project, the longest running study on bottlenose dolphins in the world, noted that 2% of the study population was lost to ingestion/entanglement conflicts with recreational fishing gear in one year. This percent, in addition to natural mortality factors, is unsustainable and if not mitigated could put the population at risk.
- In some areas, prey populations become overfished, leading to depleted populations of dolphins. In the Mediterranean and Black Seas, for example, prey depletion by commercial fisheries caused significant drops in populations of bottlenose dolphins.
- When catches decline, fishermen sometimes believe that local populations of toothed whales are the cause. In some areas, fishermen organize dolphin kills, also called drive fisheries, in an effort to control their populations.
- Probably the largest and best-known of these kills was the drive fishery on Iki Island, Japan. Over the course of 10 years, thousands of toothed whales, including bottlenose dolphins were killed. The Iki Island kills ended in 1986, although drive fisheries in Japan and other countries continue.
- SeaWorld is an active participant in the Association of Zoos and Aquariums' campaign to put a stop to the drive fishery. No animal at SeaWorld came from these hunts. Nor does any other U.S. marine institution purchase animals from this hunt. The collection method is in violation of the U.S. Marine Mammal Protection Act, which means it is illegal for any animal taken as part of this fishery to be brought into the U.S. for display.
United States federal laws do not permit people to feed and swim with dolphins or other marine mammals in the wild. These actions are considered "harassment." When people try to get close to wild marine mammals, they put the animals and themselves at risk. Feeding and swimming with marine mammals in the wild is harmful to animals and sometimes dangerous to people.
- When humans feed marine mammals in the wild, the animals may become less able or willing to search for food on their own.
- The U.S. National Marine Fisheries Service (NMFS) has serious concerns about the quality of fish being fed to wild marine mammals and has received complaints that animals are being fed harmful items such as sandwiches, cookies, candy, and chips. Fish that has not been inspected for freshness can cause illness in the animals fed. There also is strong evidence that feeding marine mammals in the wild can lead to their learning to steal fish off fishing lines. Animals that have stranded on the beach have been found with hooks and fishing lines in their stomachs.
- Reports of injuries to people by marine mammals habituated to being fed in the wild are increasing. Unsuspecting individuals have been bitten, and swimmers have been rammed and pulled under water by wild dolphins that have been taught to expect food from humans.
- Feeding, swimming with animals, or intruding on wild environments also can have a negative influence on many other normal marine mammal behaviors. People are disturbing dolphin resting areas. Scientists believe that the long-term effects of such activities may include the disruption of normal resting patterns, mother-calf nursing and bonding behaviors, and social interactions between animals.
Marine life parks, aquariums, and zoos offer many animal interaction programs that are carefully controlled, monitored by knowledgeable staff, and approved by the government, and are safe, positive experiences for both human and animal participants.
- According to the results of a 2005 public opinion poll conducted by Harris Interactive®, 91% of respondents agree that interacting with dolphins offers people a deeper understanding and appreciation of them.
- Because most people will not engage in behavior that they know will hurt animals, NMFS, the Alliance of Marine Mammal Parks and Aquariums, and SeaWorld are educating people of the harm caused by disturbing marine mammals or their habitats in the wild. The Alliance of Marine Mammal Parks and Aquariums is an international association of more than 50 marine life parks, aquariums, zoos, scientific research facilities, and professional organizations that are dedicated to the conservation of marine mammals and their environments through public display, education, and research.