Spinner dolphins are probably the most frequently encountered cetacean in nearshore waters of the Pacific Islands Region. Spinner dolphins received their common name because they are often seen leaping and spinning out of the water. The species' name, longirostris, is Latin for “long beak,” referring to their slender shaped beak or rostrum.
Regarded as one of the most acrobatic of dolphins, spinner dolphins are well known for their habit of leaping from the water and spinning up to seven times in the air before falling back into the water. Experts believe that spinner dolphins use these behaviors primarily for acoustic signaling or communication, but the activity can also be a way to remove ectoparasites, such as remoras.
Spinner dolphins are relatively small compared with other species of oceanic dolphins. They are slender, with thin, recurved flippers, and dorsal fins that usually range from slightly curved to erect and triangular. Among Gray’s spinner dolphins (the subspecies that includes Hawaiian spinner dolphins), adult females are 4.6 to 6.7 feet long and adult males are 5.2 to 6.8 feet long. They reach weights of at least 181 pounds. There is a great deal of color variation in spinner dolphins across the globe, depending on the region and subspecies of dolphin. Gray’s spinner dolphins exhibit a tripartite color pattern with countershading from dark to light. The three-part color region consists of a dark gray dorsal/back cape, a light gray side, and a white belly.
Individual dolphins are identified by their unique dorsal fins. Researchers take photographs of the dolphins' dorsal fins and then match the shape, nicks, and notches in each fin to a catalog of known individuals to obtain life history information for each animal.
Spinner dolphins feed at night on species including small fish, shrimp, and squid that are found about 650 to 1,000 feet below the surface of the water. Spinner dolphin prey species follow a vertical and horizontal migration pattern, staying in deep waters in the ocean during the day, and then moving up in the water column (vertical migration) and inshore (horizontal migration) at night. Spinner dolphins take advantage of the nightly migration that brings their prey species to shallower depths and closer to shore by feeding throughout the night.
When resting, spinner dolphins move back and forth slowly as a single unit, with the animals in tight formation but spaced just out of contact with one another. They may engage in resting behaviors for about four to five hours daily. This behavior may vary seasonally, coinciding with the shifts in day length. During rest, spinner dolphins rely on vision rather than echolocation for scanning their environment. Group movements during rest are typically in open, sandy-bottom areas where predators are more visible.
At the end of their rest period, spinner dolphins usually abruptly increase their activity level, including their swim speeds, aerial behaviors, vocalizations, and make shorter dives than when resting. In 1994, researchers described spinner dolphins swimming in a “zig-zag” pattern following their rest period. They swam toward the open waters and then doubled back into shallower waters. This back-and-forth swimming repeats a zig-zag formation over most of the area, possibly functioning as a social cue for the entire group to coordinate their movement into the deeper seas. Likewise, the dolphins’ acoustic behaviors rise and fall synchronously with the zig-zag swimming patterns. When these patterns subside, the spinner dolphins swiftly race to the offshore waters of their foraging grounds, where they are sometimes joined by bottlenose or spotted dolphins. At this point, spinner dolphins’ dive times are extended and the dolphins begin their foraging movements.
Habitat
In most places, spinner dolphins are found in areas of deep waters where they likely track prey. Although the pelagic stock of Hawaiian spinner dolphins are found in the deeper waters offshore of the islands, the rest of the Hawaiʻi population has a more coastal distribution. During daytime hours, the island-associated stocks of Hawaiian spinner dolphins seek sanctuary in nearshore waters, where they return to certain areas to socialize, rest, and nurture their young. These areas are typically in clear, calm, and relatively shallow waters. They usually have a sandy bottom that presumably provides an environment in which the dolphins are able to visually monitor for predators, as they cease echolocation while they rest. Spinner dolphins use a variety of bays and nearshore coastal waters throughout their range, but they seem to prefer certain bays.
Distribution
Spinner dolphins are found throughout the world in tropical and warm-temperate waters. Four subspecies of spinner dolphins have been described worldwide:
- Stenella longirostris longirostris (also known as Gray’s spinner dolphin)—which includes the Hawaiian spinner dolphin—in the tropical Atlantic, Indian, and western and central Pacific Oceans.
- S. l. orientalis in the eastern tropical Pacific Ocean
- S. l. centroamericana near Central America
- S. l. rosiventris, the dwarf spinner dolphin, in southeast Asia to northern Australia
The Gray’s spinner dolphin is the typical form of spinner dolphin found in most areas of the world.
Like all marine mammals, spinner dolphins are slow reproducers. They live for about 20 years, with some individuals living for at least 25 years. Spinner dolphins may mate year-round, with multiple males mating with one female. Gestation is similar to other dolphin species and lasts approximately 11 months. Spinner dolphins calve year-round, generally about once every 3 years, and lactate often for 1 to 2 years. They reach sexual maturity at around 7 years of age.
Human Interactions and Viewing Pressure
Viewing wild marine mammals in Hawaiʻi has been a popular recreational activity for both tourists and residents over the past several decades. We estimate that there are upwards of 70 tour operators that provide dolphin-directed tours focused on Hawaiian spinner dolphins. More than 100 commercial boat tour and kayak tour operations may opportunistically view these animals.
Historically, tours operated out of various harbors along most of the coasts of the main Hawaiian Islands, bringing guests to well-known locations for spinner dolphin viewing. Researchers have observed up to 13 tour boats at a time in some locations, with vessels jockeying for position on a single spinner dolphin group and more than 60 swimmers in the water attempting to closely interact with the dolphins at once. In addition, a number of residents and visitors ventured on their own, independent of commercial operators, to view and interact with spinner dolphins. Some operators and various media outlets encouraged the expectation for close interactions with wild dolphins, and routinely promoted close vessel or in-water encounters with the dolphins.
Because of these problematic interactions, NOAA Fisheries developed regulations that now prohibit swimming with, approaching, and remaining within 50 yards of spinner dolphins within 2 nautical miles from shore of the main Hawaiian Islands and in designated waters bounded by the islands of Lāna‘i, Maui, and Kahoʻolawe.
Commercial operators and individuals interested in viewing or closely interacting with Hawaiian spinner dolphins increasingly target the dolphins' essential daytime habitats. Encounters with dolphins in these areas are very common due to the predictability of spinner dolphin behavior. Viewing spinner dolphins in their natural habitat can be an educational and enriching experience if conducted safely and responsibly from a distance. However, closely approaching, swimming with, pursuing, interacting, or attempting to interact with the dolphins could disrupt their daytime behaviors. Some interactions with people in essential daytime habitats may disturb individual dolphins or resting groups, even rising to the level of “take” under the Marine Mammal Protection Act, which is illegal.
Response to disturbance varies among individuals, but ultimately diverts time and energy from fitness-enhancing activities (such as resting and recuperating after an energetically-costly night of foraging for prey). These responses may, over time, result in negative effects to the fitness of individuals or resident populations. For example, lack of consistent, undisturbed resting periods can reduce the amount of energy available for a spinner dolphin to engage effectively in foraging activities at night. Over time, this can result in overall poor body condition, which reduces the dolphin’s ability to fight off disease, protect itself from predators, successfully reproduce, or rear young.
Peer-reviewed scientific literature has documented disturbance responses by individual spinner dolphins, as well as changes to spinner dolphin group behavior patterns over time. Individual dolphin responses include:
- Increased displays such as leaping, spinning, or tail slapping when closely approached by vessels and swimmers.
- Avoidance behaviors, including increased swimming speed and moving away from swimmers and vessels, or leaving the bay completely in response to human pursuit.
- Aggressive behaviors directed at people, including charging or threat displays.
Researchers have also documented changes to spinner dolphins’ behavior patterns in essential daytime habitats, including changes to patterns associated with aerial behaviors, residence times, and distribution. Human-caused disturbances to daily behaviors may be incidents of 'take,' as defined (and prohibited) under the Marine Mammal Protection Act. The chronic nature of these problems in Hawaiʻi and the observed changes to behavioral patterns over time are a cause for concern for this dolphin population.
Marine Debris
Marine debris is a growing concern within the marine environment, as it poses multiple threats to the marine ecosystem. For instance, spinner dolphins may ingest (either directly or through prey items) or become entangled in marine debris. These interactions may cause:
- Drowning
- Debilitation
- Limited predator avoidance
- Internal or external wounds
- Skin lesions or sores
- Blockage of the digestive tract, resulting in starvation that often leads to death
- Reductions in quality of life and/or reproductive capacity
- Impairment of feeding capacity
- Introduction and/or concentration of damaging or toxic compounds to the animal
The severity of the effects of debris interactions on all dolphin populations remains unclear because many deaths likely occur undetected at sea.
We do not have sufficient information to determine the severity of the threat of direct ingestion of large debris on spinner dolphin populations. Some data indicates that spinner dolphin prey species are consuming very small plastics. In 2010, researchers analyzed mesopelagic fish in the North Pacific Central Gyre and found that 35 percent of the fish—many of which were lantern fish (spinner dolphins’ main prey)—had ingested plastic. Larger fish generally had more pieces of plastic in their guts than smaller fish. We are particularly concerned about the ability for plastic debris to absorb organic pollutants that may be toxic to the marine organisms that ingest them. These contaminants can bioaccumulate as they move up the food chain to top predators such as spinner dolphins. Scientists have also found high levels of butyltin and organochlorine (chemical compounds found in some plastics) in migrating lantern fish species in the Western North Pacific, which may indicate a cause for concern for predators, such as spinner dolphins.
Human-Made Noise
Humans introduce sound intentionally and unintentionally into the marine environment. This could be from commercial and recreational ocean activities, navigation, oil and gas exploration and acquisition, research, and military activities. Spinner dolphins use sound to communicate, navigate, locate prey, and locate predators, which can be impacted by introduced anthropogenic noise. How severely noise exposure affects dolphins and whales depends on factors including:
- Noise source
- Decibel level
- Distance between the source and the animal
- Characteristics of the animal (for example, hearing sensitivity, behavioral context, age, sex, and previous experience with sound source)
- Time of day or season (which affects how sound travels through the water)
In marine mammal populations, noise can seriously disrupt communication, navigational ability, and social patterns.
Disease
Spinner dolphins, like all marine mammals, can be susceptible to widespread disease. Outbreaks in spinner dolphins are not commonly reported, but scientists have previously detected serious diseases, such as toxoplasmosis and cetacean morbillivirus. Although the number of spinner dolphin deaths attributed to these diseases is fairly low (many are thought to be unreported), thousands of other marine mammal species have died from these diseases worldwide.
