Protected Resources

Spinner Dolphin

©FAO/FIGIS

Scientific name: Stenella longirostris longirostris
Pacific Island names: nai'a (Hawaiian), mumua (Samoan)

Stock Assessment / Estimated Breeding Population

The best available population estimate is approximately 3,300. Scientists have proposed 6 separate stocks for the Hawaiian Islands. Two distinct stocks in the Northwestern Hawaiian Islands: Kure and Midway atoll; Pearl and Hermes Reef; and three in the Main Hawaiian Islands: Niihau/Kauai; Oahu, Maui, Lanai, Molokai, Kahoolawe; and the Island of Hawaii. The sixth stock includes all other animals within the EEZ, including those at French Frigate Shoals. These stocks are reproductively isolated populations, with few or no genetic exchange occurring (Andrews, 2009).

Legal Protection

All marine mammals are protected from "take" under the Marine Mammal Protection Act (MMPA). The MMPA states that the essential habitats used by marine mammals should be protected, and marine mammals should be protected from the harmful actions of man. Responsible viewing guidelines provided by NMFS can be found at http://www.fpir.noaa.gov/PRD/prd_laws_policies_guidelines1.html. Spinner dolphins are currently not listed as "threatened" or "endangered" under the Endangered Species Act (ESA).

Natural History

Spinner dolphins are probably the most frequently encountered cetacean in waters of the Pacific Islands Region. They get their common name from the impressive spins they perform during social encounters. Spinners spend their daylight hours in coastal waters, generally in calm bays. They use these areas to rest, care for their young and to avoid predators, before traveling to deeper water at night to hunt for food. Spinner dolphins have what is called a "fission fusion social pattern." They "fuse" to form large schools of hundreds of animals when feeding at night and split off into much smaller groups, sometimes of only a dozen individuals, when socializing and resting during the day.

Physical Description

The spinner dolphin is a small dolphin that lives in open ocean and coastal waters.
Individuals will generally grow from 4 to 7 feet in length and weigh between 100 and 165 pounds.
Each has a long narrow beak, dark grey dorsal cape, medium grey flanks and a light grey/whitish belly.

Individual dolphins are identified by their unique dorsal fins. Researchers take photographs of the animals' dorsal fin, and the shape, nicks and notches in each fin can then be matched to a catalog of known individuals to obtain life history information for each animal. See the PIFSC PIPIN page for more information (http://www.pipin.org/community/)

Diet

Spinner dolphins feed at night on mesopelagic fish, shrimp and squid that are found about 650-1,000 feet below the surface of the water. The mesopelagic boundary layer stays in deep waters in the ocean during the day. At night they move up in the water column (vertical migration), and inshore (horizontal migration). Dolphins follow these diel migrations of their prey in order to maximize foraging efficiency.

Drawing: Spinner dolphins rest in shallow bays during the day and feed in
deeper waters at night. Dave Johnston/PIFSC

Lifespan

Lifespan is estimated to be about 20 years.

Reproduction

Spinner dolphins do not have a mating or birthing season. They mate, give birth, and care for their young year round.

Distribution

Spinner dolphins are found worldwide in temperate to tropical waters. Stenella longirostris longirostris is a subspecies that resides in waters throughout NOAA's Pacific Islands Region. New stock assessment data shows that there are six stocks in the Hawaiian archipelago; 2 in the NWHI and 3 in the MHI, with one stock encompassing all other animals in the EEZ, including those at French Frigate Shoals.

Potential Threats

Current research shows that pursuit and close approach of boats, swimmers and other ocean users to spinner dolphins may have negative impacts on their health and behavior.
Other potential threats include entanglement in marine debris, anthropogenic (human made) noise, and fisheries interactions.

Current Management Issues

The Protected Resources Division is working on drafting an Environmental Impact Statement (EIS) on the potential rulemaking under the MMPA to provide more protection to Hawaiian spinner dolphins. The Notice of Intent (NOI) (Sept 2006, pdf 67kB) to prepare the EIS was published in the Federal Register on October 2, 2006.

For more information, please click here to visit our Spinner Dolphin - Human Interaction EIS and Rule-making page.

To learn more about the potential impact of swimming with wild spinner dolphins, please visit our webpage entitled "Swimming with Wild Spinner Dolphins"

Viewing Guidelines

Remain at least 50 yards from spinner dolphins.
Limit your time observing to 1/2 hour.
Spinner dolphins should not be encircled or trapped between boats or shore.
If approached by a spinner dolphin while on a boat, put the engine in neutral and allow the animal to pass. Boat movement should be from the rear of the animal.

Important Phone Numbers

Marine Mammal Stranding/Entanglement Hotline: 1-888-256-9840

More Information

Frequently Asked Questions (FAQ): Hawaiian Spinner Dolphins (Jan 2011, pdf 246 kB)
If you would like to be updated on current research and upcoming meetings concerning the Hawaiian spinner dolphin, please join the NAIANEWS listserv: http://listserver.afsc.noaa.gov/read/all_forums/subscribe?name=naianews
The Pacific Islands Fisheries Science Center (PIFSC) has created the following Pacific Islands Photo-ID Network (PIPIN) website to present general information about spinner dolphins in the Hawaiian Islands, photo-id techniques, news regarding spinner dolphin science and conservation and links to cetacean websites: http://pipin.org/community
Advanced Notice of Proposed Rulemaking (ANPR) (Apr 2006, pdf 64kB)
Federal Register Notice of Intent (Oct 2006, pdf 67kB)
Swimming with Wild Spinner Dolphins webpage