Kenneth Norris has been an instrumental leader in the research of whales and dolphins, and has published many groundbreaking papers on their social patterns and echolocation skills. In Hawaii, Norris conducted extensive research on spinner dolphins (Stenella longirostris) located in the Hawaiian archipelago. Norris’s paper from 1980 entitled, “Behavior of the Hawaiian Spinner Dolphin, Stenella longirostris” compiles observational data on spinner dolphin social structure, feeding behavior, vocalizations, aerial behavior, predation, and distribution in the Hawaiian archipelago. This study was particularly valuable as most of what was previously known about dolphins came from observations of captive dolphins, where normal behavior and movement can never truly be observed.
The Genus Stenella is composed of three major species or species complexes: The spinner dolphin (S. longirostris), the striped dolphin (S. coeruleoalba), the spotted dolphin (S. attenuata). Hawaiian spinner dolphins are moderate-sized, slim-bodied, and long-beaked odontocetes. Adults reach at least 2 m TL and about 55-62 kg. Spinner dolphins have a dark gray cape over the dorsal surface, a light gray lateral field, and the white belly below. The beak, flukes, dorsal fin, and pectoral fin are all dark gray. In the eastern and central pacific, Perrin (1975) has discerned four geographical forms of spinner dolphins:
1) A Costa Rican long-snouted form occurring close to the Central American coast
2) An eastern form occupying the open sea from the America coast out to long. 115° W
3) A whitebelly form occupying the open ocean both south and west of the eastern form (and overlapping with it to some extent) to about long. 145° W and nearly to lat. 5° S.
4) An Hawaiian form localized around the Hawaiian island chain
Hawaiian Spinner dolphins are most closely related to the adjacent whitebelly form; only the Costa Rican and eastern forms are strikingly different from the Hawaiian spinner dolphin. Spinner dolphins occur throughout the Hawaiian chain from its northwestern most limit at Kure Atoll (lat 25°40’ N, long 175°38’ W) to its southernmost limit at South Point or Ka Lae on the island of Hawaii (lat 18°49’ N, long 155°41’ W).
Norris’s study observed spinner dolphins from Kure Atoll to the Hawaiian Islands, in particular a school permanently living Kealakekua Bay on the Kona coast of the Island of Hawaii. The bay seems to be able to hold only 60-70 animals. An observation camp was established on the Greenwell Ranch at the edge of the cliff overlooking Kealakekua Bay. Two observation sites were used for recording and observation and several vessels were used for observations at sea, including the brigantine Westward and the motor sailors RV Hikino and the RV Imua. Underwater observations were made in a specially built underwater observation vehicle. Acoustic recordings were made with a hydrophone deployed from a stationary skiff located near a school of animals or from the MOC. Dolphin radio tracking was achieved through dolphin dorsal fin radios and a handheld direction finder.
Scars and markings were used to distinguish individuals and school structures. Spinner dolphins show scars from large sharks and from the small squaloid shark: Isistius brasiliensis. The dolphins frequently boast scars from meetings with boat propellers and from becoming entangled in fishing lines or plastic pollution. Schools vary in the number of individuals and their composition, usually gathering in small to moderate schools of 6 to 250 animals. Large schools differ from each other based on their composition of older adults or juvenile groups. Norris suggests that some subgroup structure may persist over long periods and that familial lineages may be important as has been observed in captivity. Instead of finding tightly knit schools of constant size that habitually occupy a given cove like they expected, Norris and his observers found coves occupied by schools of highly variable numbers and composition that would often merge to form large feeding groups offshore. Thus, Norris terms smaller groupings of less than about 30 animals “schools” while recognizing that such schools change in size and structure from time to time.
Spinner dolphins occasionally seek schools of spotted dolphins for refuge during rest in the open sea. Unlike spinner dolphins, spotted dolphins feed during the day, allowing spinner dolphins to associate with alert animals while they rest. The daily cycle of spinner dolphins includes nighttime feeding, morning approach to shore, morning-midday rest, and travel to feeding grounds near dusk. The primary feeding grounds seem to be to the north and west of Kealakekua Bay off the shallows of the island. Spinner dolphins feed upon scattering layer fishes, squid, and shrimp.
There are three noticeable features of the distribution of spinner dolphins in the Hawaiian chain: The distribution is discontinuous; some coasts have several areas where dolphins congregate while other coasts have stretches of no dolphins for several kilometers. Second, certain coves or shallow areas are regular aggregation sites while others seem to be used more infrequently. Third, some areas consistently carry more animals than others. Rest areas are shallow sandy areas usually 50 m or less in depth, are usually composed of a mixture of open sandy bottom with scattered coral formations, and usually water greater than 500 m depth can be reached within a few kilometers. Norris infers that rather than having a “home cove”, rest areas are chosen by dolphins not only for physical characteristics such as depth, bottom type, and perhaps calm water but also for their accessibility to nighttime feeding areas.
Spinner dolphins exhibit aerial patterns thought to be associated with sound production upon reentry. Each aerial pattern is typical of a specific school activity level, and each aerial behavior is structured to make noise. Tail slaps may be the loudest. Spinner dolphins not only spin and leap from the water, but also perform other aerial behavior including the tail over head leap, the backslap, the headslap, the noseout, and the tailslap. Spinner dolphins get their name from the characteristic spins they perform. As the dolphin breaches the water, they flex in order to spin around a longitudinal axis during the leap. As many as four revolutions may be made in a leap. Spins are usually performed in a series of descending intensity, with most series consisting of three or four spins. The greater the alertness, the more frequent the spins. In feeding schools, spinning and other high energy aerial behavior occur almost continuously. Vocal behavior varies based on how alert the group of dolphins is. Alert schools produce clicks, pure-tone whistles, squeals and a variety of burst-pulse signals described as barks, moos, or chirps. Resting schools are nearly silent, only sporadically emitting clicks. A school of dolphins swimming outside Kealakekua Bay during longshore movement would be able to detect animals in the bay without having to enter.
While this study was comprehensive and provided a significant amount of data to the public, Norris targeted several areas in which he would like further research completed. The not ideal positioning of observational land camps, and limited ability for underwater observation prevented thorough data to be gathered on the following “problem areas”: acoustic signaling, school structure, energetics, and social relationships.
Photo Credit: Figure is from Norris and Dohl’s 1980 paper