Spinner dolphins have a widespread distribution in tropical and subtropical oceans around the world, living in both the open ocean and coastal environments. They are known to forage for food in groups during the night when their prey migrate toward the surface, and rest in shallow waters during the day. Spinner dolphins are commonly found around Hawaii due to concentrations of prey brought to the surface by slow, nutrient-rich bottom currents that move up the slopes of the islands.
The purpose of Marc O. Lammers’ 2004 study was to determine the patterns of habitat use and behavior of a population of spinner dolphins around the island of Oahu. Occurrence, movement, and behavioral patterns were examined on the western and southern shores of Oahu over a five-year period, from June 1996 to November 2001. The southern shore has much rougher waters and is more heavily navigated by boats. Observations of the spinner dolphins were made by visually following the movements of various groups and recording their behaviors for periods of several hours. The results follow.
On the western shore of Oahu, spinner dolphins searched for specific locations each morning, each one sheltered from winds and with a white, sandy bottom. These locations serve as gathering sites to meet up after foraging and to socialize. On the southern shore of Oahu, no specific locations were searched for. The dolphins were found in larger groups, most likely to better protect themselves from predators in the rougher waters and without the presence of white sand. In both locations, smaller subgroups of dolphins were formed throughout the day until early afternoon. Spinner dolphins near the western and southern shores of Oahu move around quite a bit during the day, either milling about or traveling along the coast. They do most of their resting at midday and early afternoon, staying nearshore in the daylight to minimize predation. In the late afternoon, they rejoin or meet up with new groups to prepare for foraging. They move offshore with multiple false starts and an abundance of whistles, thought to serve the purpose of alerting all members of the group. On the western shore, the pods of dolphins spread out widely, travel to one of three sites, and have been shown to join up with other species of dolphins for extended dives. Prey moves both upward and shoreward at night, making it possible for the spinner dolphins to make shallower dives and conserve more energy.
Human interaction has a great effect on the spinner dolphins of Hawaii. Socially active groups of dolphins often tolerate human presence, but the more coordinated groups shy away and will leave an area if they become forced to interact. With the increase in dolphin tours and visitors to the Hawaiian islands, local populations of spinner dolphins could become less frequent near the shores, choosing to remain at sea. Therefore, tour industry operators and regulatory agencies should be notified in order to preserve the presence of such beautiful creatures near the Hawaiian shores.
For more information, see the reference below:
Lammers, M. O. 2004. “Occurrence and Behavior of Hawaiian Spinner Dolphins (Stenella longirostris) Along Oahu’s Leeward and South Shores.” Aquatic Mammals 30(2): 237-250.