SPINNER DOLPHIN
Stenella longirostris (Gray 1828)

Order Cetacea : Family Delphinidae

DESCRIPTION. A small dolphin that averages <1.8 m in length and 75 kg in weight; maximum size about 2.1 m and 95 kg. Very slender body with a long, narrow beak that is black above and white below. Coloration dark gray dorsally fading to lighter gray on the sides; belly white; a dark stripe extends from the flipper to the eye. Average total teeth 224, more than any other Texas cetacean.

DISTRIBUTION. Worldwide in tropical, subtropical, and warm temperate zones. Known in Texas from strandings along Padre Island National Seashore. In the Gulf of Mexico, spinner dolphins occur in waters generally deeper than 100 m.

SUBSPECIES. The subspecies in the Gulf is thought to be S. l. centroamericana.

HABITS. Spinner dolphins derive their name from a habit of leaping from the water and warping their bodies into graceful curves, or spinning lengthwise before splashing back into the water. The motives for this behavior are not known, but such actions are often sufficient to distinguish this species.

They usually occur in groups of 30 to several hundred but may number into the thousands. Spinner dolphins feed on mesopelagic fish, squid, and shrimp.
Adult females give birth to a single calf at 2-year intervals. Parturition usually occurs in early summer but can occur in any season. Gestation is 11 months and calves are about 75 cm long at birth.

Two mass strandings of spinner dolphins have been reported in the Gulf of Mexico. One stranding of 36 animals occurred in 1961 on Dog Island, Florida, and the other in 1976 was near Sarasota, Florida, and involved 50–150 spinners that beached themselves at several points during an extremely low tide. The dolphins came ashore with much "squealing and crying," but the noise later subsided and the animals were quite passive on the beach. Several of the animals were returned, apparently successfully, to the sea; however, others merely stranded again, and at least 10 died.

In the eastern tropical Pacific, this species is often caught and drowned in large numbers by the tuna fishing industry. Over the last 20 years the total population in those waters has declined about 80%, from 2 million to 400,000, because of incidental catch. Gulf of Mexico populations are not subjected to this pressure.

POPULATION STATUS. Common; strandings and observations. The spinner dolphin is another offshore, deepwater dolphin that has only occasionally stranded or been sighted near the Texas coast. The most recent stranding period (2002–2014) included five strandings of this dolphin from three different counties along the Texas coast (Galveston, Nueces, and Kleberg). During the GulfCet surveys, spinner dolphins were sighted in every season except fall. Group sizes ranged from 9 to 750 animals. At least 15,000 spinner dolphins are believed to occur in the northern Gulf, generally in waters much deeper than 100 m, and most of the sightings have come from east and southeast of the Mississippi Delta.

CONSERVATION STATUS. The spinner dolphin does not appear on the USFWS or TPWD list of concerned species, and it is considered to be "data deficient" by the IUCN. The data available from the Gulf would seem to indicate this dolphin is relatively abundant with no reason to question its conservation status.

 

From The Mammals of Texas, Seventh Edition by David J. Schmidly and Robert D. Bradley, copyright © 1994, 2004, 2016.  Courtesy of the University of Texas Press.