Spinner Dolphin (Stenella longirostris)

 

Description


The spinner dolphin is a small cetaceans with a slim build. Adults are typically 129235 cm and reach a body mass of 2379 kg. This species has an elongated rostrum and a triangular or sub-triangular dorsal fin. Spinner dolphins generally have tripartite colour patterns. The dorsal area is dark gray, the sides light gray, and the underside pale gray or white. There is also a dark band that runs from the eye to the flipper, bordered above by a thin light line. However, the spinner dolphin has more geographic variation in form and coloration than other cetaceans. In the open waters of eastern Pacific, dolphins have relatively small skulls with short rostra. There is also a dwarf form of spinner dolphin that occurs around southeast Asia. In these same subspecies, a dark dorsal cape dims their tripartite colour patterns. Further offshore, subspecies tend to have a paler and less far-reaching cape. In certain subspecies, some males may have upright fins that slant forward. Some populations of spinner dolphin found in the eastern Pacific have bizarre backwards facing dorsal fins, and males with strange humps and upturned caudal flukes.

Ecology

The spinner dolphin lives in nearly all tropical and subtropical waters between 40N and 40S. The species is primarily inhabit inshore waters, islands or banks. However, in the eastern tropical Pacific, dolphins live far from shore. Spinner dolphin may use different habitats depending on the season.

The spinner dolphin feed mainly on small mesopelagic fish, squids and sergestid shrimps and will dive 200-300m to feed on them. Spinner dolphins of Hawaii are nocturnal feeder and forage in deep scattering layers, which contain many species. The dwarf spinner dolphin may eat mostly on benthic fish in reefs and shallow water. Off Oahu, Hawaii, spinner dolphins foraging at night and cooperatively herd their prey in highly dense patches. They swim around circle of prey and a pair may swim through it to make a catch. Spinner dolphins are themselves preyed on by sharks. Other possible predators include then killer whale, the false killer whale, the pygmy killer whale and the short finned pilot whale. They are also susceptible to parasites, both external ones like barnacles and remoras, and internal ones like nematodes, trematodes, cestodes and acanthocephalans. Spinner dolphins move about the oceans in schools; groups that vary in size from just a few dolphins to over a thousand. They commonly school with other species such as Pantropical spotted dolphins, or small toothed whales. In such schools, spinner dolphins are known to undertake migrations, following prey or warm water currents. In Hawaii, spinner dolphins usually spend their days resting in shallow bays near deep water, and then move offshore at dusk and feed as they move substantial distances along the shore (8). Pelagic spinner dolphins feed primarily on small mesopelagic fish, squids and shrimps, and dive down to depths of 300 meters to catch their prey . The dwarf spinner dolphin feeds on reef fishes and other benthic organisms.

Mating in spinner dolphins appears to be promiscuous, and like many small dolphins, true courtship behaviour can be observed, such as mutual caressing between the male and female. The breeding system may vary geographically, with some populations showing a greater degree of polygyny than others . Calves are born every three years, after a gestation period of about ten months. The mother nurses the calf for up to two years, and they form a bond that lasts a lifetime. Females reach sexual maturity between four and seven years, whereas males do not reach maturity until between seven and ten years.

The purpose of the energetic spinning behaviour of the spinner dolphin is not known. It has been suggested that the large cloud of bubbles created by the powerful spin and splash landing may act as an echolocation target, to allow a widely dispersed school of dolphins to communicate. Another theory is that the spinning may dislodge hitch-hiking remoras, or the spinning may, at times, simply be play.

Behavior and life history

In certain regions, such as Hawaii and northern Brazil, dolphins spend the daytime resting in shallow bays near deep water. At dusk, they travel offshore to feed. They travel along the shore during foraging trips, and the individuals that occupy the same bay may change daily. Some individual dolphin do not anyways go to  a Bay to rest. However, in Hawaii, dolphins do seem to return to the same site each trip.
Spinner dolphins live in an open and loose social organization. The spinner dolphins of Hawaii live in family groups but also have associations with others beyond their groups. Mothers and calves form strong social bonds. Spinner dolphins seem to have a promiscuous mating system, with individuals changing partners for up to some weeks. A dozen adult males may gather into coalitions. Vocalizations of spinner dolphins include whistles, which may be used to organization the structure of the school; burst-pulse signals, which may serve to be evocative and vocative and echolocation clicks. The spinner dolphin has a 10 months gestation period, and mothers nurse their young for 12 years. Females are sexuality mature at 47 years, with 3 year calving intervals, while males are sexually mature by 710 years. Breeding is seasonal, more so in certain regions than others.

Spinning behavior

Spinner dolphins are well known for their acrobatics and aerial behaviours. A spinner dolphin comes out of the water, front first, and twists its body as ascends in the air. After it reaches its maximum height the dolphin descends back into the water, landing on its side. A dolphin can make 2-5.5 spins in one leap. The swimming and rotational speed of the dolphin spinning underwater affects the number of spins it can do while airborne. These spins may serve multiple functions. Dolphins may also make nose-outs, tail slaps, flips, head slap, "salmon leaps" and side and back slaps.

Threats

Spinner dolphins in the eastern tropical Pacific have been killed incidentally since the early 1960s by tuna purse seine fisheries. They were caught in such large numbers that the population of S. l. orientalis was reduced to less than one third of its original size. Following raised awareness of the number of dolphins killed in tuna purse seine fisheries, measures were implemented to reduce dolphin by-catch. Today spinner dolphins continue to be killed in this way, although in greatly reduced numbers. However, continued chase, capture and release of large numbers in the fishery may be preventing the population from recovering .

In Sri Lanka and the Philippines, large numbers of spinner dolphins have also been captured in gillnets and killed by harpoons for the past 20 years, and local harpoon fisheries exist in several more locations throughout the world. Incidentally captured dolphins are consumed by local people, or used as shark bait, and this has led to the development of markets and fisheries directed at dolphins. The takes in these fisheries may be unsustainable.

Conservation

The eastern tropical Pacific and Southeast Asian populations of the spinner dolphin are listed on Appendix II of the Convention on Migratory Species (CMS). This means that the spinner dolphin is a migratory species that needs, or would significantly benefit from international co-operation, and the convention encourages the range states to conclude global or regional agreements. The spinner dolphin is also listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), meaning that international trade in this species should be carefully regulated. The value of dolphins as a tourist attraction offers countries and communities an incentive to protect these beautiful animals. Fernando de Noronha National Marine Park; (an archipelago off equatorial Brazil) was established in 1988, to provide nominal protection to spinner dolphins and support dolphin watching tourism, and it has been reported that in Zanzibar, the value of spinner dolphins for tourism far exceeded that of using them as bait for sharks (8). It is hoped that with meaningful laws, the will and resources to enforce them, continued attention by non-governmental organizations and efforts to make the public aware of the intrinsic value of their endemic dolphins, this charismatic species will continue spinning in our oceans forever.

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