Dolphins are known to be endangered because of accidents that are occuring in the sea now and then.

Spinner Dolphins

Biology
They are relatively small dolphins when compared to other dolphins, such as the bottlenose dolphin. They usually grow to lengths of about 6 ft. - 7 ft. and also weigh approximately 130 lbs - 170 lbs. at adulthood. They have very long, slender snouts or beaks. Mating and calving occur all year-round, with gestation being that of most dolphins, around eleven months. They are best known for their above-water displays of leaping and spinning several times on their body axis. Spinner dolphins feed primarily at night on mid-water fish and squids in deep waters, while resting for most of the day.

Human Threats
Usually, the eastern spinner and other dolphins have been the target of the tuna purse-seine fishery which uses the dolphins' locations to find tuna, and the eastern spinner dolphin is considered to be one of the dolphin species most affected by this fishery as when tuna is being caught, dolphins which are oblivious to their surroundings also get caught in the net. When unnoticed by the fishermen, they will die in the net. But although some fishermen noticed it, they will not be able to save them in time.

Bottle-nose Dolphins

Biology

Adult length is from 8-12 feet (2.5-3.8 m). These dolphins may weigh as much as 1,430 pounds (650 kg) off Great Britain , though most are much smaller in other parts of the world. Males are significantly larger than females.

FEEDING

Feeding behaviors are diverse, primarily involving individual prey capture, but sometimes involving coordinated efforts to catch food, feeding in association with human fishing, and chasing fish into mudbanks. An adult bottlenose dolphin may consume 15-30 pounds (8-15 kg) of food each day. Bottlenose dolphins eat a wide variety of food, including primarily fishes, and sometimes squid and also crustaceans.

MATING AND BREEDING

Males reach sexual maturity at about 10 years of age. Females reach sexual maturity at about 5-10 years of age. The gestation period is 12 months. Calving can take place all year-round with peaks in some areas during spring and fall. Calves nurse for about 12 to 18 months and stay with their mothers for 3-6 years during this period of time, thry will learn how to catch fish and other important tasks.

How dolphins communicate:

Dolphins make a wide array of sounds, such as clicks, moans, chirps, creaks, barks, squeaks, yaps, mews, and whistles. They are excellent mimics too and have been known to make sounds resembling the engine of a motorboat, and the laugh of their trainer!

Dolphins use clicking noises in echolocation, which bounce off objects underwater. This allows them to navigate, identify preys and friends, and avoid obstacles and predators. Dolphins use whistles to maintain contact within their pod or when meeting other pods of dolphins. Their whistles may signal danger, a call for help, or simply identification. Scientists think that each dolphin has its own signature whistle, something like our names. Whistles may also help dolphins to hunt cooperatively and coordinate migratory movements.

How do scientists study marine mammals' use of sound in social interactions?

Sounds carry a long distance underwater, but we humans have a hard time pinpointing where the sounds originate. Scientists record the sounds of captive dolphins often found it impossible to decipher which animals were making which sounds, since marine mammals do not move their mouths when they vocalize.

Peter Tyack, a whale researcher at the Woods Hole Oceanographic Institution, has developed the "datalogger," a computerized instrument designed to record sounds made by captive marine mammals, such as beluga whales and dolphins. Each datalogger contains an underwater microphone linked to a small computer to store information. A datalogger is attached to a subject's back by two suction cups. As the animals vocalize, the datalogger on each animal picks up the loudest sound, revealing who is "talking."

Cheri Reechia, a graduate student at the Woods Hole Oceanographic Institution, is using the data logger to study vocalizations and social behavior among groups of beluga whales at four aquariums. Belugas long ago were nicknamed the "canaries of the sea" because sailors could hear their plaintive calls even through the wooden hulls of their sailing ships.

She believes understanding their vocal and social behavior will be helpful for captive-animal husbandry, perhaps providing information on changing social dynamics, breeding conditions, or the general health of captive belugas. Long-term, she hopes to use dataloggers to study wild beluga society, to track their movements and provide data for their protection in the ocean.