Dolphins & Porpoises

There are 33 species of oceanic dolphins (Delphinidae family) and six porpoise species (Phocoenidae family).

There are six living species of freshwater cetaceans four of which are dolphins: the Amazon, Ganges, and Indus river dolphins, the Irrawaddy dolphin, the tucuxi, and the finless porpoise. Most of these species are threatened by overfishing and habitat loss. Amazon river dolphins inhabit flooded grasslands and forests during the rainy season. Ganges river dolphins live in a river whose banks house a tenth of the world’s human population. It is common for river dolphins to have small eyes, and the Ganges river dolphin and Indus river dolphins are almost blind. A seventh species, the baiji or Yangtze river dolphin, lived in the Yangtze River. The baiji was declared extinct in 2007 because it was often caught as bycatch in other fisheries.

Yellow-fin tuna often school beneath large herds of pan-tropical spotted dolphins and spinner dolphins in the eastern Pacific Ocean. One theory explaining this pairing is that the dolphins and tuna are both feeding on surface-dwelling squid, mackerel and flying fish. When purse-seine-fishing nets are set beneath the dolphins to surround the tuna, dolphins and other animals also get tangled in the nets and die. These unintended catch are called bycatch.

Public pressure for “dolphin safe” canned tuna led to new laws in 1990, helping to reduce the numbers of dolphins killed. However, tuna fishers are still allowed to set seine nets around dolphins, provided that dolphins are not observed being killed. Studies are presently being carried out to determine how dolphins are affected by being encircled by seine nets or by being chased at high speeds by fishing vessels. Only the yellow-fin tuna fishery affects dolphins. Albacore tuna do not associate with dolphins.

Whistle-like sounds help dolphins keep in contact and communicate with each other as they travel and feed. Dolphins "echolocate" to find their food or to scan their surroundings. They direct "clicks" into the water and the clicks rebound off solid objects (fish, logs, boats) and echo back to the dolphins. Dolphins listen for the strength of the rebounding clicks to identify what the object is and its distance from them.

These clicks and whistles are created in the dolphin’s nasal passages just below their blowhole. The sounds are received by fat deposits in the lower jaw. These deposits send vibrations of the echo to the middle ear and brain for processing.

Pacific white-sided dolphins feed on herring, squid, salmon, anchovies and pollock. Hector’s dolphins of New Zealand’s coastal waters feed on schooling yellow-eyed mullet and red cod from the ocean floor. Bottlenose dolphins feed on small fishes, eels, catfishes, mullet, squid and shrimp. White-beaked dolphins of the North Atlantic have been observed feeding with fin whales on capelin.

Dolphins are definitely smart but there is much debate over intelligence tests. Do they indicate relative intelligence or reflect the adaptations each species has made to its environment? Studies of numerous species of dolphins have shown evidence of high levels of intelligence, including complex social behaviour, detailed memory, self-recognition, and the ability to learn rudimentary symbol-based artificial codes. The differences between the various dolphin species reflect the variation in their habitat, behaviour and diet rather than being an indication of their relative intelligence. Although it is tempting to think that humans and dolphins will one day be able to communicate with one another, it is unlikely to happen because intelligence in dolphins is adapted for their aquatic lifestyle.

If you see a whale or dolphin off the coast of British Columbia, we would love to hear about it. Report your sighting at www.wildwhales.org.