The true porpoises are the cetaceans nearest in size to humans. An average adult harbour porpoise, for example, is 1.5 metres long and weight 54 to 65 kilograms. At sea, however, this animal seems much smaller. On the surface of the water, only the dorsal fin and occasionally the head and back are visible. Porpoises travel in groups of fewer than 10. If you don't watch carefully or listen for their quick "puffs" at the surface, you will miss them altogether.

Most porpoises live close to shore, although an exception is the Dall's porpoise, which is found just as often in the open sea.

Next to river dolphins, porpoises are the most threatened cetaceans. They have suffered significant losses as a result of their inevitable conflict with human activities. In an apparent effort to avoid pollution or too much shipping traffic, porpoises have relocated, leaving some areas permanently. Nevertheless, thousands of porpoises drown every year in fishing nets, and many kills go unreported. When fishermen haul up a dead porpoise in their nets, they simply cut it out and toss it back into the open sea.

Compared with the high-profile dolphins and whales, porpoises have as yet been unable to attract the wide public concern that is crucial to conservation efforts. As a result, they have been mostly overlooked in terms of research funding and protection. But porpoises have yet another strike against them: their small size makes it difficult for researchers to obtain the essential identification photographs to use in their studies.

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