Most harbour porpoises die before the age of 8. A few centuries ago, this species was fairly common in European waters but, over the past few years, the number of individuals in the European Atlantic and in the Baltic and North seas has gone down, which could be for a number of reasons. They are, for example, taken in many areas of the world for human consumption, such as in America, Canada, Iceland, Russia and Turkey. The largest fishery is in Greenland, where as many as 1,500 are taken every year. They are also accidentally caught in fishing gear, such as in gillnets, trawls, traps and on the baited hooks of longlines set out for other species such as skate, halibut or conger. The harbour porpoise mostly eats fish without any spines and has a particular fondness for herrings, sardines, mackerel, cod, sole and hake as well as cephalopods such as squid. Crustaceans are generally found in its stomach when the porpoise has either been found washed up dead on the beach or is sliced up for human consumption. The harbour porpoise's main defence is its size but its second defence is speed. On occasion its third defence will be numbers, such as when a group of them are swimming together. All of these are effective against top marine predators like sharks but less effective against other species of dolphins, like the false killer whale which will, if it gets the chance, kill and eat not only harbour porpoises but also several other species of dolphin.

They are amazing creatures that have many wonderful skills but they are in a lot of danger from human activity. Hopefully, the government will act to ban nets of the kind used in pair trawling so they will, at least, have this danger removed but there are other destructive forms of fishing which should also be acted upon. If they can be kept safe from these practices they will then not be caught and can swim happily and freely on their way, providing many of us with the pleasure of seeing, at first hand, their graceful visits to our estuaries and beaches.

Sharks are the main predators of porpoises but they are also killed by several other species of dolphins.