Porpoises find their food under water with the help of sonar. In the coastal region of the Netherlands and the Baltic Sea, they eat mainly small benthic fish such as gobies. In the open North Sea they feed mostly on herring, sprat and mackerel. Porpoises in the German Wadden Sea go after small flatfish. Large flatfish are dangerous for porpoises; various porpoises that have washed ashore were found to have choked to death after eating large flatfish. Twenty years ago, porpoises in the North Sea ate mainly whiting. However, this fish species is not that prominent anymore. That is why they have switched over to gobies. Gobies are much smaller than whiting, so they need to catch much more to satisfy their needs. Scientists have found that porpoises that strand in the summer seem to have problems finding sufficient food. Porpoises eat around five kilograms of fish daily - this is 10% of their body weight. There is a theory that porpoises help each other to look for food using their sonar system. You can imagine that it is easier to find food in a large sea when the group is spread out.

Whale louse is a kind of crustacean, but it isn't as innocent as it sounds. It is a true parasite. The creatures eat the skin, fat and blood of their hosts, in this case porpoises. They survive particularly on sick and weakened animals, causing lots of problems for these victims. Whale louse can accumulate around a wound, for example. Healthy porpoises make sure that these pests don't get a chance by scratching sufficiently to get rid of them.

In 2006, a large group of scientists performed autopsies to establish the cause of death in stranded porpoises. The results showed that more than half of the animals died from drowning. Most likely, they became entangled in fishing nets from gill, trammel and standing rigging. A similar study in Denmark in 1974 showed that thousands of porpoises were victims of standing rigging nets. The drowned animals were otherwise healthy and their stomachs were often full. Drowning occurred mostly in March and April.
A second major cause of death was contagious diseases. These sick animals, with little fatty reserves and an empty stomach, washed ashore mostly in August. In 2007, the seal creche in Pieterburen performed its own internal study, commissioned by the Dutch Fishery Union. In this study, the proportion of drowned porpoises was less. Other important causes of death in that study were pneumonia and starvation.

Studies on the fatty layer of freshly stranded porpoises showed that they contained lots of toxic substances, such as a poisonous flame retardant. This substance is used in insulation foam and upholstery. The poison ends up in the sea via waste water and 'consumed' by fish. Because porpoises eat lots of fish in their lifespan, large amounts of this poison eventually build up in their blubber. This toxic material disrupts the animal's thyroid metabolism and functioning of its nervous system.

More and more fishermen are attaching underwater loud speakers to their fishing nets to keep porpoises at a distance. These 'pingers' transmit sounds which scare the animals. Since 2007, pingers have been required for fishing vessels larger than 12 meters that fish with gill and trammel nets. However, there are also disadvantages to using pingers. In some cases, the marine mammals learn that the sound is made in an area where they can easily snatch up fish. Then the pinger works more as a dinner bell and that makes it dangerous for the porpoises.