The bottom substrate consists primarily of sand and gravel in the shallow parts, and mud in deeper parts. The North Sea ecosystem is heavily influenced by human activities, including fishing, extraction of oil, gas, and gravel, and eutrofication. Although pollution levels have improved since 1985, these activities remain a reason for concern. The water masses in the North Sea originate from the Atlantic Ocean. In addition to this salty water, there is a substantial supply of fresher water from the Baltic, and river discharge. The North Sea can roughly be divided into four areas, each with a characteristic ecological profile. In the northern part, at depths between 100–200 m, we find the most important areas for Norwegian fisheries, containing cod, saithe, haddock, herring and Norway pout. In the Norwegian trench, there are adult herring and mackerel near the surface whereas the deep has a distinct fauna of its own. In the central parts, the juvenile herring replaces the adults and sprat becomes more common. Finally, in the eastern part of the Sea, there are nursery areas for herring and cod, and important sand eel areas. There are also marine mammals in the North Sea. The most common ones are minke whale, harbor porpoise, white-beaked dolphin, harbor seals and grey seals.
At the beginning of 2008, the temperatures in the North Sea were high and remained high until autumn. At the end of the year, they were about normal. The recruitment to important fish stocks continues to be poor. This is probably caused by changes in the physical and biological conditions. A shift in distribution of important zooplankton prey species is observed.