The ecosystems transgress national borders and go beyond Norwegian jurisdiction and Norwegian Exclusive Economical Zone. Hence, this is a short and simplified presentation of the ecosystems of the three sea areas.
The Barents Sea
The Barents Sea is relatively shallow, with an average depth of 230 m. The oceanographic conditions are strongly affected by sea water transport from the Norwegian Sea, which varies substantially between years. Surface temperatures observed throughout the last decade have been the highest on record. Pollution levels in the sea are generally very low, but toxic substances, which are transported into the area by currents, are found to accumulate in some top predators such as birds and mammals. Petroleum activity is very likely to increase during the coming decade.
Zooplankton production is high, but variable. A wide variety of benthic organisms are also an integral component of the ecosystem. Important pelagic fish species are capelin, polar cod, herring, and blue whiting. Capelin is a key species in the ecosystem, and undergoes large fluctuations in abundance (0.1–7.0 million tons). These fluctuations largely are due to recruitment failure caused by the predation of strong herring year classes on capelin larvae.
Cod and haddock are the most abundant demersal fish species, and the 2011 spawning stock biomasses are the highest on record for both. Redfish, Greenland halibut and long rough dab are also abundant. The most important marine mammals in the Barents Sea ecosystem are minke whales and harp seals.
The Norwegian Sea
The Norwegian Sea is dominated by two deep basins of 3000–4000 m depth. Compared to the North Sea, the Norwegian Sea is little affected by human activities although there is considerable fishing activity and an increasing activity in oil and gas extraction. Every second about 8 million tonnes of warm Atlantic water enters the Norwegian Sea. This transport equates to 8 times the sum of the global river discharge and is decisive for the mild climate in northern Europe. The ecosystem in the Norwegian Sea has a relatively low biodiversity, but the food chain is productive and some species occur in very high numbers.
The phytoplankton constitutes the bottom of the food chain and is found in enormous quantities during the intense spring bloom. The ecosystem contains a high zooplankton biomass, which is harvested by abundant fish stocks and a variety of marine mammals including minke whales as well as larger whales such as humpback whales, blue whales and fin whales. The harvest in the Norwegian Sea of mainly Norwegian spring spawning herring and mackerel is about 1.5 million tonnes annually. The bottom fauna in the Norwegian Sea is varied due to the great variation in depth. The great basins are dominated by deep-sea fauna while there are deep-sea coral reefs with a high biodiversity on the continental shelf along the Norwegian Coast.
The North Sea and Skagerrak
The North Sea and Skagerrak, including its fjords and tributaries, has a surface area of approximately 750,000 km2. It is shallow in comparison to the Barents and Norwegian Seas Two thirds of the North Sea measures less than 100 m in depth.
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.