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 a relatively shallow shelf sea, 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 some toxic substances, which are transported into the area by currents, are found to accumulate in some top predators such as birds and mammals.
Due to the warm currents from the Atlantic and upwelling of nutrient rich water, the Barents Sea contains one of the most productive ecosystems in the world. The spring blooms of phytoplankton trigger vigorous growth of zooplankton, and subsequently growth of a few, but highly abundant marine stocks throughout the food chain. 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 are primarily due to recruitment failure caused by the predation on capelin larvae by strong herring year classes.
Cod and haddock are the most abundant demersal fish species. Their abundance varies significantly due to oceanographic fluctuations and other ecosystem interactions, but at a slower rate than for the pelagic species.. The spawning stock biomass for cod has increased strongly over the past decade and is close to 2 million tonnes in 2013, the largest recorded since measurements began in 1946. 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 and the coastal shelves of surrounding land masses. Compared to the Barents Sea, the Norwegian Sea is less productive, but there are several abundant fish stocks in this vast ocean area. Immense amounts of warm water enter the Norwegian Sea with The North Atlantic current (approximately 8 million tonnes every second).
Zooplankton biomass has decreased in recent years, probably due to high abundances of pelagic fish species. The most important pelagic feeders are mackerel, Norwegian spring spawning herring and blue whiting. Their migration routes vary throughout the year and interannually. Marine mammals include minke whales as well as larger whales such as humpback whales, blue whales and fin whales. Beaked redfish is an abundant demersal species at the slopes of the Norwegian shelf. Moreover, there is a large variety in bottom fauna in the Norwegian Sea 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 spill-off from agriculture. Although pollution levels have been reduced since 1985, these activities remain a reason for attention. 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 large European river systems.
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.
The most common marine mammals in the North Sea are minke whale, harbor porpoise, white-beaked dolphin, harbor seals and grey seals.