There was a strong inflow of nutrient rich Jutland water to Skagerrak in April/May. The decline of oxygen in the Skagerrak bottom water continued in 2008, but there are good possibilities for a ventilation in the winter 2009.
North Sea and Skagerrak water masses originate with inflow of highly saline Atlantic water from the Norwegian Sea through the English Channel and freshwater runoff from land. During winter there is substantial vertical mixing in water columns of most areas, such that there is little difference in the characteristics of water masses between upper and lower layers. During summer warmer temperatures in the upper layers of the water column layer can lead to sharp temperature increases at depths from 20 to 50 m. In Skagerrak and along the Norway coast delivery of significant amounts of fresh water from the Baltic Sea and river tributaries causes the water to freshen; this less dense (lighter water) remains at upper layers of the water column, to a large extent throughout the entire year there is little mixing between deeper saltier layers and the lighter Atlantic waters. A lot of fresh water is delivered to the southern region of the North Sea, in the shallow areas along the coast with strong tidal waters the water is quite well mixed during the entire year, and creates a front between the saltier water in the central areas. Water masses in North Sea currents primarily rotate counter-clockwise. And, most of the sea water must course through Skagerrak before it continues northward as part of the Norwegian Coastal Current.
IMR routinely carries out monitoring of contaminants in open parts of the North Sea. Our investigations confirm that both radioactive and organic contaminants are present at low levels. Studies of fish have been carried out due to a big acute oil spill at the Statfjord Field in December 2007. This discharge of about 4,000 tons crude oil had little effect on fish sampled after the oil spill.
Poorly Oxygenated Bottom Water
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. Model simulations indicate that the inflow of Atlantic water was very low, both from the north and through the English Channel. There was a strong inflow of nutrient rich Jutland water to Skagerrak in April/May. The decline of oxygen in the Skagerrak bottom water continued in 2008, and the possibilities for a ventilation winter 2009 seem good.
Continued Poor Recruitment
The Institute of Marine Research has recommended no sandeel fishing in the Norwegian zone in 2009. Sandeel is an important prey species for several fish species, whales and seabirds. The recruitment to the North Sea cod, haddock and herring stocks has been poor for many years. This is probably caused by changes in the physical and biological conditions. Overexploitation might also be an important factor for the decline in recruitment in stocks like cod and sandeel. The spawning stocks of haddock, saithe and sprat are relatively good, while the spawning stock of herring is expected to be below the precautionary level in 2009. Very poor recruitment of herring has been seen for seven consecutive years. The recruitment of the North Atlantic mackerel has developed positively during the last years, but in the North Sea a strong decline is shown.