Water temperatures, both at the surface and in deeper waters in the Norwegian Sea have been above the long-term trend since around the beginning of the 2000s, peaking in 2007 at almost 1.5˚C above the long-term mean at water depths of 50–500 m. Though the 2014 level was near and slightly above and the 2015 level at and below the long-term mean, the temperature trend is still positive because of inflow of Atlantic waters at the western entrance. The heat content of Atlantic water in the Norwegian Sea has been above the long-term mean since 2000.
The decrease in the zooplankton biomass index observed during the last decade for the whole Norwegian Sea has stopped. The index increased again from 2010 to 2014, but had a drop in 2015.
Since the mid-2000s, the mackerel Scomber scombrus stock has increased both its geographic distribution during summer feeding and its stock size.
The Norwegian spring-spawning (NSS) herring Clupea harengus stock has not produced large year classes after the relatively productive period of 1998–2004, causing decreasing SSB since 2009 to around Bpa in 2016 (5 million tonnes).
The blue whiting Micromesistius poutassou biomass reached a maximum level in the mid-2000s, declining thereafter until around 2010. Since then blue whiting has shown an upward trend with production of strong year classes.
Populations of seabirds breeding (and therefore feeding) in the ecoregion have declined greatly since 1980.
The NwS is influenced by human activity; historically involving fishing as well as the hunting of marine mammals. More recently, human activities also involve transportation of goods, oil, gas, and tourism, with contaminants coming from outside the boundaries of the ecoregion.
Human-induced climate change and ocean acidification may have a large influence on the NwS in future.
Changing distributions of valuable fish stocks (e.g. mackerel and NSS herring) lead to international disputes on harvest rights and quota sharing. It may also lead to changes in spawning success and to changes in migration patterns and ecological cascades with unknown outcome. The main pressures described below are defined in the ICES glossary of human pressures.
Pelagic fishing by multinational fleets is the major activity in the ecoregion. The number of fishing vessels is declining while the sizes of the vessels are increasing. The Norwegian commercial fleet has the highest fishing activity in the shelf area, particularly along the coast of Norway and along the continental shelf edge (Figure 4a ad 4b). Icelandic vessels operate mainly with pelagic trawl in the ecoregion (Figure 4c). Other fisheries in the ecoregion are predominantly pelagic fisheries targeting NSS-herring, mackerel, and blue whiting.
Bottom trawls are regulated along the Norwegian continental slope through closed areas to avoid extended damage on fragile and vulnerable benthic communities and reef-building organisms.
The oil- and gas-related activities are managed through governmental licences. Seismic investigations occur annually and are prohibited in the Norwegian sector during the spawning periods of Northeast Atlantic (NEA) cod and NSS herring.
Non-fishing marine traffic shows a slightly increasing trend, in particular in tourist traffic. Most ships follow the main traffic lanes near the coasts (Figure 5).
Marine litter, noise, and introduced species are all pressures within the ecoregion, but their effects are considered to be of minor importance.
Figure 4. Representation of fishing activity in the Norwegian
Sea by (a) the Norwegian fleets (larger than 15 m) in 2014 with pelagic trawls
(red dots), bottom trawls (blue dots), gillnets (light green), longlines
(green), and seines (orange); (b) Norwegian and foreign fishing commercial
fleets (larger than 15 m) and fishing vessels used for research purposes from
01.01.2013 to 01.01.2016, as reported through vessel monitoring systems (VMS)
to Norwegian authorities (Sources: ICES, 2015b, 2016a; Norwegian Directorate of Fisheries; and (c) the Icelandic
fishing fleet in 2014 with midwater trawls (red dots) and bottom trawls (blue
dots; no purse-seine fishery is in the area).
Figure 5. Density plot for vessel (including fishing vessels)
movements (AIS-data) in the Norwegian Sea for
July through August 2013. The traffic seen in international waters in the
centre of the ecoregion is predominantly fishing vessels. (Source: Norwegian Maritime Authority)