The Barents Sea is a shelf sea of the Arctic Ocean. Being a transition area between the North Atlantic and the Arctic Basin, it plays a key role in water exchange between them. Atlantic waters enter the Arctic Basin through the Barents Sea and the Fram Strait (Figure 3.1.1). Variations in volume flux, temperature and salinity of Atlantic waters affect hydrographic conditions in both the Barents Sea and the Arctic Ocean and are related to large-scale atmospheric pressure systems.
Mesozooplankton play a key role in the Barents Sea ecosystem by transferring energy from primary producers to animals higher in the food web. Geographic distribution patterns of total mesozooplankton biomass show similarities over time, although some inter-annual variability is apparent. Challenges in covering the same area each year are inherent in such large-scale monitoring programs, and inter-annual variation in ice-cover is one of several reasons for this. This implies that estimates of average zooplankton biomasses for different years might not be directly comparable.
Benthos is an essential component of the marine ecosystems. It can be stable in time, characterizing the local situation, and is useful to explain ecosystem dynamics in retrospect. It is also dynamic and shows pulses of new species distribution, such as the snow crab and the king crab, and changes in migrating benthic species (predatory and scavenger species such as sea stars, amphipods and snails with or without sea anemones). The changes in community structure and composition
Most Barents Sea fish species are demersal (Dolgov et al., 2011); this fish community consists of about 70-90 regularly occurring species which have been classified into zoogeographical groups. About 25% are Arctic or mainly Arctic species. The commercial species are all boreal or mainly boreal (Andriashev and Chernova, 1995), except for Greenland halibut (Reinhardtius hippoglossoides) that is classified as either Arcto-boreal (Mecklenburg et al., 2013) or mainly Arctic (Andriashev and Chernova, 1995).
Phytoplankton development in the Barents Sea is typical for a high latitude region with pronounced maximum biomass and productivity during spring. During winter and early spring (January-March), both phytoplankton biomass and productivity are relatively low. Spring bloom is initiated during mid-April to mid-May and may vary strongly from year to year. Bloom duration is typically about 3-4 weeks and is followed by a reduction in phytoplankton biomass mainly due to nutrient exhaustion
During the 20 June to 14 August 2017 period, a sighting survey was conducted in the Barents Sea east of 28°E as part of a six-year mosaic survey of the Northeast Atlantic to estimate the regional abundance of minke whales and other cetaceans during summer. Coverage was adequate, except in the southeastern area where military restrictions re-stricted survey activity. The most often observed species was minke whale, followed by white-beaked dolphins, harbour porpoises, humpback whales, and fin whales. A few observations were also made of bowhead whales and beluga whales.
Zoogeographical groups of fish species are associated with specific water masses. Rel-ative distribution and abundance of fish species belonging to different zoogeographic groups are of interest because these fish will respond differently to climate variability and change. Since they are not commercial species, fishing does not directly contribute to changes in abundance and distribution of these species. Different zoogeographic groups also tend to differ in their trophic ecology: many of the Arctic species are small, resident, and feed mainly on invertebrates; whereas, most boreal and mainly boreal species are migratory and piscivorous.
Zero group fish are important consumers on plankton and are prey of other predators, and, therefore, are important for transfer of energy between trophic levels in the ecosystem. Estimated total biomass of 0-group fish species (cod, haddock, herring, capelin, polar cod, and redfish) was 1.92 million tonnes during August-September 2017; slightly above the long term mean of 1.76 million tonnes (Fig 3.5.1). Biomass was dominated by cod and haddock, and mostly distributed in central and northern-central parts of the Barents Sea.
The level of discarding in fisheries is not estimated, and discards are not accounted for in stock assessments. Both undersized fish and by-catch of other species can lead to discarding; fish of legal size but low market value are also subject to discarding to fill the quota with larger and more valuable species (known as high-grading). Discarding is known to be a (varying) problem, e.g., in haddock fisheries where discards are highly related to the abundance of haddock close to, but below, the minimum legal catch size.
Fishing activity in the Barents Sea is tracked by the Vessel Monitoring System (VMS). Figures 22.214.171.124-126.96.36.199 show fishing activity in 2017 based on Russian and Norwegian data. VMS data offer valuable information about temporal and spatial changes in fishing activity. The most widespread gear used in the Barents Sea is bottom trawl; but long lines, gillnets, Danish seines, and handlines are also used in demersal fisheries. Pelagic fisheries use purse seines and pelagic trawls. The shrimp fishery used special bottom trawls.
Management of the minke whale is based on the Revised Management Procedure (RMP) developed by the Scientific Committee of the International Whaling Commission. Inputs to this procedure are catch statistics and absolute abundance estimates. The present quotas are based on abundance estimates from survey data collected in 1989, 1995, 1996–2001, 2002–2007 and 2008–2013. The most recent estimates (2008–2013) are 89 600 animals in the Northeastern stock, and 11 000 animals for the Jan Mayen area, which is exploited by Norwegian whalers.
Norwegian and Russian vessels harvest northern shrimp over the stock’s entire area of distribution in the Barents Sea. Vessels from other nations are restricted to trawling shrimp only in the Svalbard zone and the Loophole — a piece of international waters surrounded by the EEZs of Norway and Russia. No overall TAC has been set for northern shrimp, and the fishery is regulated through effort control, licensing, and a partial TAC in the Russian zone only. The regulated minimum mesh size is 35mm.
Fishing has the largest anthropogenic impact on fish stocks in the Barents Sea, and thereby, on the functioning of the entire ecosystem. However, observed variations in both fish species and ecosystem are also strongly impacted by climate and trophic interactions. During the last decade, catches of most important commercial species in the Barents Sea and adjacent waters of Norwegian and Greenland Sea varied around 1.5 – 3 million tonnes and has decreased in the last years (Fig. 188.8.131.52.).
With retreating sea ice, new areas in the northern Barents Sea become available for fisheries, including bottom trawlers. Of special interest to WGIBAR is therefore the vulnerability analysis. Current knowledge on the response of benthic communities to the impact of trawling is still rudimentary. The benthos data from the ecosystem survey in 2011 has been used to assess the vulnerability of benthic species to trawling, based on the risk of being caught or damaged by a bottom trawl (see WGIBAR report 2016).
In order to conclude on the total impact of trawling, an extensive mapping of fishing effort and bottom habitat would be necessary. In general, the response of benthic organisms to disturbance differs with substrate, depth, gear, and type of organism (Collie et al. 2000). Seabed characteristics from the Barents Sea are only scarcely known (Klages et al. 2004) and the lack of high-resolution (100 m) maps of benthic habitats and biota is currently the most serious impediment to effective protection of vulnerable habitats from fishing activities (Hall 1999).
The impact of fisheries on the ecosystem is summarized in the chapter on Ecosystem considerations in the AFWG report (ICES 2016c), and some of the points are:
In most of the measured years, the biomass in the northeast part of the Barents Sea was above the total Barents Sea mean (see Fig. 3.4.7). But from 2013 and ongoing, the mean biomass was reducing, and was record low (<20 kg/n.ml) in 2016, and below the total Barents Sea mean. This decrease could be explained by the maximum distribution of the snow crab predating on the benthos, and with increasing bottom temperatures (chapter 3.1). But in 2017 the biomass increased to 116 kg/nml, the highest value recorded both with and without snow crabbiomass.
The interaction cod-capelin-polar cod is one of the key factors regulating the state of these stocks. Cod prey on capelin and polar cod, and the availability of these species for cod varies. In the years when the temperature was close to the long term mean, the cod overlap with capelin and polar cod was lower than in the recent warm years. Cod typically consume most capelin during the capelin spawning migration in spring (quarters 1+2), but especially in recent years the consumption has been high also in autumn (quarters 3+4) in the northern areas (Fig. 4.2.3).
The Barents Sea polar cod stock was at a low level in 2017. Norway conducted commercial fisheries on polar cod during the 1970s; Russia has fished this stock on more-or-less a regular basis since 1970. However, the fishery has for many years been so small that it is believed to have very little impact on stock dynamics. Stock size has been measured acoustically since 1986, and has fluctuated between 0.1-1.9 million tonnes. Stock size declined from 2010 to a very low level in 2015, increased to 0.9 million tonnes in 2016, and again declined to 0.4 million tonnes in 2017.
The Barents Sea capelin has undergone dramatic changes in stock size over the last three decades. Three stock collapses (when abundance was low and fishing moratoriums imposed) occurred during 1985–1989, 1993–1997, and 2003–2006. A sharp reduction in stock size was also observed during 2014-2016; followed by an unexpectedly strong increase during 2016-2017. Observed stock biomass in 2015 and 2016 was below 1 million tonnes, which previously was defined as the threshold of collapse.
Cod is the major predator on capelin; although other fish species, seabirds and marine mammals are also important predators. In the last 6-7 years, cod stock levels have been extremely high in the Barents Sea. Estimated biomass of capelin consumed by cod in recent years has been close to the biomass of the entire capelin stock (Fig. 4.2.3). Abundance levels of predators other than cod are also high and, to our knowledge, stable.
Eleven years (2006-2016) of capelin diet were examined from the Barents Sea where capelin is a key forage species, especially of cod. The PINRO/IMR mesozooplankton distribution shows low plankton biomass in the central Barents Sea, most likely due to predation pressure from capelin and other pelagic fish. This pattern was also observed in 2017. In the Barents Sea, a pronounced shift in the diet from smaller (<14cm) to larger capelin (=>14cm) is observed. With increasing size, capelin shift their diet from predominantly copepods to euphausiids, (mostly Thysanoessa inermis - not shown), with euphausiids being the largest contributor to the diet weight in most years (Figure 4.1.1).
Marine litter is defined as “any persistent, manufactured or processed solid material discarded, disposed or abandoned in the marine and coastal environment”. Large-scale monitoring of marine litter was conducted by the BESS survey during the 2010-2017 period, and helped to document the extent of marine litter in the Barents Sea (the BESS survey reports, Grøsvik et al. 2018). Distribution and abundance of marine litter were estimated using data from: pelagic trawling in upper 60 m; trawling close to the sea floor; and Visual observations of floating marine debris at surface.
Oceanic systems have a “longer memory” than atmospheric systems. Thus, a priori, it seems feasible to predict oceanic temperatures realistically and much further ahead than atmospheric weather predictions. However, the prediction is complicated due to variations being governed by processes originating both externally and locally, which operate at different time scales. Thus, both slow-moving advective propagation and rapid barotropic responses resulting from large-scale changes in air pressure must be considered.
Most of the commercial fish stocks found in the Barents Sea stocks are at or above the long-term level. The exceptions are polar cod and Sebastes norvegicus. Also the abundance of blue whiting in the Barents Sea is at present very low, but for this stock only a minor part of the younger age groups and negligible parts of the mature stock are found in the Barents Sea.
Concerning shellfish, the shrimp abundance is relatively stable and above the long-term mean while the abundance and distribution area of snow crab is increasing.