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.
There were no special researchers on marine mammals on board of Norwegian vessels during ecosystem survey. However, the Norwegian observers of seabirds on boards «Eros», «Johan Hjort», and «Helmer Hansen», as far as possible in parallel also did observations of marine mammals.
During the 2016 Barents Sea Ecosystem Survey (BESS) 96 fish species from 33 families were recorded in the both pelagic and bottom catches, some taxa were recorded at genus or family level only (Prokhorova et al., 2017). In the period 2004–2015 a total of 106 species were caught in demersal trawls during the BESS (Johannesen et al., 2017).
Most of the fish in the Barents Sea are demersal (Dolgov et al., 2011). The demersal fish community consists of about 70–90 regularly occurring species. These 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).
Zero group fish are important consumers on plankton and prey of other predators and therefore an important element in the transfer of energy between trophic levels in the ecosystem. The total biomass of 0-group (cod, haddock, herring, capelin, polar cod, and redfish), was 1.95 million tonnes in August-September 2016, which is slightly above the long-term mean of 1.76 million tonnes (Figure 3.5.1). The biomass was dominated by capelin and herring. Most of the biomass was distributed in the central and northern-central part of the Barents Sea.
Benthos is one of the main components of the marine ecosystems. It can be stable in time, characterizing the local situation, and is able to show the ecosystem dynamics in retrospective. 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 mesozooplankton plays a key role in the Barents Sea Ecosystem by channelling food from primary producers to animals higher in the foodweb. The main features of the distribution patterns show similarities across years, although some be-tween-year variability is apparent.
The phytoplankton development in the Barents Sea is typical for a high latitude region with a pronounced maximum in biomass and productivity during spring. During winter and early spring (January-March) both phytoplankton biomass and productivity are quite low. The spring bloom is initiated during mid-April to mid-May and may vary strongly from one year to another. The bloom duration is typically about 3–4 weeks and it is followed by a reduction of phytoplankton biomass mainly due to the exhaustion of nutrients and grazing by zooplankton. Later in autumn when the increasing winds start to mix the upper layer and bring nutrients to the surface a short autumn bloom can be observed. However, the time
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 (Jørgensen et al., 2015). Current knowledge of 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 (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 substratum, depth, gear, and type of organism. Seabed characteristics from the Barents Sea are only scarcely known 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.
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 benthos biomass in the northeast part of the Barents Sea (NE) was above the total Barents Sea mean (Figure 4.6.1). But from 2013 and ongoing, the mean biomass (see also black arrows in Figure 3.4.3 lower panel) was reducing, and was record low (<20 kg/n.ml) in 2016, and below the total Barents Sea mean.
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 (Figure 4.2.3).
The Barents Sea polar cod stock is in 2016 at an intermediate level. Norway conducted commercial fisheries for polar cod during the 1970s, and 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. The stock size declined from 2010 to a very low level in 2015.
The Barents Sea capelin stock has undergone drastic changes in size during the last three decades. Three stock collapses (when the abundance was low and fishing has been stopped) occurred in 1985–1989, 1993–1997, and 2003–2006. A strong reduction in stock size was also observed in 2014-2016, and in 2015 and 2016 the stock biomass has been below 1 million tonnes which earlier has been defined as a threshold for collapse.
Cod is the main predator on capelin, although other fish species as well as seabirds and marine mammals are also important predators. In the last 6-7 years there has been an extremely high cod stock level in the Barents Sea. Estimated biomass of preyed capelin by cod in recent years has been equivalent to the biomass of the entire capelin stock (Figure 4.2.3). Under good conditions the capelin stock tolerated a high grazing pressure; the biomass produced during the year was equivalent to the standing stock biomass measured in autumn. The abundance of predators other than cod is also at high and, to our knowledge, stable levels.
Ten years (2006-2015) of capelin diet was examined from the Barents Sea where capelin is a key forage species, especially of cod (Gadus morhua). The PINRO/IMR mesozooplankton distribution shows low plankton biomass in the central Barents Sea, most likely due to predation pressure from capelin. In the Barents Sea, a pronounced shift in the diet from smaller (<14 cm) to larger capelin (≥14 cm) is observed.
According to the national monitoring program, every year PINRO conducts research of pollution level in the Barents Sea. The objective of the research is to collect data on the potential anthropogenic impact on bioresources and on the ecosystem of the Barents Sea in general, to obtain data, filling the gaps in quality assessment of the Barents Sea environment, and to develop an infobase for future monitoring.
The level of discarding in the fisheries is not known, and no discards are accounted for in the assessments. Both undersized fish and bycatch of other species can lead to discarding, and also low-paid fish just above the minimum size has been subject to discarding in order to fill the quota with larger and better paid fish (known as highgrading).
The fishing activity in the Barents Sea is among other monitored by Vessel Monitoring System (VMS) data. Figures 126.96.36.199-188.8.131.52 show fishing activity in 2016 from Russian and Norwegian data. VMS data might give us valuable information about temporal and spatial changes in fishing activity. The most widespread gear used in the Barents Sea is bottom trawl, but also longline, gillnets, Danish seine and handline are used in the demersal fisheries. The pelagic fisheries use purse-seine and pelagic trawl, shrimp fishery used special bottom trawls.
The management of this species is based on the Revised Management Procedure (RMP) developed by the Scientific Committee of the International Whaling Commission. The inputs to this procedure are catch statistics and absolute abundance estimates. The present quotas are based on abundance estimates calculated from surveys conducted in 1989, 1995, 1996–2001, 2002–2007 and 2008–2013.
Norwegian and Russian vessels harvest northern shrimp in the Barents Sea over the stock’s entire area of distribution. Vessels from other nations are restricted to fish only in the Svalbard zone and the loophole. 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 35 mm.
Fishing is the largest human impact on the fish stocks in the Barents Sea, and thereby on the functioning of the whole ecosystem. However, the observed variation in both fish species and ecosystem is also strongly affected by as 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 mill. tonnes and has decrease in the last years (Figure 184.108.40.206.).
The long-term variation of the mean biomass of the NW, SW and the total Barents Sea show strong correlations (Figure 5.3.1). This might indicate that the western Barents Sea are driven by a factor common for the total Barents Sea. If comparing fluctuations of bottom temperature (Figure 5.3.1) with the benthos biomass (Figure 5.3.2) both shows clear decreasing values during 2007-2010. But previous studies show that benthos has a delay of approximately 3-7 years for macrobenthos to environmental variables (Lyubina et al., 2013; Denisenko, 2013), but with the more long-lived megabenthos, we suggest this delay to no less than 7 years.
Natural mortality of capelin is currently very high. The main predator for capelin is cod. The size of the cod stock is probably a main factor in the decline in capelin stock size. However, the relationship between changes in stock size of cod and capelin is not very strong. Historical data show that the probability of increase of capelin stock to a high level is low when the cod stock is large (Figure 5.2.1).
The 2016 capelin year class was strong at the 0-group stage and preliminary reports from the 2017 winter survey state that 1-group capelin was abundant and widely distributed.
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.