ICES AFWG REPORT 2014 35 1 Ecosystem considerations The aim of this chapter is to identify important ecosystem information influencing the fish stocks, and further show how this knowledge may be implemented into the fish stock assessment and predictions. There has been steady development in this aspect over the last years. Hopefully, the gathering of information on the ecosystem in this chapter will lead to a better understanding of the complex dynamics and interactions that takes place in the ecosystem, and also supports the development of an ecosystem based management of the Barents Sea. The ecosystem approach to management is variously defined, but in principle it puts emphasis on a management regime that maintains the health of the ecosystem along- side appropriate use of the marine environment, for the benefit of current and future generations (Jennings, 2004). Along with fishery, changes in the Barents Sea ecosystem are mainly caused by variations in the ocean climate. A warm period is characterized by increased impact of warm Atlantic water masses in the Barents Sea contributing to advection of zooplankton, faster growth rate in fish and emergence of abundant year classes (Dalpadado et al., 2002). A cold period is, conversely, in general characterized by reduced primary biological production in the Barents Sea and emergence of weak year classes of commercial species. In addition, inter-species trophic relations are an important factor that influences the abundance dynamics of commercial species. Movement towards an ecosystem approach to the fishery management in the Barents Sea should include (Filin and Røttingen, 2005): More extensive use of ecosystem information in the population parameters applied in assessment and prognosis ( i.e. in the models) Expansion of the use of multi-species models for fishing management. In addition, information about state and important changes within the ecosystem should be taken into consideration in the decision process to maintain sustainable ex- ploitation (e.g. in the process of setting the TAC). This chapter is in general based on the “Joint Norwegian-Russian environmental status 2008, report on the Barents Sea Ecosystem” (Stiansen et al., 2009), affiliating more than 100 scientists from 24 institutions in Norway and Russia. It is further based on two preliminary reports on current situation in the Barents Sea ecosystem. The first, WD20 (McBride et al.), is an IMR-PINRO update of the Joint Norwegian-Russian report (Sti- ansen et al., 2009) and the other, WD21 (Johannesen et al.) is the report from the new ICES working group on integrated assessment in the Barents Sea (WGIBAR). Addi- tional information is also gathered from other ICES WG’s reports and WD’s to this AFWG assessment. Text, figures and tables taken from these reports (i.e. Stiansen et al., 2009, McBride et al. WD20 and Johannesen et al. WD21) are in general not further cited in this chapter. Further, information is also taken from the joint Norwegian-Russian book “The Barents Sea – ecosystem, resources, management” (Jakobsen and Ozhigin 2011). Some of the stocks in the AFWG assessment relates strongly to the coastal ecosystem for most of their lifecycle (e. g. Anglerfish and coastal cod), or for parts of it (e.g. spawn- ing and larvae stage for NEA cod). This chapter, however, does not presently describe the coastal ecosystem and its relationship with the open ocean ecosystem. In the future important features of the coastal ecosystem related to the AFWG stocks are intended to be included in this chapter. 36 36 ICES AFWG REPORT 2014 1.1 General description of the Barents Sea ecosystem (Figure 1.1-1.2, Ta- bles 1.1-1.10) 1.1.1 Geographical description The Barents Sea is on the continental shelf surrounding the Arctic Ocean. It connects with the Norwegian Sea to the west and the Arctic Ocean to the north and the Kara Sea to the east. Its contours are delineated by the continental slope between Norway and Spitsbergen to the west, the top of the continental slope towards the Arctic Ocean to the north, Novaya Zemlya archipelago to the east, and the coasts of both Norway and Russia to the south (Figure 1.1). It covers an area of approximately 1.4 million km2, has an average depth of ca 230 m, and a maximum depth of about 500 m at the western end of Bear Island Trough (Figure 1.1). Its topography is characterized by troughs and basins (300 m – 500 m deep), separated by shallow bank areas, with depths ranging from 100-200 m. The three largest banks are Central Bank, Great Bank and Spitsbergen Bank. Several troughs over 300 m deep run from central Barents Sea to the northern (e.g. Franz Victoria Trough) and western (e.g. Bear Island Trough) continental shelf break. These western troughs allow the influx of Atlantic waters to the central Barents Sea. 1.1.2 Meteorological and oceanographic conditions Atmospheric forcing exerts influence on marine ecosystems through winds and air-sea interactions. Variations in large-scale atmospheric circulation cause changes in upper ocean circulation, ice extent and hydrographic properties of the water column. The cli- matic conditions of the Barents Sea are determined by both Atlantic and Arctic climatic systems. During cold seasons, a typical feature of atmospheric pressure is a low-pres- sure trough stretching from Iceland to the central Barents Sea. Pressure lows frequently travel along it bringing warm air from the Atlantic towards Novaya Zemlya archipel- ago. The southern Barents Sea is usually dominated by southwesterly winds, which contribute to increased advection of warm Atlantic water into area. In the northern part of the sea, cold northeasterly winds predominate. During the summer in the Barents Sea horizontal gradients of pressure are relatively small; as a result, light winds of dif- ferent directions blow over the Barents Sea. The general pattern of ocean circulation (Figure 1.1) is strongly influenced by the to- pography, and is characterized by inflow of relatively warm Atlantic water, and coastal water from the west. This Atlantic water current divides into two branches: 1) a south- ern branch that flows parallel to the coast and eastwards towards Novaya Zemlya; and 2) a northern branch that flows into the Hopen Trench. The Coastal Water has more fresh-water runoff and a lower salinity than the Atlantic water; it also has a stronger seasonal temperature signal. In the northern region of the Barents Sea, more fresh and cold Arctic waters flow from northeast to southwest. Atlantic and Arctic water masses are separated by the Polar Front, which is characterized by strong gradients in both temperature and salinity. In the western Barents Sea the front position is stable; in the eastern Barents Sea the front position varies seasonally and inter-annually. There is large inter-annual variability in ocean climate related to variable strength of the Atlan- tic water inflow and exchange of cold Arctic water. Ice cover has a strong seasonal and inter-annual variation, ranging from almost ice free conditions to cover more than half the sea. Thus, seasonal variations in hydrographic conditions can be quite large. In addition, there is an eastward coastal current along the Norwegian and Russian coastline, characterized by lower salinity and variable temperature. ICES AFWG REPORT 2014 37 1.1.3 Primary production In the biogeochemical cycles of the ocean, a multitude of processes are catalyzed by Bacteria and Archaea (single-cell microorganisms), and the functioning of these cycles in the Barents Sea does not differ qualitatively from those at lower latitudes. Both bac- teria and viruses show highly variable abundance in the Barents Sea. The situation in the ice-covered areas in the north remains to be investigated. The Barents Sea is a spring bloom system. During winter, primary production is close to zero. Timing of the phytoplankton bloom varies throughout the Barents Sea and there may also be a high inter-annual variability. The spring bloom starts in the south- western areas and spreads north and east with the retracting ice. In early spring, the water is mixed from surface to bottom. Despite adequate nutrient and light conditions for production, the main bloom does not occur until the water becomes stratified. Stratification of water masses in different areas of the Barents Sea may occur in several ways; 1) through fresh surface water from melting ice along the marginal ice zone; 2) through solar heating of surface layers in Atlantic water masses; or 3) through lateral dispersion of waters in the southern coastal region (Rey, 1981). As in other areas, dia- toms are also the dominant phytoplankton groups in the Barents Sea (Rey, 1993). Dia- toms particularly dominate the first part of the spring bloom, and the concentration of diatoms can reach up to several million cells per liter. They require silicate for growing. Other phytoplankton groups, such as flagellates, take over when the silicate is con- sumed. An important flagellate species in the Barents Sea is Phaeocystis pouchetii, but other species may, however, dominate the spring bloom in different years. 1.1.4 Zooplankton In the Barents Sea ecosystem, zooplankton forms a link between phytoplankton (pri- mary producers) and fish, mammals and other organisms at higher trophic levels. Zo- oplankton biomass in the Barents Sea can vary significantly between years. Crustaceans constitute the major part of this biomass, especially the calanoid copepods of the genus Calanus. Calanus finmarchicus, is the most abundant zooplankton species in Atlantic waters and C. glacialis is most abundant zooplankton species in Arctic wa- ters. Calanoid copepods are largely herbivorous, and feed particularly on diatoms (Mauchline, 1998). They can account for more than 80 % of the zooplankton biomass in some regions, especially in the spring.