The combined effects of climate change and fishing are complicated by complex trophic interactions (see e.g. Hjermann et al. 2007). However, based on the present situation, known trophic interactions and ecosystem effects of fishing and climate change, some possible scenarios can be outlined; these are discussed below. Although a continued warming is likely in the longer term, short term cooling might occur due to natural fluctuations, and medium and long-term prospects may therefore differ.
Irrespective of temperature development, we have some knowledge about the short-term (<5 year) development, based on the present stock size and age composition of the main fish stocks. The cod stock will stay at a stable, but high, level in the coming years, the recent growth in stock size is not likely to continue as the incoming year classes (2006-2008) are below average. The large capelin stock together with a reasonable amount of other prey should ensure enough food for the large cod stock in the coming years. The haddock stock is at a historic high level, but will probably decrease from 2010 onwards due to reduced recruitment. It is unknown whether haddock, which mainly feed on benthic organisms, will be food-limited at such high stock sizes. There are no strong year classes of herring in the Barents Sea at present, and we do not know when the next strong year class will occur. Several researchers support the view that high herring abundance in the Barents Sea seems to be a necessary but not sufficient condition for a capelin collapse, whereas others suggest that a multitude of factors are involved, including climatic fluctuations, predation from fish and marine mammals and fisheries. Based on the view that predation from herring is an important factor, and taking into account the lag between the occurrence of a strong herring year class and a capelin collapse, a capelin collapse is not likely to happen before 2012.
A large spawning stock, low harvesting and continued warming is favourable for NSS herring, and the stock can therefore be expected to increase further in the future. A large herring stock has a strong impact on the marine environment. NSS herring consume a considerable part of the copepod production in the Norwegian Sea (Dommasnes et al. 2004). An increasing stock is therefore likely to reduce the biomass of copepods, with possible consequences for the biomass of zooplankton that is transported into the Barents Sea. Herring is also an important predator on eggs and larvae of several fish species (Gjøsæter and Bogstad 1998, Godiksen et al. 2006, Segers et al. 2007, Huse et al. 2008). In the Barents Sea, a large herring stock has negative consequences for the recruitment of capelin (Gjøsæter and Bogstad 1998, Hjermann et al. 2004b). Although a high abundance of juvenile herring did not prevent the current capelin “outbreak”, a continued increase in herring might be expected to have long-term effects on capelin by affecting the frequency and amplitude of the capelin fluctuations, and possibly reduce its dominating role in the ecosystem.
If alternative prey is not present, a severely reduced capelin stock will have a strong negative impact on top predators in the Barents Sea, as observed in the late 1980s (Gjøsæter et al. 2009). A low capelin stock might for example have negative impacts on a range of seabird and sea mammal species in the area (Hamre 1994, Sakshaug et al. 1994). For some species, alternative prey such as juvenile herring, polar cod and crustaceans might provide foraging alternatives, but a low stock of capelin generally means that ice-edge feeding top-trophics must travel further to access food (see e.g. Barrett and Krasnov 1996, Barrett 2002). A low capelin stock is also associated with increased cannibalism in cod (Gjøsæter et al. 2009, Yaragina et al. 2009). The adverse effect of cannibalism might be counteracted to a degree by increased cod recruitment due to increased water temperature (e.g. Ottersen and Sundby 1995), and an increased abundance of alternative prey. As long as the harvesting of cod is kept below the long-term sustainable limit, and a large herring stock does not impair cod recruitment, the NEA cod stock might continue to be relatively strong, even with capelin at low levels. Intensive fishing has, however, reduced the cod’s ability to affect the large fluctuations in the stocks of capelin and juvenile herring.
A marked increase in primary production north of the polar front is an expected consequence of continued warming (Ellingsen et al. 2008). This new production will support an increased zooplankton community and enhance benthic production. How the benthic community will respond to the increased input of organic matter, will however, depend partially on how these communities have been impacted by trawling. Capelin is the major consumer of secondary production in the Arctic Barents Sea (Orlova et al. 2002, Dalpadado et al. 2003). A reduced capelin stock might initiate a trophic cascade resulting in an increase in the zooplankton standing stock (see Dalpadado et al. 2003; Orlova et al., 2001), and possibly a subsequent decrease in the biomass of phytoplankton. Reduced consumption by capelin could be compensated for by an expansion and increase in the stock of polar cod (Orlova et al. 2009), and an increase in the abundance of omnivorous and carnivorous crustaceans such as krill and amphipods (see Dalpadado et al. 2001, 2008; Drobysheva and Yaragina, 1990). The response will, however, depend on how these species will be impacted by warming and the continued thinning of sea ice. During the recent periods of low capelin abundance, krill/amphipods and polar cod were apparently unable to compensate for the reduced consumption of zooplankton (see Dalpadado et al. 2003). Moreover, with a reduced capelin stock, less arctic production will be transported to the Norwegian and Murman coasts during capelin spawning. This might have long-term consequences for the coastal ecosystems.
Predictions for the development of the Barents Sea ecosystem on a time scale of more than 5 years are associated with large uncertainties. Although our understanding of the system has increased considerably in recent years, a number of important questions are still unresolved. Some of these are:
• How will warming impact oceanographic drivers of ecosystem function responsible for determining quality, quantity, and timing of primary production?
• How will warming affect the match/mismatch between phytoplankton, zooplankton and the spawning of major fish stocks?
• How will a large NSS herring stock affect the zooplankton community and the recruitment of cod and capelin?
• Will the capelin stock continue to fluctuate?
• How will top-predators respond to changes in the abundance of pelagic fishes?
• Will changes in the abundance of pelagic fishes cause a trophic cascade?
• How will the benthic community respond to changes in organic input, combined with fish trawling, temperature increase and invasive predatory species?