Fish in the Barents Sea can be classified into planktivorous, benthivorous and piscivorous, but many of them have a wide diet and a diet that changes with size. Fish species that feed on the same prey and that overlaps spatially are potential competitors. Capelin and polar cod overlap on the border of their feeding areas which is in the southeastern and central areas of the Barents Sea in cold years and in the northeastern areas in warm years. Capelin and polar cod have similar food ranges and rhythms of feeding, therefore, food competition may arise between them. For example, in the Admiralty Island area food competition was very pronounced in 2007 (Figure 2.6.5), resulting in different food used by the two species.
However, north of the Novaya Zemlya shallows, food supply was high in this the same year. Here competition was probably low even though capelin and polar cod of all sizes fed on copepods (Figure 2.6.5) resulting in different food used by the two species.
Capelin is a key species in the Barents Sea ecosystem, and one of the main components in the transfer of trophic energy from lower to higher trophic levels. Capelin can have important impact on zooplankton abundance (above), and on the feeding behaviour and condition of their predators. Many predators in the Barents Sea, including cod, sea birds and marine mammals have capelin as their preferred prey. In years when capelin abundance is low, the predation on alternative prey is larger (Figure 2.6.4 and 2.6.6). Pelagic fish can be both competitors to, and prey for, marine mammals. This is also the case for cod. The strength and type (e.g., competition or predation) of interactions between pelagic fish, cod, sea birds and marine mammals seems to depend largely on the relative abundances of pelagic fish and zooplankton within the ecosystem at a given time (Haug et al., 2002; Sivertsen et al., 2006; Gjøsæter et al., 2009).
Capelin abundance fluctuate dramatically in the Barents Sea system, and have impacts on the distribution, and inter-annual diets of many marine mammal and sea birds species as well as piscivorous fish. Gjøsæter et al. (2009) studied the ecosystem effects of the three capelin stock collapses which have taken place since the mid-1980s. These stock collapses occurred in 1985-1989, 1993-1997, and 2003-2006. When capelin biomass was drastically reduced, its predators were affected in various ways. The cod experienced increased cannibalism, the growth was reduced and the maturation delayed. Sea birds experienced increased rates of mortality and total recruitment failures, and some breeding colonies were abandoned for several years. Harp seals experienced food shortage, increased mortality because they invaded the coastal areas and were caught in fishing gears, and recruitment failures. The three capelin collapses affected the predators differently. The effects were most serious during the 1985-1989 collapse and could hardly be traced during the last collapse. It was concluded that these differences likely result from increased availability of alternative food sources during the two last periods of collapse.
Cod is the most important predator on fish in the Barents Sea. Cod has a wide diet and feed both on pelagic and demersal fish, zooplankton and benthos, such as shrimp. The consumption by cod in the period 1984-2008, is presented in Figure 2.6.6. Overall, capelin is the most important prey of cod. In 2008 the proportion of capelin was 40%, followed by krill (13%), polar cod, haddock, shrimp, cod and hyperiid amphipods. Cannibalism might be important for cod recruitment and is now at an intermediate level, while the consumption of haddock by cod is at a record high level.