According to the ICES criteria (ICES 2008b) the stocks of NEA cod, NEA haddock, northern shrimp (Pandalus borealis) and capelin have full reproductive capacity and are harvested within sustainable limits. The stocks of NEA Greenland halibut (Reinhardtius hippoglossoides), golden redfish (Sebastes marinus) and deep-sea redfish (Sebastes mentella) have been fished down to very low levels. These threatened species are long-lived and have low potential growth rates. Although the fisheries at present are strongly regulated, the rebuilding of these stocks will take many years.
Norwegian coastal cod is of special concern. The spawning stock biomass and recruitment are at historically low levels, and according to the ICES advice, no catch should be taken from this stock in 2009, and a recovery plan should be developed and implemented.
Fishery by-catch is a serious problem in the protection of endangered, long-lived species with low reproductive rates (Hall et al. 2000, see e.g. Casey and Myers 1998). In the Barents Sea, the by-catch of deep-sea redfish in the shrimp fishery has been a serious problem of this type. This has been addressed recently via the introduction of legal limits being set for by-catch, and by using sorting grids in the shrimp fishery. Norwegian coastal cod is taken in the ordinary NEA cod fishery. Several restrictions on the fisheries close to the coast and within fjords have been implemented to reduce the fraction of coastal cod in the catches. The effect of these restrictions is still unknown. Diving seabirds, and specifically auks and sea ducks, are taken as by-catch in the coastal gillnet fisheries (Strann et al. 1991, Zydelis et al. 2009). The by-catch of common guillemots (Uria aalge) in the gillnet fisheries for cod in Troms and Finnmark during spring is a special concern (Strann et al. 1991, Christensen-Dalsgaard et al. 2008). The problem is probably highly variable from year to year, depending on the spatial overlap between the fisheries and the aggregations of the birds. In some years, the number of birds taken can be large (Strann et al. 1991), which might have effects on the already threatened population.
Disturbance from trawling and dredging has wide-ranging impacts on the diversity, and productivity of benthic communities (Jennnings and Kaiser 1998). In the Barents Sea, particular attention has been paid to biotic habitats generated by aggregations or colonial growth of single species. Such habitat-generating species are represented by a wide range of taxonomic groups (e.g. Porifera, Polychaeta, Cnidaria, Mollusca and Bryozoa; see reviews in Jennings 1998, Auster and Langton 1999, Kaiser and de Groot 2000, Moore and Jennings 2000), house a high diversity of associated species, and are examples of whole communities that can be managed within restricted areas. For obvious reasons these biota are seriously threatened by bottom trawling, and there is a strong need for protection. Damage from bottom trawling and other forms of seafloor disturbance (e.g. petroleum activities) is not limited to colonial species, but will impact all species with a life span that does not favor reproduction between disturbance events. More generally, frequent disturbance of soft-sediment communities leads to the proliferation of smaller benthic species with faster life histories. Because the larger species are removed, the depth of bioturbation and habitat complexity is reduced, resulting in a reduced benthic production (Kaiser et al. 2000, Jennings et al. 2001). However, benthic production is also highly dependent on primary production, and temporal changes are often masked by e.g. climatic changes (Jennings et al. 2001). Studies indicate that the benthic biomass in the Barents Sea has been reduced by as much as 70% in some areas (Denisenko 2001, chapter General background description of the ecosystem - Biotic components - Benthos). Parts of this reduction can be attributed to reduced primary production and perhaps increasing populations of invasive opportunistic decapods, king crab (Cunningham 1969; Anisimova et al 2005; Jørgensen and Primicerio 2007) and snow crab, which forage on a wide variety of benthic animals. However, increased bottom trawling is probably also an important factor, and disturbance of benthic communities from bottom trawling might accordingly have a substantial impact on the Barents Sea ecosystem.
Fishing affects the demography of targeted species and consequently imposes selection pressure on the stocks (Olsen et al. 2004, Jørgensen et al. 2007). In the Barents Sea, the onset of the industrial trawl-fishery induced a shift from harvest of old, large NEA cod to younger and smaller fish, as well as increasing the overall harvesting rate. The result has been a marked shift in the age and size distribution of the stock, from a stock dominated by old, large individuals to a stock dominated by young and small individuals (Ottersen et al. 2006). The shift in harvest strategy has also induced a change in selection pressure, favouring earlier maturation, and age and size at spawning have decreased accordingly (Ottersen et al. 2006). This shift may, however, have been less severe than claimed by those authors, as it has recently been shown that the determination of age at maturation by otolith reading has changed over time (Zuykova et al. 2009). The NEA cod is a dominant predator within the Barents Sea (Dolgov 2009, Yaragina and Dolgov 2009). Cod is a highly plastic and omnivorous predator, which feeds extensively on capelin, krill, northern shrimp, polar cod (Boreogadus saida), juvenile herring, and young cod and haddock (Yaragina and Dolgov 2009). Feeding is concentrated to the most abundant and favourable prey item, and changes in the relative abundance of prey items result in a “switch” in feeding preferences (Yaragina and Dolgov 2009). “Switching” to the most abundant prey item stabilize the system by dampening outbreaks in the prey populations. Moreover, at times when prey is generally scarce, cannibalism on younger ager classes quickly regulates the cod population to the availability prey (Hjermann et al. 2004a, Gjøsæter et al. 2009, Yaragina et al. 2009). Frequency-dependent “switching” behaviour combined with facultative cannibalism underline the importance of cod as a predator species with a stabilizing role in the ecosystem (see e.g. Frank et al. 2005, Casini et al. 2009). The fishery-induced change in the abundance and size-distribution of NEA cod has changed its role as a top-predator. Similar to other northern shelf ecosystems, the Barents Sea is therefore likely to be susceptible to large outbreaks and fluctuations in the stocks of small pelagic schooling fish such as capelin and herring.
