For several centuries, fishing have played an important role in the function and dynamics of the Barents Sea ecosystem. In early periods fisheries were purely coastal and had limited effects on the ecosystem. With the development of offshore fisheries the impact on fish stocks and the ecosystem increased rapidly. At present, large-scale fisheries are one of the main factors determining the state and dynamics of the Barents Sea ecosystem.
The over-exploitation of demersal fish stocks, such as cod, haddock, redfish and Greenland halibut in the 1950s was the first large-scale disturbance in the Barents Sea ecosystem caused by fisheries. Technical management measures were introduced to limit the catches and to restore the depleted stocks. Despite positive effects of these measures, the redfish and Greenland halibut stocks did not fully recover. In 1970-1980, fisheries expanded from targeting demersal fish, which are top predators in the system, to target small pelagic fish and shrimps at intermediate trophic levels. As a response to this expansion, fishery management introduced a system which included both total allowable catch (TAC) and various technical means for protection of juveniles. During the last decades the exploitation of all main commercial species in the Barents Sea has generally corresponded to maximum allowable catches. As the overall impact by fisheries may influence the ecosystem stability (Filin et al., 2008), there is a need for including ecosystem components into fisheries management. The Barents Sea ecosystem seems to be particularly vulnerable when high fishing pressure coincides with adverse environmental conditions, such as in the middle of the 1980’s when the capelin stock collapsed. It should be noted, however, that overfishing was not the main cause for the capelin collapse in the mid-1980s (see chapter Overall picture).
Current fishing mortality contributes to keep stocks at a reduced abundance levels. However, stocks of cod, capelin, haddock and shrimp in the Barents Sea are currently managed sustainably, in the sense that fishing mortality is below the precautionary limits (Fpa) set by ICES, and stock sizes above the corresponding precautionary limit (Bpa). The stock sizes of Greenland halibut and the two redfish species (Sebastes mentella and S. marinus) are below Bpa partly due to overfishing. There are, however, signs of recovery in the S. mentella stock.
Fisheries in the Barents Sea do not only influence the targeted stocks. Due to strong species interactions fisheries removal of one stock may influence the abundance of other stocks. For example, herring collapses have positively influenced capelin abundance. Reduced stock sizes due to fisheries removal may also lead to changing migration patterns. Due to density dependent migrations, fish stocks cover greater areas and migrate longer distances when abundances are high compared to low. Fisheries also reduce the average fish size, age and age at maturity (further discussed in chapter Evolutionary effect of fishing on maturity in cod). The reduced size and age of the cod stock may actually have altered the ecological role of cod as top predators in the Barents Sea.
Other indirect impact of fisheries include bycatch of non-targeted fish species, marine mammals and seabirds and gostfishing caused by lost fishing gear.
Fisheries’ impact on seabirds and marine mammals
Fisheries have effects on seabirds and marine mammal populations in two different ways; through bycatch in fishing equipment as mentioned above and through removal of prey. Knowledge of the scale of seabird bycatch in the Barents Sea is fragmentary. Special incidents like the bycatch of large numbers of guillemots during spring cod fisheries in Norwegian areas have been documented (Strann et al., 1991). Gillnet fishing affects primarily coastal and pelagic diving seabirds, while the surface-feeding species will be most affected by long-line fishing (Furness, 2003). The population impact of bycatch will vary with the time of year, the status of the affected population, and the sex and age structure of the birds killed. Even a numerically low bycatch may be a threat to red-listed species such as Common guillemot, White-billed diver and Steller’s eider. Also small marine mammals, e.g. seals and porpoises, are caught in fishing gears. The extent of this bycatch is not known, but bycatch is currently thought to be a threat to small coastal harbour porpoise populations living along the Finnmark coast.
The greatest impact of fisheries on seabirds and marine mammals may, however, be through the effect on their food base. Interactions between these top predators and fisheries are complex. In recent decades reduced prey availability has been a serious threat to many seabird populations, although the direct cause of these changes may be difficult to determine. Both fisheries and climatic variations are likely important. The capelin collapse in the 1980s also adversely affected both seabird and marine mammal populations (Vader et al., 1990, Nilssen et al., 1998).
Fisheries’ impact on benthos
Fisheries affect benthic communities through bottom trawling and dredging. Particularly areas with biotic habitats generated by aggregations or colonial growth of single species are vulnerable. Such habitat-generating species are represented by a wide range of taxonomic groups, e.g. Porifera, Polychaeta, Cnidaria, Mollusca and Bryozoa (e.g., reviews in Jennings, 1998; Auster and Langton, 1999; Kaiser and de Groot, 2000; Moore and Jennings, 2000). These biotic habitats house a high diversity of associated species and are examples of whole communities that can be managed within restricted areas. Trawling and dredging also affect single species with a life span which does not favour reproduction between the trawling events.
By reducing abundances of larger long-living and deep burrowing seston-feeders, and increasing abundances of small detritophagous animals, fisheries change the structure of the sublittoral communities. The damage to coral reefs are well known (Fosså et al., 2002), but the effects on soft sediment communities have only recently been quantified. In already disturbed areas, where the fauna comprise opportunistic, short-lived (r-selected) organisms, the trawl damage is less than in more pristine areas (Olsgard et al., 2008). Furthermore, combined effects of climate variability, trawling and dredging are believed to be the main factors reducing the benthos biomass up to 70 % in some areas of the Barents Sea (Denisenko 2001; 2007). In general, the response of benthic organisms to disturbance differs with substrate, depth, gear, and type of organism (Collie et al.; 2000). Effects of trawling may therefore be hard to determine, and global indicator species may be hard to find. The challenge for management is to determine levels of fishing that are sustainable and not degradable for benthic habitats in the long run.
Whaling and hunting
The great abundance and diversity of marine mammals in the Barents Sea attracted the attention of the earliest European explorers to the region. Industrial whaling started in the Barents Sea in the early 17th century. This resulted in abrupt reductions in whale abundances, and was the first pronounced, large-scale human induced change in the Barents Sea ecosystem. As a result of unregulated hunting the stocks of right whales in the Northeast Atlantic were almost extirpated around 1800-1850, and the northern right whale stock in the Northeast Atlantic has subsequently gone extinct. The closely related bowhead whale has not gone extinct, but it is critically endangered and has not shown signs of recovery despite its protected status. The current population in the Barents Sea is estimated to number between 10 and 100 individuals. Walruses, seals and polar bears were initially taken largely as a by-catch of northern whaling, but these animals have also been the subjects of significant commercial harvests over time in the Barents Sea and adjacent areas. When protected on Svalbard in 1952, about 200 walruses remained at Svalbard. The population is still low and Red-Listed. Harbour seals and grey seals as well as harbour porpoise have been exploited throughout their range by coastal people from early human history in the region, and ringed seals, bearded seals, harp seals and white whales have also been harvested since the 1400s in coastal areas of the Barents Sea (Alekseeva, 2008).
Most marine mammal species in the Barents Sea are currently protected from exploitation. Only harp seals and minke whales are harvested commercially by Norway and this is done within sustainable limits. In Norwegian areas, sport hunting for ringed and bearded seals occur without any significant effects on the populations. However, there is a potential risk to small populations of coastal-living seals within Norwegian territories in the southern Barents Sea because of policies aimed to reduce the populations to avoid conflicts with inshore fisheries and aquaculture. Both grey and harbour seals have been hunted at levels that are almost certainly not sustainable in Troms and Finnmark counties in recent years (Nilssen and Haug, 2007; Frie and Kondakov, 2008; Nilssen et al., 2009). In the White Sea, ringed, bearded and harp seals and beluga whales are all hunted based on quota systems. Current quotas for White Sea harp seals are not sustainable (Chernook and Boltnev, 2008), but the actual harvest level during the past couple of decades has been well below the calculated sustainable limit (ICES, 2008). In 2009 there was no commercial harp seal hunt in this region. The small beluga harvest (100-150 individuals) is likely within sustainable limits. The other hunts are difficult to assess due to no recent available abundance data. West Ice harp and hooded seals have been the subject of centuries of commercial harvest; though due to precipitous declines in the latter species since WWII, hooded seals are now Red-Listed both in Norway and internationally and the West Ice quota (Norwegian and Russian hunting area) is set at zero (ICES, 2008; Salberg et al., 2008).
Despite what is, in hindsight, a repeated, tragic history of over-exploitation, the marine mammal community of the Barents Sea region is still rich in species, and some populations, particularly among the pinnipeds, are very abundant (Table 2.4.2, Figure 2.4.27). However, the large-scaled removal of top predators must have influenced the intermediate and upper trophic levels of the Barents Sea ecosystem. It is likely that predation pressure on krill and small pelagic fish species decreased, which may have benefited other top predators in the system such as gadoid fish. However, due to limited knowledge on the ecosystem prior to this removal, we do not know how the removal has changed the system.
Harvesting of seabirds has a long tradition in the Barents Sea region, and used to be widespread and important (Gavrilo 2008, Strøm et al., 2008). Today, the extent of harvesting is reduced and subject to strict regulations. Egging, down collection and harvesting of adult birds and chicks were important commercially and as a food supply in the past for the rural residents of coastal northern Norway (Wold, 1981; Bakken and Anker-Nilssen, 2001). In Svalbard, common eiders have been harvested since the 16th century, but reliable harvest data exist only from the middle of the 18th century onwards (Norderhaug, 1982). Large amounts of eggs and down were collected and the population declined greatly before it was protected in 1963. Hunters also visited seabird colonies where they collected eggs and adult birds. At Bear Island, 50,000-60,000 eggs were collected annually between 1952-1958, mainly from common and Brünnich’s guillemots. This activity was stopped in 1971 (Rossnes, 1981). The most extensive harvest in the Russian seabird colonies occurred in Novaya Zemlya.
Commercial seabird harvest in Russia by Russians and foreigners (mostly Norwegians) started in the 19th century, and up to the start of the 2000 century, tens of thousands of birds and their eggs were collected annually (Sidorov, 1873; Ukhtomski, 1881). Seabird harvest peaked in 1920s – 1950s. At Bezymyannaya Bay (one of the largest seabird colonies on Novaya Zemlya) 342,500 Brunich’s guillemot eggs were collected and more than 12,000 adult birds were killed in 1933 (Krasovski, 1937). During World War II, guillemots and their eggs were collected on Novaya Zemlya as a valuable food supply for the starving citizens of Archangelsk. In the late 1940s the harvest was restricted and protected areas were established. Nevertheless, dramatic decline in exploited colonies were observed. The commercial harvest was closed in 1954 after a nuclear testing ground had been established on the archipelago and the local population had been transferred to the mainland. No seabird colonies along the Murman coast have been harvested extensively.