The Barents Sea is on the continental shelf surrounding the Arctic Ocean. It joins the Norwegian Sea in the west, the Arctic Ocean in the north and the Black Sea in the east. Its contours are the continental slope between Norway and Spitsbergen to the west, and the top of the continental slope towards the Arctic Ocean in the north. And it is drawn by the Novaya Zemlya archipelago to the east and the coasts of both Norway and Russia in the south. It covers an area of approximately 1.4 million km 2 and has an average depth of approximately 230 m and a maximum depth of approximately 500 m at the western end of the Bear Island Trough.
Its topography is characterized by pits and basins 300-500 m deep, separated by shallow coastal areas, ranging in depth from 100 to 200 m. The three largest banks are the Central Bank, the Big Bank and the Spitsbergen Bank. From the Middle Barents Sea to the north and west to the breaking of the continental shelf there are several pits over 300 m deep. These western troughs allow Atlantic waters to flow into the center of the Barents Sea. The Barents Sea is shared between Russia and Norway, and there is a long history of relatively successful cooperation in fisheries management even in times otherwise marked by political tensions.
Bentos and Shellfish
A wide variety of organisms are found on the sea floor. Some are buried in sediments, others are attached to a substrate, some are slow and sluggish, others are roaming and fast. More than 3050 species of benthic invertebrates inhabit the Barents Sea. Benthic ecosystems in the sea have important value in both direct economic terms and ecosystem functions. Comb, shrimp and king crab are harvested in the region. The snow crab can be considered a potential commercial species in the Barents Sea. Many benthos species, such as sea cucumbers, snails, and bivalves, are also interesting for biological research or as a potential food source. Major fish species such as haddock, cod, catfish, and most flatfish feed primarily on benthos.
Many benthic animals, especially bivalves, filter particles from the ocean and effectively remove particles from the water column. Others clean up dead organisms, returning valuable nutrients to the water column. Detritus feeders and other active diggers regularly move sediment at the bottom. So the sediment increases the oxygen content and overall productivity just like terrestrial worms. The decline in the total biomass of benthos from 1924-1935 to 1968-1970 occurred in most of the sea and has been attributed to climate change by many researchers. However, the mechanism behind this biomass reduction is not clear. Northern shrimps are found in the deepest parts of the Barents Sea and Spitsbergen waters. The most concentrated concentrations are at depths between 200 and 350 meters. This species feed mainly on leftovers, but also with clean food. It is also important as a nutrient for many fish species and seals.
The red king crab originated in the Barents Sea in the 1960s. It is still an important commercial species, and adult red king crabs are opportunistic omnivores. The snow crab is an invasive species deliberately brought to this area. The first recordings of this species in the Barents Sea were in 1996. Snow crabs have been found in the stomachs of cod, haddock, wolf and spiny skates since 2003, indicating that the abundance and settlement density of crab has increased significantly. Icelandic comb is a slow-growing species common in all shallow areas. It is usually associated with a hard-bottomed substrate and is most commonly in areas with strong currents. The comb is a filter feeder and therefore highly dependent on seasonal phytoplankton production. This also has an effect on its growth and its lifespan is 30 years or more.
Eight species of squid are found living in the Barents Sea. The flying squid Todarodes sagittatus has been an important fishing resource in Norwegian waters for various periods until 1988. However, it has almost disappeared from the waters since then and only occasional hunts have been recorded. Gonatus fabricii is another type of squid that is abundant in the open waters of Barents and the Norwegian Sea. This species is an important food for several bird and marine mammal species, but can possibly be seen as a potential fishery resource.
Marine mammals as major predators and keystone species are important components of the Barents Sea ecosystem. Twenty-five marine mammal species occur regularly in the Barents Sea. Some of these are as follows:
• Seven pinnipeds (seals and walruses),
• 12 large mammal marine animals (large whales),
• Five small marine mammals (dolphins),
Polar bear (Ursus maritimus),
Some of these species are not full-time residents in the Barents Sea, but use temperate zones for mating, calving and feeding. Others live in the Barents Sea year-round, such as the white-billed dolphin. Some marine mammals such as the Beluga whale, the other whale Balaena mysticetus and the blue whale Balaenoptera musculus are rare due to their historical high exploitation. Marine mammals are important predators for commercial fish species in the Barents Sea. However, consumption estimates are associated with high levels of uncertainty. Minke whales and harp seals have the largest consumption, and together they can consume around 5 million tons of crustaceans, capelin, herring, arctic cod and other gadoid fish annually. Functional relationships between marine mammals and their prey appear to be closely related to fluctuations in marine ecosystems. It is thought that both minke whales and harp seals vary between krill, capelin and herring, depending on the availability of different prey species.
The only marine mammal species harvested commercially in the Barents Sea are harp seals and minke whales. Harp seal pup production estimates are based on data collected during the traditional Russian multispectral weather survey. According to these studies, the abundance of war seal production in the White Sea has decreased sharply since 2004. One of the most important factors that has caused the abundance of harp seals to decrease in recent years is the reduced ice size due to warming. Changing ice conditions are responsible for the redistribution of animals during the juvenile period. Abnormal ice conditions in the White Sea likely also led to higher natural mortality in juveniles.
A benchmark assessment for North East Atlantic (NEA) cod was conducted in 2015. The assessment recommended that the Expanded Survivor Analysis (XSA) model continue to be used as the main tool for NEA cod assessment. Also some changes in the model configuration have been proposed. The main model used in the assessment of key stocks is the Extended Survivor Analysis (XSA). The model is often adapted for age and natural mortality data, and most work similar to typical Virtual Population Analysis (VPA) retrospective calculation models. Back calculations in these applications are the same, but they differ in the statistical methods used to adjust to the indices of population size. For North East Atlantic cod, saithe, XSA is used as the main assessment method. For the Norwegian Coastal cod, the separable VPA (SVPA) model proposed by it was used. This model assumes that the fishery mortality rate can be broken down into an annual component common to each age of the same year and a component common to many years.
Haddock and beaked red fish (Sebastes mentella) are evaluated using statistical age-catching (SCAA) models. Unlike VPA methods, SCAA is a forward-looking calculation method. With the introduction of the SAM (REF) model used for whiting in the Barents Sea, ICES is more widely used in fishing. Where age data are uncertain or not available, an alternative model, age and length structured globally applicable Field Separated Generic Ecosystem Toolbox model or GADGET is used for assessment. The model is currently used for golden red fish, Sebastes norvegicus and Greenland halibut.
Author: Ozlem Guvenc Agaoglu