Olsen, E., Aanes, S., Mehl, S., Holst, J. C., Aglen, A., and Gjøsæter, H. 2010. Cod, haddock, saithe, herring, and capelin in the Barents Sea and adjacent waters: a review of the biological value of the area. – ICES Journal of Marine Science, 67: 87–101. Cod, haddock, saithe, herring, and capelin are the most important fish species in the Barents Sea and adjacent waters. Ecosystem-based management requires species-specific knowledge of the biological value and vulnerability throughout their life history and distributional range. For each of the five species and four annual quarters, the spawning (egg) areas, nursery areas for larvae and juveniles, and feeding grounds for adults are described and mapped. Areas of eggs (spawning) and larvae were the most important because these are the life stages when fish are most vulnerable to anthropogenic impact. The greatest overlap of spawning areas was from Røstbanken in the south to the Varanger Peninsula in the northeast, and overlap of larval distribution was more extensive.

Themisto abyssorum and Themisto libellula were the dominant amphipod species observed in the central and the northern Barents Sea during [1984][1985][1986][1987][1988][1989][1990][1991][1992][1993][1994][1995][1996]. T. abyssorum was predominant in the subarctic waters, T. libellula in the Arctic waters. A third species, Themisto compressa, was rare and was restricted to the Atlantic waters. Our study showed peak abundances of T. abyssorum and T. libellula in summer and in early autumn. High abundances were usually associated with Polar Front waters. T. libellula has a more near surface distribution than T. abyssorum.Followed by a decrease in the capelin stock from 1985-1987 there was an increase in the abundance of Themisto abyssorum and T. libellula, probably due to the reduced grazing pressure from capelin (Mallotus villosus). In the mid-1980s and 1990s when the capelin stock was at extremely low levels, cod (Gadus morhua) switched from capelin to alternative prey such as amphipods and krill. Detailed analysis of amphipods in the cod stomachs from 1984-1999, showed that cod fed mainly on Themisto spp., especially on T. libellula. Themisto species were consumed by most age groups of cod.With an increase in the capelin stock from 1987-1991, a corresponding decrease in the abundance of Themisto abyssorum and T. libellula was observed. During 1993-1996 when the capelin stock again was at low levels, the abundance of these two amphipod species increased. The increase in abundance was less pronounced in the mid-1990s for T. libellula probably owing to higher grazing pressure from cod and other predators such as harp seal (Phoca groenlandica) and arctic sea birds. The stock size of cod in the mid-1990s was about twice the amount as in the mid-1980s. These results illustrate strong predator-prey interactions between macrozooplankton species as amphipods and capelin and cod in the Barents Sea. The amphipod populations in the Barents Sea appear to be to a large extent controlled by predation.

An overview of the estimates of consumption by predators on the main fish stocks in the Barents Sea is given. The main predators are cod (Gadus morhua), harp seal (Phoca groenlandica) and minke whale (Balaenoptera acutorostrata). The results indicate that cod is the most important predator, consuming about as much food annually as harp seals and minke whales combined. The consumption estimates, together with data on the amount of fish removed by commercial fisheries, are compared to estimates of the abundance and removal through natural mortality of the various species of fish prey. The consistency between these estimates is discussed. The natural mortality values for cod and haddock used in assessments are found to be reasonably consistent with the consumption estimates. The consumption of capelin is found to be higher than what is available for predation in years of low capelin abundance, while in years of high herring abundance the consumption of herring does not explain all the mortality. The way in which the consumption estimates are and can be utilised in the assessment and management of fish stocks in the Barents Sea using multispecies models and approaches is described.

Data (trawl, acoustic, CTD) from scientific surveys along the Norwegian coast, in the North Sea, in the Barents Sea, west of the British Isles, and in the Irminger Sea are used. The vertical density distributions of blue whiting, cod, haddock, redfish, saithe, capelin, and herring are described in relation to environmental conditions and physiological limitations. The first four surveys mainly cover banks and shelf areas shallower than 500 m. The last two surveys, aimed at blue whiting and redfish, mainly cover shelf edge and deep-sea areas with depths from 200 to 1300 m and from 440 to 3000 m. In regard to cod some information from data-storage tags is used.For physoclists the relative vertical profile of each acoustic sample i.e. acoustic-area backscattering coefficient (s A ), is expressed in terms of the relative pressure reduction level from seabed up to surface. Thus relative vertical profiles with different bottom depths are normalized and are made compatible for a discussion in terms of the free vertical range (FVR). This restriction to rapid vertical movement is evident in the physoclist species studied. For samples in the shelf area, the profiles show that blue whiting, haddock, saithe, cod, and redfish are mainly distributed within the bottom half of the water column. Some fish adapt to pelagic living especially in areas with high acoustic density and where the bottom is deep. Here a pelagically living fish is defined as an individual fish having a current free vertical range that does not include the seabed.For demersal fish, day and night relative vertical profiles are corrected for unequal day and night losses in the bottom acoustic dead zone, which is the zone near the seabed where echoes from fish cannot be discriminated from the sea bottom echo. Day and night samples are separated by the sun's passing 5 below the horizon. In most years evidence of diurnal vertical migration is found for all investigated species. In many cases of demersal fish there is a higher relative acoustic density (s A -values) in the mid-range of the bottom half of the water column in the daytime as opposed to the night-time. At night there is a degree of separation, one group of fish descends to aggregate near the seabed and another ascends. Inter-annual variations in the diel movement from different parts of the stock are discussed in relation to the inter-annual variations in age composition of the stock.

Consumption of different age groups of juvenile, Norwegian, spring-spawning herring (Clupea harengus) by northeast Arctic cod (Gadus morhua) in the Barents Sea in 1992-1997 is estimated using cod stomach content data. We present a new approach to the problem of estimating consumption by fish. The new method is based on the estimation of digestion time for single prey items based on the difference between fresh weight at ingestion and weight in the stomach at time of sampling. Estimation is based on a gastric evacuation model for cod and area-specific sea temperatures. This is used to estimate the time (t max ) it takes for a prey to become digested to a stage where length is no longer measurable. Predation rate is then estimated for all prey with digestion time ≤t max as number of prey eaten in the time range defined by t max . This rate is combined with estimates of the proportion of the cod stock consuming the prey and area-specific abundance of cod, giving consumption of herring on a seasonal and yearly basis. The consumption estimates differ from those obtained using current methods. Predation mortality of herring is estimated directly from the consumption estimates by combining them with acoustic herring abundance data.Résumé : Une analyse des contenus stomacaux de morues nous a permis d'estimer la consommation des diverses classes d'âges des jeunes harengs de Norvège à reproduction printanière (Clupea harengus) par les morues arctiques (Gadus morhua) dans la mer de Barents en 1992-1997. La méthodologie nouvelle que nous proposons pour estimer la consommation par les poissons est basée sur l'évaluation du temps de digestion des différentes proies individuelles d'après la différence entre la masse humide à l'ingestion et la masse dans l'estomac au moment de l'échantillonnage. L'estimation se base sur un modèle d'évacuation gastrique chez la morue et sur les températures de la mer spécifiques à chaque région. Ces données permettent d'estimer le temps (t max ) nécessaire pour qu'une proie soit digérée au point où la longueur devient impossible à mesurer. Le taux de prédation peut alors être calculé pour toutes les proies dont le temps de digestion est ≤t max comme étant le nombre de proies mangées dans l'intervalle de temps défini par t max . Ce taux est ensuite combiné à des estimations de la proportion du stock de morues qui consomme la proie et aux abondances de morues en fonction de la région pour ainsi estimer la consommation de harengs sur une base saisonnière et annuelle. Ces estimations de consommation diffèrent de celles obtenues par les méthodes courantes. La mortalité des harengs due à la prédation est alors estimée directement à partir des estimations de consommation en les combinant à des données acoustiques d'abondance des harengs.[Traduit par la Rédaction] 359 Johansen et al.