The Barents Sea (BS) is a high-latitude shelf ecosystem with important fisheries, high and historically variable harvesting pressure, and ongoing high variability in climatic conditions. To quantify carbon flow pathways and assess if changes in harvesting intensity and climate variability have affected the BS ecosystem, we modeled the ecosystem for the period 1950–2013 using a highly trophically resolved mass-balanced food web model (Ecopath with Ecosim). Ecosim models were fitted to time series of biomasses and catches, and were forced by environmental variables and fisheries mortality. The effects on ecosystem dynamics by the drivers fishing mortality, primary production proxies related to open-water area and capelin-larvae mortality proxy, were evaluated. During the period 1970–1990, the ecosystem was in a phase of overexploitation with low top-predators’ biomasses and some trophic cascade effects and increases in prey stocks. Despite heavy exploitation of some groups, the basic ecosystem structure seems to have been preserved. After 1990, when the harvesting pressure was relaxed, most exploited boreal groups recovered with increased biomass, well-captured by the fitted Ecosim model. These biomass increases were likely driven by an increase in primary production resulting from warming and a decrease in ice-coverage. During the warm period that started about 1995, some unexploited Arctic groups decreased whereas krill and jellyfish groups increased. Only the latter trend was successfully predicted by the Ecosim model. The krill flow pathway was identified as especially important as it supplied both medium and high trophic level compartments, and this pathway became even more important after ca. 2000. The modeling results revealed complex interplay between fishery and variability of lower trophic level groups that differs between the boreal and arctic functional groups and has importance for ecosystem management.

Year-class strength of Barents Sea capelin has been monitored closely since the early 1970s and during this 45 years period three short periods of almost total recruitment failure leading to three stock collapses have been observed. These events triggered much attention since there was a large commercial fishery for capelin, but also because of observed ecosystem effects attributed to the first of these collapse events. This attention motivated research to clarify mechanisms behind the recruitment failures, and many papers have been published regarding the causes of these events. Here, we review this literature and try to put the various investigations into context. Most of the research conducted gives evidence in favour of a hypothesis that was formulated after the first recruitment failure event in the mid-1980s that predation on capelin larvae was the main cause of recruitment failure. Most studies also support the hypothesis that young herring (Clupea harengus) was the main predator on capelin larvae, but other predators like young-of-the-year cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) probably also played a role. Investigations of the effect of predators such as haddock, red king crab (Paralithodes camtschaticus), diving birds, and capelin on the demersal capelin eggs have also been reviewed. Usually, these predators are found to consume capelin eggs, but most likely not to an extent that would affect the recruitment to a noticeable degree. It is concluded that the predation on capelin larvae is the main reason for the observed recruitment failures, although predation from the predators reviewed here can hardly be the only reason for almost total recruitment failures observed in some periods.

The spawning distribution of Barents Sea capelin Mallotus villosus off northern Norway in 2002 was restricted to a narrow area at the easternmost spawning grounds. There was an increase in abundance and a marked shift in aggregation area from east towards west during 11 March to 4 April, as the capelin went from prespawning through spawning to spent stages. The capelin stomachs were either empty or contained capelin eggs, often mixed with sand. Both the occurrence of cannibals and stomach fullness increased with stage of maturity, being highest in spent fish, and higher in males than in females. No differences were found between cannibals and non-cannibals of the same sex when adjusted for total length (L T ), age, L T at age and condition factor. At similar stomach fullness, the females had consumed more eggs than the males. This was attributed to a higher proportion of empty and broken eggs and less sand in females compared to males. In the full stomachs, the mean consumption in females and males was 623 and 334 eggs, respectively, whereas the respective maximum egg consumption was 871 and 683 eggs. The minimum estimates (given no digestion) of mean AE 95% CL egg consumption in spent females and males were 75Á4 AE 6Á9 and 58Á4 AE 12Á0 eggs, respectively, implying a minimum mortality of 1-2% of the total egg production caused by cannibalism.

The invasive red king crab Paralithodes camtschaticus preys on lumpsucker Cyclopterus lumpus eggs. We tested the hypothesis this egg consumption may hamper the recruitment of lumpsucker. Methods applied included field work, laboratory experiments and modelling of egg consumption. Crabs were sampled and feeding behaviour was studied by means of a remotely operated vehicle and SCUBA divers in a field survey carried out in Varanger Fjord, Norway, in 2003. Laboratory experiments were carried out in 2006 to study the digestion of lumpsucker eggs by red king crabs, and a stomach evacuation model was fitted to the experimental data. Using data from the field and laboratory studies, an egg consumption model was used to quantify the amount of lumpsucker eggs consumed by king crabs in Varanger Fjord. The uncertainty in model input data was assessed using a Monte Carlo simulation. Sex or sampling area did not significantly affect egg predation. A total of 7.9% of all crab stomachs contained an average of 20 lumpsucker eggs, but the number of eggs per stomach varied widely. The average time required to evacuate lumpsucker eggs at 6°C in the laboratory experiment was ~10 h. In 2003, king crabs in Varanger Fjord consumed lumpsucker roe equivalent to approximately one-third of commercial catches during the same period. Red king crab predation on lumpsucker eggs may hamper lumpsucker recruitment in northern Norwegian waters.

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We studied whether incorporation of terms for environmental factors (temperature, herring and 0‐group cod) into the stock recruitment functions for the Barents Sea capelin (Mallotus villosus) would increase the ability to predict recruitment. We also investigated the effects of alternative estimates of the capelin spawning stock (SSB) and juvenile herring on model predictability. By using nonlinear regression, the modified Beverton–Holt and Ricker stock recruitment models were fitted to time series of capelin spawning stock, capelin recruitment, juvenile herring, 0‐group cod and temperature from 1973 to 2000. The modified Beverton–Holt model with a term for juvenile herring fit capelin recruitment data well. Inclusion of terms for temperature and 0‐group cod abundance did not significantly improve the model fit. The best model predicts an almost proportional relationship between the spawning stock and the capelin recruitment when the abundance of juvenile herring is high.