Biodiversity and exploitation of the main fish stocks in the Norwegian - Barents Sea ecosystem

Juvenile herring and capelin are the main stocks of plankton feeders in the Barents Sea, the cod is the dominant predator. Warm climate favours recruitment of herring and cod, but large stocks of juvenile herring hamper survival of the capelin fry. Since the early 1970s, the herring stock has been grossly overexploited, which could have led to an imbalance in the state of the predatorprey relationships in the Barents Sea. In the 1970s and early 80s, however, cod could feed on capelin which had excellent growth and recruitment conditions when the herring was absent. The consequences of the reduced herring stock were triggered in the mid 1980s, when excellent recruitment conditions for herring and cod occurred. Three abundant year classes of cod were recruited, but the herring stock was not sufficiently large to take full advantage of the favourable recruitment conditions. Given the lack of juvenile herring and a reduced capelin stock, the rapidly growing cod stock grazed down all other available prey species in the area, including its own progeny, and starved cod, seabirds and seals have in later years appeared on the north Norwegian coast. The capelin fishery collapsed, and the traditional coastal cod fisheries have been struck by the most serious crisis on record.     

Ecological significance of 0-group fish in the Barents Sea ecosystem

Investigations into the 0-group fish in the Barents Sea have been carried out since 1965, with the goal of estimating the abundance of 0-group fish. 0-group abundance indices have been used in the assessment of the recruitment level and in recruitment variability studies. However, the ecological importance of the 0-group fish in the Barents Sea has been less studied. Although 0-group capelin, herring, cod and haddock are widely distributed in the Barents Sea, the central area seems to be the most important, accounting for approximately 50–80% of the annual biomass. The total biomass of the four most abundant 0-group fish species can be up to 3.3 million tonnes, with an average of 1.3 million tonnes (1993–2009). Wide distribution and high biomass of pelagically distributed 0-group fish make these fishes an important element in the energy transport between different trophic levels and different geographical areas, having a critical impact on the entire Barents Sea ecosystem. In recent years, capelin have shown a pronounced northward shift in biomass distribution, and several successive strong year classes occurred during warm temperature conditions. Cod biomasses were unexpectedly low during warm years and were positively correlated with spawning stock biomass, while the correlation with temperature was not significant. Haddock and herring show, as expected, increasing biomass with increased temperature when the spawning stock is at a sufficiently high level.     

Temperature-driven changes in early life-history stages influence the Gulf of Riga spring spawning herring (<Emphasis Type="Italic">Clupea harengus m</Emphasis>.) recruitment abundance

Processes occurring during early life-history stages influence the year-class abundance of marine fish. We found that the abundance of 1-year-old spring spawning herring is statistically significantly determined by the number of post-flexion herring larvae in the Gulf of Riga (Baltic Sea). The abundance of consecutive developmental stages of larvae: yolk-sac, pre-flexion, flexion and post-flexion strongly correlated with each other, indicating that factors which already influence the yolk-sac stage are important in determining the abundance of post-flexion herring larvae. Winter air temperature before spawning determined the timing of maximum abundance of pre-flexion herring larvae, but not their main prey: copepod nauplii, implying that different mechanisms governing major preconditions for the formation of year-class strength. The abundance of post-flexion larvae displayed a potential dome-shaped relationship with sea surface temperature experienced after hatching. We suggest that increased summer temperatures, which exceed the physiological optimum negatively, affect the survival of post-flexion herring larvae. Overall, future climate warming poses an additional risk to larval herring survival and this may lead to a reduction in those herring stock which rely on recruitment from shallow coastal areas.     

Climate-Driven Ichthyoplankton Drift Model Predicts Growth of Top Predator Young

Climate variability influences seabird population dynamics in several ways including access to prey near colonies during the critical chick-rearing period. This study addresses breeding success in a Barents Sea colony of common guillemots Uria aalge where trophic conditions vary according to changes in the northward transport of warm Atlantic Water. A drift model was used to simulate interannual variations in transport of cod Gadus morhua larvae along the Norwegian coast towards their nursery grounds in the Barents Sea. The results showed that the arrival of cod larvae from southern spawning grounds had a major effect on the size of common guillemot chicks at fledging. Furthermore, the fraction of larvae from the south was positively correlated to the inflow of Atlantic Water into the Barents Sea thus clearly demonstrating the mechanisms by which climate-driven bottom-up processes influence interannual variations in reproductive success in a marine top predator.     

Feeding Ecology of Northeast Atlantic Mackerel,Norwegian Spring-Spawning Herring and Blue Whiting in the Norwegian Sea

The Norwegian spring-spawning (NSS) herring (Clupea harengus), blue whiting (Micromesistius poutassou) and Northeast Atlantic (NEA) mackerel (Scomber scombrus) are extremely abundant pelagic planktivores that feed in the Norwegian Sea (NS) during spring and summer. This study investigated the feeding ecology and diet composition of these commercially important fish stocks on the basis of biological data, including an extensive set of stomach samples in combination with hydrographical data, zooplankton samples and acoustic abundance data from 12 stock monitoring surveys carried out in 2005–2010. Mackerel were absent during the spring, but had generally high feeding overlap with herring in the summer, with a diet mainly based on calanoid copepods, especially Calanus finmarchicus, as well as a similar diet width. Stomach fullness in herring diminished from spring to summer and feeding incidence was lower than that of mackerel in summer. However, stomach fullness did not differ between the two species, indicating that herring maintain an equally efficient pattern of feeding as mackerel in summer, but on a diet that is less dominated by copepods and is more reliant on larger prey. Blue whiting tended to have a low dietary overlap with mackerel and herring, with larger prey such as euphausiids and amphipods dominating, and stomach fullness and feeding incidence increasing with length. For all the species, feeding incidence increased with decreasing temperature, and for mackerel so did stomach fullness, indicating that feeding activity is highest in areas associated with colder water masses. Significant annual effects on diet composition and feeding-related variables suggested that the three species are able to adapt to different food and environmental conditions. These annual effects are likely to have an important impact on the predation pressure on different plankton groups and the carrying capacity of individual systems, and emphasise the importance of regular monitoring of pelagic fish diets.     

State-dependent spawning migration in Norwegian spring-spawning herring

Norwegian spring-spawning herring Clupea harengus winter in fjords of northern Norway, whereas the subsequent spawning occurs at various locations along the coast with a main bulk off the south-western coast. The distance of the southward spawning migration tends to increase with the length and condition of the fish. The costs and benefits of the southward migration were modelled in terms of fitness (number of surviving offspring). The model assumes that larvae have increasing growth and survival rates further south as they pass through warmer water during the northward drift in the coastal current. In agreement with the observed spawning distribution, optimal spawning grounds were predicted off the southwestern coast and farther south with increasing fish length and condition. The present study suggests that homing is not a successful strategy for these herring, and instead the selection of spawning grounds depends on individual internal state (length, condition), the cost of migration and the probability of larval survival.     

Spatially structured interactions between a migratory pelagic predator, the Norwegian spring-spawning herring Clupea harengus L., and its zooplankton prey

Spring-spawning herring Clupea harengus was patchily distributed over large parts of the Norwegian Sea in May 1995–2005, during the early phase of the annual feeding migration. Overall, herring tended to be found in areas with intermediate biomasses of zooplankton prey, intermediate water temperatures and relatively high salinities. Herring had more food in their stomachs in areas of relatively low water temperature and high herring abundance. Hydrographical conditions revealed that herring was feeding mainly within Atlantic water masses, and more intensely in western and northern regions of the Norwegian Sea. Zooplankton biomass was patchily distributed, and was generally higher towards the western parts of the Norwegian Sea. Here, zooplankton biomass in year _{i +1} was also negatively associated with herring spawning stock biomass in year _i , while there was no evidence for such an association in the eastern region; indicating that herring may have a geographically structured 'top–down' effect on the recruitment of its zooplankton prey. The fact that herring was not typically associated with the areas containing the greatest zooplankton biomasses may reflect that the fish had not yet reached the most profitable feeding grounds or alternatively that herring was depleting zooplankton biomass.     

Spatial and Temporal Variability in the Abundance and Size Structure of Larvae of the White Sea Herring (<Emphasis Type="Italic">Clupea pallasii marisalbi</Emphasis>) in Onega and Kandalaksha Bays of the White Sea

According to the data of ichthyological surveys conducted in Onega and Kandalaksha bays of the White Sea in June 2015, the abundance and pattern of spatial distribution of larvae of the White Sea herring Clupea pallasii marisalbi are comparable with those of 2012. Aggregations of herring larvae detected at a distance of 12?14 km from the coast in the apex part of Kandalaksha Bay are probably the result of their mass drift caused by fresh floodwater discharge. In coastal waters of the bays adjacent to the littoral part, the abundance of herring larvae above the depths less than 5 m varies considerably due to their drift under effect of alongshore and/or tidal currents. The White Sea herring larvae reach high abundance only in the inlets (Chupa, Knyazhaya, Belaya, and Maikova inlets) with the river runoff; their length increases with the distance from spawning grounds. In different years, the main bulk of herring larvae in Knyazhaya Inlet is concentrated at depths about 12–15 m at 6?8°C, or moved to the upper 5-m quasi-homogeneous layer when the water temperature at the depth was 0?1°C.     

Species composition and distribution of ichthyoplankton in the White Sea in July 2003

On the basis of an ichthyoplankton survey made in July 2003 in Kandalaskha Gulf, the Basin, the Gorlo Strait, and the Voronka the species composition and distribution of fish eggs and larvae are investigated. The ichtyoplankton comprises 17 fish species, of which the majority (11) were boreal forms. Other fish were represented by arctic species (2) and arctic-boreal species (4). The traits of distribution of abundantly caught species are considered in detail: larvae of the White Sea oligovertebrate herring Clupea pallasii marisabli, eggs of the dab Limanda limanda, larvae of the northern sand lance Ammodytes marinus, and of the capelin Mallotus villosus villosus. The presence of all larvae of White Sea herring only within Kandalaskha Gulf is considered to be evidence supporting the hypothesis suggested by Dmitriev (1946) on the absence of their drift beyond large gulfs of the White Sea. The natural barrier for mutual exchange with early developmental stages of fish between the White Sea and the Barents Sea in summer may be a developed hydrologic front in the boundary area of the Basin and Gorlo Strait, discovered in the course of the survey. This front causes their death by thermohaline shock in the abrupt gradient zone.     

Interannual variations in condition indices of larval Norwegian spring-spawning herring

The sea water temperature off the Norwegian coast was lower in 1994 than in 1992 and 1993. RNA/DNA ratios of Norwegian spring spawning herring increased with increasing larval dry weight all years, except for larvae sampled south of 62°N in 1994. The RNA/DNA ratios indicated that each year, only a small portion ( 0.7%) of the larvae were starving. RNA/DNA ratios and temperature were negatively correlated in 1992, but in other years no significant correlations were found. Residuals of In RNA v . In DNA and In W v. L _s were poorly correlated in all years, but residuals of In RNA v. In DNA and In DNA v. In W were negatively correlated in all years. Principal component analysis showed that the RNA/DNA ratio and DNA (% of weight) were correlated with different axes. Abundance data for herring at the early larval and 0-group stages in 1992–1994 indicated higher mortality in 1994 compared with the other years. The data do not indicate that average larval condition was poorer in 1994 than in other years. However, the variability in larval condition was higher in 1994 than in other years, and the condition of later larval stages was relatively lower in 1994 than in other years.     

Two adjacent inversions maintain genomic differentiation between migratory and stationary ecotypes of Atlantic cod

Atlantic cod is composed of multiple migratory and stationary populations widely distributed in the North Atlantic Ocean. The Northeast Arctic cod (NEAC) population in the Barents Sea undertakes annual spawning migrations to the northern Norwegian coast. Although spawning occurs sympatrically with the stationary Norwegian coastal cod (NCC), phenotypic and genetic differences between NEAC and NCC are maintained. In this study, we resolve the enigma by revealing the mechanisms underlying these differences. Extended linkage disequilibrium (LD) and population divergence were demonstrated in a 17.4‐Mb region on linkage group 1 (LG1) based on genotypes of 494 SNPs from 192 parents of farmed families of NEAC, NCC or NEACxNCC crosses. Linkage analyses revealed two adjacent inversions within this region that repress meiotic recombination in NEACxNCC crosses. We identified a NEAC‐specific haplotype consisting of 186 SNPs that was fixed in NEAC sampled from the Barents Sea, but segregating under Hardy–Weinberg equilibrium in eight NCC stocks. Comparative genomic analyses determine the NEAC configuration of the inversions to be the derived state and date it to ~1.6–2.0 Mya. The haplotype block harbours 763 genes, including candidates regulating swim bladder pressure, haem synthesis and skeletal muscle organization conferring adaptation to long‐distance migrations and vertical movements down to large depths. Our results suggest that the migratory ecotype experiences strong directional selection for the two adjacent inversions on LG1. Despite interbreeding between NEAC and NCC, the inversions are maintaining genetic differentiation, and we hypothesize the co‐occurrence of multiple adaptive alleles forming a ‘supergene’ in the NEAC population.     

Pantophysin (Pan I) locus divergence between inshore v. offshore and northern v. southern populations of Atlantic cod in the north‐east Atlantic

More than 6000 cod Gadus morhua , sampled in coastal and offshore waters stretching from the Barents Sea down to the North Sea, were analysed for frequencies of alleles at the scnDNA pantophysin locus ( Pan I)[formerly called synaptophysin ( Syp I)]. The significant allele frequency difference between the two major stocks of cod in Norway, north‐east Arctic cod (NEAC) and Norwegian coastal cod (NCC), was upheld in all years of the investigation (1993 to 2001), and applied both to larval cod and post‐juveniles of various ages. On a north‐south axis, the appearance of a latitudinal cline of post‐juvenile (≥1 year) allele frequencies was exposed. The intermediate allele frequencies in coastal areas of northern Norway, seem to a large extent to be caused by intermingling of the two stocks, although the existence of populations of coastal cod with alternative Pan I frequencies could not be ruled out. The role of selection is yet unresolved. Depth of the sampling location seemed to have an effect on the allele frequencies and their temporal stability, while there was no indication of seasonal variation in the frequencies. Breeding structure was the most likely cause for upholding the extreme divergence in Pan I frequencies between NEAC and NCC.     

Feeding, Ichthyophonus sp. infection, distribution and growth history of Norwegian spring-spawning herring in summer

After its collapse in the late 1960s, 1991 was the first year of systematic herring investigations after the Norwegian Sea stock returned to the traditional summer-feeding areas. Of 11 prey groups three dominated the diet; Calanus finmarchicus, Parathemisto spp. and Sebastes spp. There was a high prevalence of Ichthyophonus sp. at stations in the central and western parts of the Norwegian Sea where the cpue was low. This suggests that infected individuals were unable to return from the feeding to the wintering area of the Norwegian fjords. Highest cpue were recorded between 68–73° N and 5–15° E. Age groups 2, 3 and 8 were most frequently caught and age group 8 (the 1983 year class) dominated the samples. Herring occurred in dense concentrations where C. finmarchicus dominated diet. 0-group fish dominated diet at stations along the coast from 66–73° N and out to c . 300 km off the coast. Mature herring overlapped in distribution with 0-group fish and Sebastes spp. contributed largely to herring diet in this area. The first rich year-class after the collapse, the 1983 year class, had a faster growth than rich year classes from past herring periods, but slower than poor ones.     

At the rainbow’s end: high productivity fueled by winter upwelling along an Arctic shelf

Herein we document findings from a unique scientific expedition north of Svalbard in the middle of the polar night in January 2012, where we observed an ice edge north of 82°N coupled with pronounced upwelling. The area north of Svalbard has probably been ice-covered during winter in the period from approximately 1790 until the 1980s, a period during which heavy ice conditions have prevailed in the Barents Sea and Svalbard waters. However, recent winters have been characterized by midwinter open water conditions on the shelf, concomitant with northeasterly along-shelf winds in January 2012. The resulting northward Ekman transport resulted in a strong upwelling of Atlantic Water along the shelf. We suggest that a reduction in sea ice and the upwelling of nutrient-rich waters seen in the winter of 2012 created conditions similar to those that occurred during the peak of the European whaling period (1690–1790) and that this combination of physical features was in fact the driving force behind the high primary and secondary production of diatoms and Calanus spp., which sustained the large historical stocks of bowhead whales (Balaena mysticetus) in Arctic waters near Spitsbergen.     

Maritime Delimitation in the Arctic: The Barents Sea Treaty

During a visit to Norway by the Russian president in the spring of 2010, the president and the Norwegian prime minister surprisingly announced agreement on a delimitation line in the Barents Sea ending almost 40 years of negotiations. The agreement was signed in Murmansk on 15 September 2010. This article presents the background of the dispute and undertakes an assessment of the agreement and its implications for the Barents Sea, Svalbard, and other Arctic maritime delimitations.     

Thermocline Regulated Seasonal Evolution of Surface Chlorophyll in the Gulf of Aden

The Gulf of Aden, although subject to seasonally reversing monsoonal winds, has been previously reported as an oligotrophic basin during summer, with elevated chlorophyll concentrations only occurring during winter due to convective mixing. However, the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) ocean color data reveal that the Gulf of Aden also exhibits a prominent summer chlorophyll bloom and sustains elevated chlorophyll concentrations throughout the fall, and is a biophysical province distinct from the adjacent Arabian Sea. Climatological hydrographic data suggest that the thermocline, hence the nutricline, in the entire gulf is markedly shoaled by the southwest monsoon during summer and fall. Under this condition, cyclonic eddies in the gulf can effectively pump deep nutrients to the surface layer and lead to the chlorophyll bloom in late summer, and, after the transition to the northeast monsoon in fall, coastal upwelling driven by the northeasterly winds produces a pronounced increase in surface chlorophyll concentrations along the Somali coast.     

Long-term changes in Krill biomass and distribution in the Barents Sea: are the changes mainly related to capelin stock size and temperature conditions?

Krill plays a significant role in the Barents Sea ecosystem, providing energy transport between different trophic levels. The current paper presents the results of a long-term study (1980–2009) based on pelagic trawl catches from August to September. Our investigations show that the krill species were distributed widely in the Barents Sea and that the largest krill concentrations were restricted to the west-central and eastern parts of the Barents Sea. The current paper presents the relative biomass indices, and the estimates must be interpreted as minimum biomass. The mean annual krill biomass was estimated to be 22 million tonnes in wet weight, with the highest values being as much as 48 million tonnes. Capelin is the largest pelagic stock, and in some years, their biomass can amount to 4–7 million tonnes, which can impose high predation pressure on krill. When their biomass is high, capelin may consume close to 26 million tonnes annually. The predation from pelagic (herring and blue whiting) and bottom (cod and haddock) fish species was much lower, being 9 and 1 million tonnes, respectively. A negative relationship between krill biomass and capelin stock size above 74°N was observed during the study period. However, during the last decade, the krill biomass has increased despite heavy predation from capelin in some years. A positive significant linear relationship between the mean annual Kola temperature and the krill biomass seems to indicate that the recent warming conditions have favourable impacts on the krill populations in the Barents Sea.     

Environmental drivers of mussels flesh yield in a coastal upwelling system

Eastern boundary coastal upwelling ecosystems (EBUEs) are highly sensitive to climate variability, particularly to coastal wind change. Here, we test the response of the flesh yield of blue mussels cultured in the northern boundary of the Iberian–Canary current EBUE to climate-related variables. Significant relationships were found between the annual mean, seasonal build-up and phenology of the mussel flesh yield with meteorological variables such as continental runoff, intensity and direction of coastal winds, and solar radiation. Our analysis shows that better flesh yields occur during years characterised by dry winters, accompanied by early springs and followed by summers dominated by strong northerly winds that produce intense upwelling. Compared with other EBUEs, upwelling has weakened in the study area over the last fifty years, implying an overall decrease in mussel flesh yield. However, future climate scenarios suggest that coastal upwelling will intensify over the 21th century, particularly during the summer months, which would lead to a recovery of mussel flesh yield.     

The contribution of single and colonial cells of Phaeocystis pouchetii to spring and summer blooms in the north-eastern North Atlantic

Studies of the phytoplankton ecology in different localities in north-Norwegian fjords, the White Sea and the Barents Sea were carried out in spring and early summer to investigate the contribution of single and colonial stages of Phaeocystis pouchetii to phytoplankton abundance. Three different types of flagellated and four colonial cells were observed in all localities. P. pouchetii was rare under the ice of the Barents and White Seas, but their abundance increased rapidly during ice retreat. Single cell C dominated over colonial cell C, often by 50 times or more. The highest share of colonial cells was encountered in April in northern Norwegian fjords, in May in the Barents Sea and in May–June in the White Sea. At times the single cell dominated the total P. pouchetii biomass in Balsfjord (April 1999, 2001) with hardly any colonies present. In the White Sea colonies of P. pouchetii were less abundant than in the other regions. Cell carbon of P. pouchetii colonies appears never to be as dominating in the north-eastern North Atlantic as P. globosa blooms in coastal regions such as the southern North Sea. However, the lobal matrix of P. pouchetii colonies appears to be less solid than that of P. globosa and partly dissolution of the colony matrix during handling and storage of fixes samples induces uncertainty about the absolute numbers of P. pouchetii colonial cell counts. Despite of that, single cells of P. pouchetii seem to dominate significantly over colonial cell biomass at most sites and during some years and in some regions colonial cells seem rare. We speculate that top-down regulation of Phaeocystis spp. blooms possibly determines the ratio between single and colonial cells.     

An analysis of allozyme variation in herring <Emphasis Type="Italic">Clupea pallasii</Emphasis> from the White and Barents Seas

An analysis of genetic variation is made according to four allozyme loci of reproductive (spawning) groups of oligovertebrate herring Clupea pallasii collected in various bays of the White Sea and the south-eastern part of the Barents Sea in 1995–2002. The temporal stability of genetic characteristics during several years is shown. The analysis of genetic variation revealed a significant difference between herring from the south-eastern part of the Barents Sea and spring-spawning and summer-spawning herring from inner bays of the White Sea. The analysis of geographic variation of genetic characteristics of spawning aggregations revealed the change in frequencies of alleles of loci LDH-2* and MDH-4* from the north-east to the south-west along the coast.