The distribution and biology of the cestode Eubothrium parvum in capelin, Mallotus villosus, (Pallas) in the Barents Sea, and its use as a biological tag

Samples of Eubothrium parvum were obtained from capelin Mallotus villosus at 55 stations throughout the Barents Sea and from Balsfjord, North Norway. The parasite is distributed widely throughout the Barents Sea, but both incidence and intensity of infection are higher in the regions off Murmansk and the Kola peninsula, and Spitsbergen. E. parvum exhibits a seasonal peak in maturation and probably also in acquisition of new infections. The incidence of infection is greatest in 1 + fish, whereas the intensity is more independent of host age. It is suggested that the parasite requires only a single intermediate host, a plank-tonic copepod, and its distribution in relation to age of host is a reflection of the dietary preference shown by young capelin for copepods. The frequency distribution of E. parvum in capelin was over-dispersed in Balsfjord, where infection levels of between 1 and 28 parasites per fish were encountered in all samples, but under-dispersed in the Barents Sea, where infections of more than four parasites per fish were never found and even infections with three and four parasites were very local. It is suggested that the underdispersion is due to a very low probability of infection in the open waters of the sea. Although the presence of E. parvum cannot be used as a biological tag for capelin, its abundance and frequency distribution can. The difference in frequency distribution and the failure to find any heavily infected fish in the Barents Sea confirm the suggestion that the capelin of Balsfjord form a local isolated population, which does not migrate into the Barents Sea. The differences in infection levels within the Barents Sea suggest the further possibility that there are at least two stocks of capelin there, but this requires further investigation and confirmation.     

Climate and cyclic hydrobiological changes of the Barents Sea from the twentieth to twenty-first centuries

The Barents Sea is a transition zone between North Atlantic and Arctic waters, so its marine ecosystem is highly sensitive to climate dynamics. Understanding of marine biota response to climate changes is necessary to assess the environmental stability and the state of marketable biological resources. These processes are analyzed using a database from the Murmansk Marine Biological Institute which holds oceanographic and hydrobiological data sets collected for more than 100?years along the meridional Kola Transect in the Barents Sea. The data demonstrate high variability in thermal state of the upper layer of the Barents Sea, which is regulated by varying the inflow of Atlantic water and by regional climate. At irregular intervals, cold periods with extended seasonal ice cover are followed by warm periods. The most recent warm period started in the late 1980s and reached its maximum from 2001 to 2006. These cyclic changes in hydrologic regime across the twentieth century and first decade of the twenty-first century are reflected (with a specific lag of 1–5?years) by changes in species composition, as well as abundance and distribution of boreal and arctic groups of macrozoobenthos and fish fauna. For instance, cod and cod fisheries in the Barents Sea are closely linked to the marine climate. Furthermore, Kamchatka crab stock recruitment benefited from the warm climate of 1989 and 1990. In general, studies in this region have shown that climatic dynamics may be assessed using biological indices of abundance, biomass, and migration of marine organisms, including commercial species.     

The occurrence of some metazoan parasites of Atlantic cod, Gadus morhua L., in relation to age and sex of the host in Balsfjord (70°N), North Norway

Age and sex differences in the occurrence of some metazoan parasites were investigated in 243 cod (Gadus morhua L.) from Balsfjord, a subarctic fjord in northern Norway. Thirteen species of parasite were studied, comprising seven nematodes, three parasitic copepods, one acanthocephalan, one cestode and one digenean. The nematode Anisakis simplex showed significant increases in both prevalence and mean abundance with host age, while the digenean Hemiurus levinseni showed a significant increase in mean abundance with age. The increases probably result from a long parasite life-span for A. simplex and changes in the feeding pattern of cod with age for H. levinseni. Prevalence of the nematode Contracaecum sp. and mean abundance of the acanthocephalan Echinorhynchus gadi were significantly higher in male than in female cod, whereas mean abundance of H. levinseni was significantly higher in female cod. These results suggest that there may be differences in feeding behaviour between male and female cod in Balsfjord. Accepted: 14 November 1999     

Search for seasonality in occurrence of parasites of cod,Gadus morhua L. in a fjord at 70°N

Samples of cod from the subarctic fjord, Balsfjord, in Norway, were collected in spring, summer and autumn from spring 1986 to spring 1987. The macroparasite fauna was investigated for seasonality in prevalence, mean intensity and abundance. Anisakis simplex had maximum mean intensity and abundance in the autumn. Echinorhynchus gadi had minimum prevalence, mean intensity and abundance in the autumn, but this was not statistically significant. None of the remaining 11 species of parasites demonstrated seasonality according to our definition.     

Demersal fish assemblages and spatial diversity patterns in the Arctic-Atlantic transition zone in the Barents Sea

Direct and indirect effects of global warming are expected to be pronounced and fast in the Arctic, impacting terrestrial, freshwater and marine ecosystems. The Barents Sea is a high latitude shelf Sea and a boundary area between arctic and boreal faunas. These faunas are likely to respond differently to changes in climate. In addition, the Barents Sea is highly impacted by fisheries and other human activities. This strong human presence places great demands on scientific investigation and advisory capacity. In order to identify basic community structures against which future climate related or other human induced changes could be evaluated, we analyzed species composition and diversity of demersal fish in the Barents Sea. We found six main assemblages that were separated along depth and temperature gradients. There are indications that climate driven changes have already taken place, since boreal species were found in large parts of the Barents Sea shelf, including also the northern Arctic area. When modelling diversity as a function of depth and temperature, we found that two of the assemblages in the eastern Barents Sea showed lower diversity than expected from their depth and temperature. This is probably caused by low habitat complexity and the distance to the pool of boreal species in the western Barents Sea. In contrast coastal assemblages in south western Barents Sea and along Novaya Zemlya archipelago in the Eastern Barents Sea can be described as diversity "hotspots"; the South-western area had high density of species, abundance and biomass, and here some species have their northern distribution limit, whereas the Novaya Zemlya area has unique fauna of Arctic, coastal demersal fish. (see Information S1 for abstract in Russian).     

DNA HETEROZYGOSITY AND GROWTH RATE IN THE ATLANTIC COD GADUS MORHUA (L)

Relationships between growth rate and the degree of individual heterozygosity at ten nuclear RFLP loci were examined in two populations of the Atlantic cod, Gadus morhua, from northern Norway. A highly significant positive correlation was observed between growth rate and DNA heterozygosity in one population (Balsfjord) but not in the other (Barents Sea). Our results provide support for an important prediction of the associative overdominance hypothesis that heterozygosity-fitness correlations can be detected at neutral genetic markers and suggest that environmental conditions might play a dominant role in the manifestation of the correlation.     

Distribution and diet of 0-group cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) and haddock (<Emphasis Type="Italic">Melanogrammus aeglefinus</Emphasis>) in the Barents Sea in relation to food availability and temperature

Distribution of 0-group cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) in August–September 2005 and 2006 was mainly restricted to the Atlantic waters of the western and central areas of the Barents Sea. The main distribution of 0-group fish overlapped largely with areas of high biomass (>7 gm⁻² dry weight) of zooplankton. The copepod Calanus finmarchicus and krill Thysanoessa inermis, which are dominant zooplankton species in both Atlantic and boreal waters of the Barents Sea, were the main prey of 0-group cod and haddock. The main distribution, feeding areas and prey of 0-group cod and haddock overlapped, implying that competition for food may occur between the two species. However, though their diet coincided to a certain degree, haddock seems to prefer smaller and less mobile prey, such as Limacina and appendicularians. As 0-group fish increased in size, there seems to be a shift in diet, from small copepods and towards larger prey such as krill and fish. Overall, a largely pelagic feeding behaviour of 0-group cod and haddock was evident from this study.     

Disentangling structural genomic and behavioural barriers in a sea of connectivity

Genetic divergence among populations arises through natural selection or drift and is counteracted by connectivity and gene flow. In sympatric populations, isolating mechanisms are thus needed to limit the homogenizing effects of gene flow to allow for adaptation and speciation. Chromosomal inversions act as an important mechanism maintaining isolating barriers, yet their role in sympatric populations and divergence with gene flow is not entirely understood. Here, we revisit the question of whether inversions play a role in the divergence of connected populations of the marine fish Atlantic cod (Gadus morhua), by exploring a unique data set combining whole‐genome sequencing data and behavioural data obtained with acoustic telemetry. Within a confined fjord environment, we find three genetically differentiated Atlantic cod types belonging to the oceanic North Sea population, the western Baltic population and a local fjord‐type cod. Continuous behavioural tracking over 4 year revealed temporally stable sympatry of these types within the fjord. Despite overall weak genetic differentiation consistent with high levels of gene flow, we detected significant frequency shifts of three previously identified inversions, indicating an adaptive barrier to gene flow. In addition, behavioural data indicated that North Sea cod and individuals homozygous for the LG12 inversion had lower fitness in the fjord environment. However, North Sea and fjord‐type cod also occupy different depths, possibly contributing to prezygotic reproductive isolation and representing a behavioural barrier to gene flow. Our results provide the first insights into a complex interplay of genomic and behavioural isolating barriers in Atlantic cod and establish a new model system towards an understanding of the role of genomic structural variants in adaptation and diversification.     

Species composition and structure of the ichthyofauna of the Barents Sea

The species composition and brief characteristic of some elements of structure of the ichthyofauna of the Barents Sea within its geographic boundaries are represented. During the whole historic period of observations in the Barents Sea, 182 species and subspecies of fish were recorded, belonging to 59 families, 28 orders, and 5 classes. Most species and subspecies belong to the boreal complex (59.3%), occur principally in the bottom layers (56.6%), more than a half feed on bottom and demersal invertebrates (52.2%), and are commercial species (52.7%). In the Barents Sea, 21 species and subspecies are commercial. Their ration in catches depends on the integral impact of natural and anthropogenous factors. In the arctic zone of the Barents Sea, the part of noncommercial species makes by biomass 1.18%; in the boreal zone—0.26%; in the Pechora Sea—10.6%.     

Mitochondrial DNA polymorphism of atlantic cod of the Barents and White seas

High level of intraspecific polymorphism of the mtDNA cytochrome b gene in the Atlantic cod Gadus morhua L. from the northeastern part of the species range (Barents and White seas) is detected. The absence of genetic differentiation between the samples from different areas of the Norwegian and Barents seas is shown, and the fact that these samples belong to the same population of Northeast Arctic cod is confirmed. Significant differences in the frequences of mitochondrial DNA haplotypes between the cod from the Barents and White seas, as well as from the northwestern part of the range, are found. The cod Gadus morhua kildinensis Derjugin, 1920 from Mogilnoye Lake and Gadus morhua marisalbi Derjugin, 1920 from the White Sea were revealed to have common haplotypes with the cod from the Barents Sea, confirming the recent origin of these forms from the Atlantic cod G. morhua.     

Population genetic substructure in blue whiting based on allozyme data

The genetic population structure of the pelagic gadoid blue whiting Micromesistius poutassou throughout its east Atlantic distribution range was explored using polymorphisms at the tissue enzyme loci IDHP-2 * and PGM-1 *. The study included 5025 individuals from 65 locations in the east Atlantic and the Mediterranean. Significant geographic heterogeneity in allele frequencies was demonstrated at both loci ( IDHP-2 *: P=0·030, PGM-1 *: P=0·005). The degree of genetic differentiation ( F _ST= 1·1%) in blue whiting appears to be at the same level as for the demersal gadoids cod and haddock. Several separate reproductive units were indicated at the fringes of the distribution range, i.e. in the Barents Sea, and in one Norwegian fjord (Romsdalsfjord).     

Early Environmental Influences on Growth of Arcto-Norwegian Cod, Gadus Morhua, From The 0-group To Adults

A high growth rate for Arcto-Norwegian cod, Gadus morhua, in the Barents Sea and adjacent areas from the larva period to the 0-group enhances survival and ultimately recruitment to the fishery. However, it appeared that high growth rates for a cohort through the 0-group were not continued as the cohort ages. Based on survey data, there was a significant negative correlation between the average length at the 0-group and its average length at ages 2 through 8. We provided evidence suggesting that this phenomenon was caused by the inter-annual variability in inflow of warm, prey-rich Atlantic water into the Barents Sea from the Norwegian Sea. Enhanced inflow provided favorable conditions for cod growth during the larva and juvenile pelagic intervals. However, this same strong inflow carried a proportion of the cohort farther to the east in the Barents Sea, where the bottom water is colder than in the west. The colder conditions experienced by such cohorts, as compared to cohorts that have a more westerly settlement, led to slower growth prior to age 2. Slow growth during this interval appeared to be the reason for these cohorts' relatively smaller mean length at older ages.     

Parasites as Biological Tags for Stock Discrimination of Beaked Redfish (Sebastes mentella): Parasite Infra-Communities vs. Limited Resolution of Cytochrome Markers

The use of parasites as biological tags for discrimination of fish stocks has become a commonly used approach in fisheries management. Metazoan parasite community analysis and anisakid nematode population genetics based on a mitochondrial cytochrome marker were applied in order to assess the usefulness of the two parasitological methods for stock discrimination of beaked redfish Sebastes mentella of three fishing grounds in the North East Atlantic. Multivariate, model-based approaches demonstrated that the metazoan parasite fauna of beaked redfish from East Greenland differed from Tampen, northern North Sea, and Bear Island, Barents Sea. A joint model (latent variable model) was used to estimate the effects of covariates on parasite species and identified four parasite species as main source of differences among fishing grounds; namely Chondracanthus nodosus, Anisakis simplex s.s., Hysterothylacium aduncum, and Bothriocephalus scorpii. Due to its high abundance and differences between fishing grounds, Anisakis simplex s.s. was considered as a major biological tag for host stock differentiation. Whilst the sole examination of Anisakis simplex s.s. on a population genetic level is only of limited use, anisakid nematodes (in particular, A. simplex s.s.) can serve as biological tags on a parasite community level. This study confirmed the use of multivariate analyses as a tool to evaluate parasite infra-communities and to identify parasite species that might serve as biological tags. The present study suggests that S. mentella in the northern North Sea and Barents Sea is not sub-structured.     

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.     

Ecology of Arctic cod <Emphasis Type="Italic">Boreogadus saida</Emphasis> (Gadiformes,Gadidae) and its fishery potential in Kara Sea

According to the bottom trawl-survey data, 97% of the ichthyomass in the southwestern region of the Kara Sea are composed of the Arctic cod Boreogadus saida; its stock is significantly higher than the previously registered resources. The Arctic cod is most unevenly distributed across the water area and capable to form the high-density aggregations, which can be caught by the targeted trawls. A wide range of the age composition (0+?6+), the size-age composition, and the growth rates of the Arctic cod in the trawl catches in the Kara Sea, which are different from those in the fish in the adjacent Barents Sea, can indicate their assignment of the Arctic cod in these seas to different populations.     

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.     

Comparative analysis of genetic variability of white sea cod (Gadus morhua marisalbi) at allozyme and microsatellite markers

Variability of cod spawning and feeding schools from Kandalaksha Bay of the White Sea, was examined at six allozyme and eight microsatellite loci. The degree of genetic differentiation at allozyme loci constituted θ = 0.36% [95% bootstrap interval 0.0458; 0.6743]. The differentiation estimates obtained using microsatellite markers were higher, θ = 1.33% [0.057; 3.11]. It was demonstrated that the level of genetic diversity in the White Sea cod was lower than that established for the Atlantic cod from Barents Sea using the same set of allozyme and microsatellite markers. The genetic data obtained support the opinion that the White Sea cod is a reproductively independent group formed as a result of the Holocene dispersal of Atlantic cod.     

Resource‐driven colonization by cod in a high Arctic food web

1. Climate change is commonly associated with many species redistributions and the influence of other factors may be marginalized, especially in the rapidly warming Arctic.
2. The Barents Sea, a high latitude large marine ecosystem in the Northeast Atlantic has experienced above‐average temperatures since the mid‐2000s with divergent bottom temperature trends at subregional scales.
3. Concurrently, the Barents Sea stock of Atlantic cod Gadus morhua, one of the most important commercial fish stocks in the world, increased following a large reduction in fishing pressure and expanded north of 80°N.
4. We examined the influence of food availability and temperature on cod expansion using a comprehensive data set on cod stomach fullness stratified by subregions characterized by divergent temperature trends. We then tested whether food availability, as indexed by cod stomach fullness, played a role in cod expansion in subregions that were warming, cooling, or showed no trend.
5. The greatest increase in cod occupancy occurred in three northern subregions with contrasting temperature trends. Cod apparently benefited from initial high food availability in these regions that previously had few large‐bodied fish predators.
6. The stomach fullness in the northern subregions declined rapidly after a few years of high cod abundance, suggesting that the arrival of cod caused a top‐down effect on the prey base. Prolonged cod residency in the northern Barents Sea is, therefore, not a certainty.

     

Ecological and genetic impact of Atlantic cod larval drift in the Skagerrak

We evaluate the hypothesis that Atlantic cod larvae are passively transported by sea currents from off-shore spawning areas to settle in coastal waters, a hypothesis which has recently gained support from genetic analysis of cod in the North Sea-Skagerrak area. Such larval transport has been suggested to be an important mechanism behind the commonly observed low spatial genetic differentiation in many marine organisms. Here, we apply an ARMAX(2,2) model for juvenile abundance and use long-term monitoring data from the Skagerrak coast, constituting 54 continuous annual series from 1945 to 1997. Analysing the model, we find that the product of the size of the North Sea breeding stock and the strength of the net inflow of North Sea waters had a significant, positive effect on the abundance of coastal juvenile cod. The peak effect occurs during the month of March, just after spawning, when eggs and larvae remain pelagic and sensitive to currents. In contrast, we find no evidence of any direct effect of the North Sea spawning stock alone. Our analyses indicate that 15-20,000 0-group larvae from the North Sea reach each fjord per year, on average. This corresponds to about 1-10% of the total 0-group population in each fjord on average. These findings clearly demonstrate a direct link between larval drift and gene flow in the marine environment.