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.     

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.     

Long-term variations of biotic and abiotic conditions in the ecosystem of the Barents Sea

Synchronism of year-class strength was noted for the majority of commercial fish in the Barents Sea. The reason for this is probably connected with a common factor, namely the intensity of water inflow which influences spawning efficiency, zooplankton food production, fish larval drift to the feeding grounds, and consequent survival of juvenile fish. Consequently, the established regular relationships of hydrological and weather processes in the ecosystem can serve as basis for long-term fishing forecasts. The overall pattern of the Barents Sea water circulation, long-term climatic changes in this region, and their effect on the fish stock reproduction are considered, using Arctic cod as an example.     

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.     

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.     

Effect of size-selective mortality on growth of coastal cod illustrated by tagging data and an individual-based growth and mortality model

In a release experiment with cod in Norway, the apparent mean growth rates of 3+ cod, calculated by sampling the released cohorts at different ages, were very slow (<0·08 mm day⁻¹). However, when individual growth rates of individual tagged cod of the same size range were measured, the mean growth rates were much faster (0·24 mm day⁻¹). These observations were attributed to size-selective fishing mortality and were illustrated by an individual based simulation model of a cohort of cod with variable individual growth rates. The effects on mean length at age of the surviving cohort of increasing fishing intensity were demonstrated. The model showed that size-selective fishing with the observed individual growth variation, removed the fastest-growing individuals at proportionally higher rates than the slower-growing ones, leading to decreased apparent mean growth rate. The fishing pattern which gave the optimum yield, changed when individual variation was included, and when the apparent growth rate was used in the model the yield per recruit reduced dramatically. This study has shown that individual growth heterogeneity and size-selective mortality are factors which should be considered in future fisheries management models.     

Biomass of scyphozoan jellyfish, and its spatial association with 0-group fish in the Barents Sea

An 0-group fish survey is conducted annually in the Barents Sea in order to estimate fish population abundance. Data on jellyfish by-catch have been recorded since 1980, although this dataset has never been analysed. In recent years, however, the ecological importance of jellyfish medusae has become widely recognized. In this paper the biomass of jellyfish (medusae) in 0–60 m depths is calculated for the period 1980–2010. During this period the climate changed from cold to warm, and changes in zooplankton and fish distribution and abundance were observed. This paper discusses the less well known ecosystem component; jellyfish medusae within the Phylum Cnidaria, and their spatial and temporal variation. The long term average was ca. 9×10⁸ kg, with some years showing biomasses in excess of 5×10⁹ kg. The biomasses were low during 1980s, increased during 1990s, and were highest in early 2000s with a subsequent decline. The bulk of the jellyfish were observed in the central parts of the Barents Sea, which is a core area for most 0-group fishes. Jellyfish were associated with haddock in the western area, with haddock and herring in the central and coastal area, and with capelin in the northern area of the Barents Sea. The jellyfish were present in the temperature interval 1°C<T<10°C, with peak densities at ca. 5.5°C, and the greatest proportion of the jellyfish occurring between 4.0–7.0°C. It seems that the ongoing warming trend may be favourable for Barents Sea jellyfish medusae; however their biomass has showed a recent moderate decline during years with record high temperatures in the Barents Sea. Jellyfish are undoubtedly an important component of the Barents Sea ecosystem, and the data presented here represent the best summary of jellyfish biomass and distribution yet published for the region.     

Diet of juvenile cod (age 0–2) in the Barents Sea in relation to food availability and cod growth

Diet investigations were carried out on 0-, 1- and 2-year-old Northeast Arctic cod (Gadus morhua) sampled in the Barents Sea during 1984–2002. Stomach-content analyses showed that the 0 and 1 group cod fed mainly on crustaceans, with krill and amphipods composing up to 70% of their diet. Krill (Thysanoessa spp. and Meganyctiphanes norvegica) and amphipods (Themisto spp.) were mainly found in cod stomachs sampled in the central and close to the Polar Front region in the Barents Sea where these prey organisms are reported to be abundant in summer. A shift in the main diet from crustaceans to fish was observed from age 1 to age 2. The diet of 2-year-old cod mainly comprised capelin (Mallotus villosus) and other fish, and to a lesser degree, krill and amphipods. Shrimp (mainly Pandalus spp.) was also an important prey in both age 1 and 2 cod. A statistically significant positive relationship was obtained between capelin stock size and the amount of capelin in the diet of 2-year-old cod. Results from this study also show that the larger age-2 cod preyed more on capelin in winter and that larger cod (>22 cm) prefer larger capelin (>12 cm). During periods of low capelin abundance, the 2-year-old cod shift their diet more to crustaceans, such as krill and amphipods. A positive significant relationship was also obtained between Total Fullness Index (TFI) and the amount of capelin in the diet and between TFI and the growth of 2-year-old cod, indicating that the growth of age-2 cod is to a large extent dependent on the amount of capelin consumed. Growth of age-1 cod was also positively correlated to TFI.     

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.     

Growth rates of young-of-year shovelnose sturgeon in the Upper Missouri River

Information on growth during the larval and young‐of‐year life stages in natural river environments is generally lacking for most sturgeon species. In this study, methods for estimating ages and quantifying growth were developed for field‐sampled larval and young‐of‐year shovelnose sturgeon Scaphirhynchus platorynchus in the upper Missouri River. First, growth was assessed by partitioning samples of young‐of‐year shovelnose sturgeon into cohorts, and regressing weekly increases in cohort mean length on sampling date. This method quantified relative growth because ages of the cohorts were unknown. Cohort increases in mean length among sampling dates were positively related (P < 0.05, r² > 0.59 for all cohorts) to sampling date, and yielded growth rate estimates of 0.80–2.95 mm day⁻¹ (2003) and 0.44–2.28 mm day⁻¹ (2004). Highest growth rates occurred in the largest (and earliest spawned) cohorts. Second, a method was developed to estimate cohort hatch dates, thus age on date of sampling could be determined. This method included quantification of post‐hatch length increases as a function of water temperature (growth capacity; mm per thermal unit, mm TU⁻¹), and summation of mean daily water temperatures to achieve the required number of thermal units that corresponded to post‐hatch lengths of shovelnose sturgeon on sampling dates. For six of seven cohorts of shovelnose sturgeon analyzed, linear growth models (r² ≥ 0.65, P < 0.0001) or Gompertz growth models (r² ≥ 0.83, P < 0.0001) quantified length‐at‐age from hatch through 55 days post‐hatch (98–100 mm). Comparisons of length‐at‐age derived from the growth models indicated that length‐at‐age was greater for the earlier‐hatched cohorts than later‐hatched cohorts. Estimated hatch dates for different cohorts were corroborated based on the dates that newly‐hatched larval shovelnose sturgeon were sampled in the drift. These results provide the first quantification of growth dynamics for field‐sampled age‐0 shovelnose sturgeon in a natural river environment, and provide an accurate method for estimating age of wild‐caught individuals. Methods of age determination used in this study have applications to sturgeons in other regions, but require additional testing and validation.     

The trout (Salmo trutta) population of the Afon Cwm, a small tributary of the Afon Dyfi, mid-Wales

Data are presented from a 10-year (1984 to 1993) study of a Salmo trutta population in the Afon Cwm, a small tributary of the Afon Dyfi, mid-Wales. The stream is a spawning and nursery area for sea trout. Growth of trout within the stream can be summarized by a von Bertalanffy growth coefficient ( K ) of 0·310, with asymptotic length (1∞) 21·6 cm and with length at age 1 of 7·6 cm. Mean population density in the whole stream varied from year to year between 0·05 and 0·60 0-group trout m⁻² and between 0·05 and 0·70 older trout m⁻². Mean biomass varied, between years, from 0·1 to 3·5 g m⁻² for 0-group and from 1·3 to 10·4g m⁻² for older trout. Loss between 3 and 5 months of age appeared to be proportionate at about 50 to 60% and instantaneous loss rate from 5 to 53 months of age varied from 0·04 to 0·10 month⁻¹ and was positively correlated with cohort number at 3 months of age. Production between 3 and 53 months of age varied between cohorts from 3 to 8 g m ⁻² live weight.     

Feeding habits of harp seals (Phoca groenlandica) during early summer and autumn in the northern Barents Sea

The feeding habits of harp seals (Phoca groenlandica) in the Barents Sea were examined in studies conducted during June 1991, September 1990 and 1991, and October 1992. Analyses of stomach and intestinal contents were carried out and concurrent estimates of prey abundance were made using trawl gear. Harp seals appeared to feed at low intensity in the pack ice belt during the first half of June. There was little potential prey in the water column, but prawns (Pandalus borealis), capelin (Mallotus villosus) and polar cod (Boreogadus saida) were abundant close to the bottom. In September, the seals sampled in the northern pack ice areas of the Barents Sea fed on the pelagic amphipod Parathemisto libellula, krill (Thysanoessa spp.), prawns and, to a lesser extent, on fish species such as polar cod, sculpins (Cottidae) and snailfish (Liparidae). Trawling revealed that large quantities of Parathemisto libellala were present in the upper layers of the water column. Fish, mainly capelin and polar cod, were less abundant and occurred in deeper waters. In mid-October, the diet of seals in the northern Barents Sea consisted mainly of amphipods (Parathemisto sp.). Later in October, when increasing pack ice cover forced the harp seals to move south, the diet seemed to change from amphipods to fish prey, predominantly capelin and polar cod.     

Distinction among North Atlantic cod Gadus morhua stocks by tissue fatty acid profiles

The fatty acid (FA) profiles of the white muscle and heart tissues of cod Gadus morhua from five locations, Faroe Bank, Faroe Plateau, North‐West Iceland, Norway–Barents Sea and Denmark–Skagerrak, were population dependent. The interregional differences of FAs were significantly dissimilar (P < 0·01) in most cases. By way of a rapid and simple analytical method, the stock dependence and harvest location of individual G. morhua were chemometrically determined by multivariate principal component analysis. The difference among the stocks was correlated with the average water temperature at the harvest locations. It thus appears that the tissue FA profile is a phenotypic trait that is partly temperature driven.     

Seasonal distribution and dive behaviour of harp seals (<Emphasis Type="Italic">Pagophilus groenlandicus</Emphasis>) of the White Sea–Barents Sea stock

Eight adult female harp seals (Pagophilus groenlandicus) of the White Sea–Barents Sea stock were tagged with satellite-linked dive recorders during the nursing period and followed from breeding in late February 1995 until moulting in late April 1995. Another ten adult harp seals of both sexes were tagged and followed from moult in early May 1996 until breeding in late February the following year. Between breeding and moult the seals were distributed along the coasts of Kola of Russia and eastern Finnmark of Norway, coinciding in time and space with the spawning capelin (Mallotus villosus). Between moulting and breeding they encircled the entire Barents Sea, mostly in open water, using the water column from 20 to 300 m, and in so doing by and large reflecting the annual migrations of the capelin. Capelin is therefore assumed to be the main source of prey for the White Sea–Barents Sea stock of harp seals, to be substituted, in part, by amphipods (e.g. Themisto libellula) in mid-summer and polar cod (Boreogadus saida) and herring (Clupea pallasii) in late autumn and winter. These data provide a baseline for the evaluation of the effects of future climatic change in the rich Barents Sea ecosystem.     

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.

     

Structure,biomass distribution,and energetics of the pelagic ecosystem in the Barents Sea: A synopsis

Biomass distribution and energetics of trophic levels in the pelagic ecosystem of the Barents Sea are presented as averages over several years for the whole Barents Sea using data from the research programme Pro Mare in 1984–1989 and mathematical ecosystem models. Average biomasses range from more than 3 tonnes carbon km^–2 (zooplankton) to 0.1 kg C km^–2 (polar bears) and P/B ratios from 300 (bacteria) to 0.035 (minke whales). However, the Barents Sea ecosystem is in a far from steady state with, for instance, capelin stocks ranging from 30–700 kg C km^–2 between years and cod stocks from 150–700 kg C km^–2. As a general rule, the various fish stocks grow adequately, albeit at different rates, in warm years characterized by large influxes of Atlantic water and high zooplankton productivity. The skewed populations distribution which arises in warm years may lead to grave imbalances in cold years and even to the collapses of stocks, such as of capelin in the eighties. The food requirements of average-sized stocks of cod, seabirds and marine mammals correspond to more than twice the average productivity of capelin. Thus other species of pelagic fish (herring, polar cod) and zooplankton obviously play major roles as prey for these animals.     

Thermal growth potential of Atlantic cod by the end of the 21st century

Ocean warming may lead to smaller body sizes of marine ectotherms, because metabolic rates increase exponentially with temperature while the capacity of the cardiorespiratory system to match enhanced oxygen demands is limited. Here, we explore the impact of rising sea water temperatures on Atlantic cod (Gadus morhua), an economically important fish species. We focus on changes in the temperature‐dependent growth potential by a transfer function model combining growth observations with climate model ensemble temperatures. Growth potential is expressed in terms of asymptotic body weight and depends on water temperature. We consider changes between the periods 1985–2004 and 2081–2100, assuming that future sea water temperatures will evolve according to climate projections for IPCC AR5 scenario RCP8.5. Our model projects a response of Atlantic cod to future warming, differentiated according to ocean regions, leading to increases of asymptotic weight in the Barents Sea, while weights are projected to decline at the southern margin of the biogeographic range. Southern spawning areas will disappear due to thermal limitation of spawning stages. These projections match the currently observed biogeographic shifts and the temperature‐ and oxygen‐dependent decline in routine aerobic scope at southern distribution limits.     

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.     

Atlantic cod <Emphasis Type="Italic">Gadus morhua</Emphasis> (Gadiformes: Gadidae) from the Gulf of Ura: Morphobiological characteristic

Analysis of variation of biological and morphometric characteristics of cod Gadus morhua from the Gulf of Ura-a water area in the composition of Motovskii Bay of the Barents Sea was performed. The material was collected from 1999 to 2006 during spawning of the Atlantic cod. Parameters such as length, weight, sex composition, maturity stage, age, otolith structure, and variation of plastic characters were analyzed. It was shown that the Gulf of Ura, parallel to individuals of junior age groups, is inhabited by cod in the spawning state aged 5–6 months and older with coastal and Atlantic types of otolith.