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.     

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.     

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.     

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.     

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.     

Temporal Dynamics of Top Predators Interactions in the Barents Sea

The Barents Sea system is often depicted as a simple food web in terms of number of dominant feeding links. The most conspicuous feeding link is between the Northeast Arctic cod Gadus morhua, the world''s largest cod stock which is presently at a historical high level, and capelin Mallotus villosus. The system also holds diverse seabird and marine mammal communities. Previous diet studies may suggest that these top predators (cod, bird and sea mammals) compete for food particularly with respect to pelagic fish such as capelin and juvenile herring (Clupea harengus), and krill. In this paper we explored the diet of some Barents Sea top predators (cod, Black-legged kittiwake Rissa tridactyla, Common guillemot Uria aalge, and Minke whale Balaenoptera acutorostrata). We developed a GAM modelling approach to analyse the temporal variation diet composition within and between predators, to explore intra- and inter-specific interactions. The GAM models demonstrated that the seabird diet is temperature dependent while the diet of Minke whale and cod is prey dependent; Minke whale and cod diets depend on the abundance of herring and capelin, respectively. There was significant diet overlap between cod and Minke whale, and between kittiwake and guillemot. In general, the diet overlap between predators increased with changes in herring and krill abundances. The diet overlap models developed in this study may help to identify inter-specific interactions and their dynamics that potentially affect the stocks targeted by fisheries.     

Zooplankton biomass variation in relation to climatic conditions in the Barents Sea

Over the last two decades, there have been large changes in the zooplankton biomass in the Barents Sea. These biomass variations are mainly attributed to predation pressure and environmental factors (e.g. advective transport). When stock size of capelin (Mallotus villosus), a major planktivorous fish in the Barents Sea ecosystem, was quite low as in 1986 and 1994, the zooplankton biomass showed marked increase. However, the increase in the zooplankton biomass occurred in different water masses during 1986 and 1994. In 1986, a climatically cold year, the plankton biomass was highest in the Arctic waters of the northeastern Barents Sea. This is probably due to the increase in larger Arctic amphipod species, such as Themisto libellula. In 1994, a climatically warm year, the zooplankton biomass was high in the Atlantic waters of the southwestern Barents Sea. The large increase in zooplankton biomass in the Atlantic waters in 1994 was presumably due to the higher inflow of advected organisms, e.g. Calanus spp., as well as high temperatures, which may lead to high growth rates of zooplankton. Throughout the studied region, the plankton biomass in the "cold year" of 1986 was generally much lower than in the "warm year" of 1994.     

The ability of gadoids to take advantage of a short-term high availability of forage fish: the example of spawning aggregations in Barents Sea capelin

During 11 March to 4 April 2002, the distribution of Barents Sea capelin Mallotus villosus along the coast of Finnmark, northern Norway, was covered four times by combining acoustic survey with trawling, synoptically and simultaneously sampling capelin and its main fish predators; cod Gadus morhua, haddock Melanogrammus aeglefinus and saithe Pollachius virens. The surveys demonstrated how these gadoid predators were able to exploit such a short‐term abundance of forage fish. The predator aggregation as well as the stomach fullness and proportion of capelin in their diet followed the capelin spawning migration, increasing in areas and periods with increasing capelin abundance. Capelin clearly constituted most of the biomass in stomachs of cod (97%), haddock (87%) and saithe (96%). The stomach fullness was highest in cod and lowest in haddock, although in areas with low capelin abundance, saithe had more capelin in their stomachs. The total length (L_T) of capelin in predator stomachs increased with predator L_T, but the proportion of capelin in the diet was not influenced by predator L_T. The capelin in predator stomachs was significantly smaller than capelin in the trawl hauls, also when compared within the same sex, indicating feeding selectivity towards weaker individuals. Female capelin, being significantly smaller than the males, predominated in the diet of haddock, whereas in cod and saithe the sex ratio was more equal. Male capelin predominated in the predator diet during the pre‐spawning period, whereas the females predominated as the spawning commenced. During the overall study period, most of the female capelin in predator stomachs was in a pre‐spawning or a spawning stage, whereas the majority of the males appeared to be spent. Regardless of sex, the percentage of spent, relative to pre‐spawning or spawning capelin in the diet of the predators, followed the capelin spawning dynamics, increasing with time as the spawning progressed.     

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.     

Recruitment prediction for Pacific herring (Clupea pallasi) on the west coast of Vancouver Island, Canada

The accurate prediction of recruitment to the fishery is a very important tool within the management structure of any fish stock being exploited. In the case of the Pacific herring, Clupea pallasi, fishery in Canada, a forecast of the abundance of each herring stock is particularly important for formulating an annual catch quota. The sustainable management of the fishery and the resource is based in part on accurate recruitment forecasting because Pacific herring are short-lived and so the recruitment contributes a significant part of the total spawning run targeted by the fishery each year. Several factors are believed be important in determining the success of recruitment besides spawners biomass. Since herrings are “r” strategists, conditions related to the egg, the planktonic, or even the juvenile stage might determine the future level of recruitment. Recently a formula that defines conditions for a semi-quantitative level of recruitment forecast was elaborated using genetic algorithms and current study attempts to improve on this model. Using salinity in two quarterly periods during the planktonic and pre-recruit stages, temperature and spawning biomass for the west coast of Vancouver Island stock, classification rules that define recruitment in 3 different levels (low, medium and high) were developed with a genetic algorithm, setting low and high boundaries for each condition. A 75% success in classifying recruitment was obtained. The model was shown to be particularly effective at predicting when the recruitment would be low, which could be important from the perspective of the Precautionary Approach and the sustainable management of this stock.     

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.     

Biological features of the common fish species in Olyutorsky-Navarin region and the adjacent waters of the Bering Sea: 2. Families Macrouridae,Clupeidae, and Osmeridae

Biological features of the four common fish species, giant grenadier Albatrossia pectoralis (Macrouridae), Pacific herring Clupea pallasii (Clupeidae), Pacific rainbow smelt Osmerus mordax dentex, and Pacific capelin Mallotus villosus catervarius (Osmeridae), were studied under the 20-year dataset (1995?2015). These species inhabit the northwestern Bering Sea in the summer–autumn period and form the schoolings in the Olyutorsky-Navarin region. The size–age parameters of the fish caught by different sampling gear, as well as the peculiarities of the body length and body weight dynamics, spawning periods, spawning range, and conditions, were analyzed. The largest specimens of giant grenadier, Pacific herring, and Pacific rainbow smelt were observed in the catches performed by the bottom setline and the gill nets; the smallest fish were found in the trawl catches. The body length and body weight of Pacific herring were larger in the pelagic trawls compared to the bottom trawls; an opposite pattern was observed for the Pacific capelin. The abundant year-class in the species with short life cycle (capelin and herring) is well tracked on the longterm plots of the fish body size; this is accompanied by the decrease of their biological parameters. Herring stock covers large growing grounds; smaller body size was observed for the herring grazing in the coastal waters; young specimens dominate here.     

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.     

Food web dynamics affect Northeast Arctic cod recruitment

Proper management of ecosystems requires an understanding of both the species interactions as well as the effect of climate variation. However, a common problem is that the available time-series are of different lengths. Here, we present a general approach for studying the dynamic structure of such interactions. Specifically, we analyse the recruitment of the world's largest cod stock, the Northeast Arctic cod. Studies based on data starting in the 1970-1980s indicate that this stock is affected by temperature through a variety of pathways. However, the value of such studies is somewhat limited by the fact that they are based on a quite specific ecological and climatic situation. Recently, this stock has consisted of fairly young fish and the spawning stock has consisted of relatively few age groups. In this study, we develop a model for the effect of capelin (the cod's main prey) and herring on cod recruitment since 1973. Based on this model, we analyse data on cod, herring and temperature going back to 1921 and find that food-web effects explain a significant part of the cod recruitment variation back to around 1950.     

Local and large-scale climatic variables as predictors of the breeding numbers of endangered Lesser Black-backed Gulls on the Norwegian Coast

In the 1970s and 1980s, the nominate subspecies of the Lesser Black-backed Gull (Larus fuscus fuscus) showed a dramatic drop in breeding numbers on the Norwegian Coast, and in 2000, the population in some colonies was only 10–20% of the population in 1980. This decline has been attributed to the collapse in the stock of Norwegian spring spawning herring (Clupea harengus). In this study, we examined whether local climate (sea and air temperatures), winter NAO (North Atlantic Oscilliation), and the year-class strength and size of 0-group herring could predict the relative changes in breeding numbers between years, mainly after the population collapse. Breeding birds were counted in 19 of the years between 1980 and 2007 in an archipelago on the coast of Helgeland, northern Norway. The best model predicting changes in breeding numbers for the period between 1980 and 2005 (for which data on 0-group herring was available) included mean local air temperature in winter (January–March) and winter NAO, explaining 57% of the variation between years, while the other factors had little effect. When also adding the years 2006–2007 (no herring data), the best model included only mean air temperature in winter, explaining 41% of the variation. In conclusion, the high positive correlation between breeding numbers and climatic factors probably resulted from a higher availability of important fish prey after mild winters, for which 0-group herring presently may only account for a limited proportion. However, this prey might have been of much more importance prior to the population decline.     

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.     

Tendencies in the Herring Population Development of the Baltic Sea

During the 1960's a change in population structure of Baltic herring started which resulted in the dominance of spring spawners in the entire Baltic since the beginning 1970's. Autumn spawning herring is very rare in the yields of fisheries since. This development has been accompanied by a likely increase of the total stock biomass of herring in the area. Yields of herring fisheries increased remarkable up to 1984, partly as a result of increased fishing effort. Recent developments of stock biomasses point to dependencies on fluctuations of growth rates. Growth is influenced by several environmental factors but is very likely especially dependent on abundance of food and on temperature. Eutrophication of the Baltic Sea increased perhaps the abundance of food for the planktonfeeding herring but it may have been contributing to the depletion of autumn spawning herring via the declining oxygen content of bottom water layers during the past 20 years.     

Growth of the Barents Sea capelin,Mallotus villosus,in relation to climate

Synopsis In order to investigate a possible relationship between temperature and fish length growth in the Barents Sea capelin stock, estimates of environmental temperature in the feeding season were compared to estimates of length growth during the same season. The mean temperature of the capelin feeding area was calculated by averaging the temperatures in September for the depth interval 10–200 m in statistical rectangles. The estimates of capelin growth were obtained for the same rectangles using backcalculation of length from otoliths. Correlation coefficients for the relationship between water temperature and growth were 0.70 and 0.53 for two- and three-year-olds, respectively, when all the material was considered, and between 0.85 and 0.91 for within-year data. In addition, a close correspondence between feeding area and growth rates was found.     

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.     

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.