Barents Sea Stock Research Articles

Are state-space stock assessment model confidence intervals accurate? Case studies with SAM and Barents Sea stocks

Our main contribution is to examine the reliability of confidence intervals using the SAM state-space fish stock assessment model used for the assessment of many stocks by the International Council for the Exploration of the Seas. We focus on frequentist statistical inferences and more specifically on inference conditioned on specific values of the state-space model random effects drawn from their process distribution. This is somewhat consistent with simulation self-test procedures that are commonly used to examine the reliability of state-space assessment model results. However, recent research has indicated that some estimation bias may be expected in the conditional setting. Hence, we also investigate recently proposed bias corrected confidence intervals appropriate for the conditional inference setting. The SAM simulation coverage probabilities of 95% confidence intervals for SSB and Fbar were usually slightly larger than 95%, but in a small number of years these coverage probabilities could be much smaller than 95%. The bias corrected confidence intervals were more reliable. When averaged over years, the SAM and bias corrected confidence interval coverage probabilities were similar for the Northeast Artic cod and saithe case studies, but the bias corrected confidence intervals performed much better overall for the haddock case study.

Size-fractioned zooplankton biomass in the Barents Sea: macroecological patterns across biogeography, climate, and varying ecosystem state (1989-2020)

The zooplankton community in the Barents Sea was monitored during autumn cruises from 1989-2020, with biomass determined as the depth-integrated dry weight (g m-2) of 3 size fractions (>2 mm, 1-2 mm, <1 mm). A large data set of 4543 sampling stations encompassing a subdivision of the Barents Sea was spatially averaged into 15 polygons based on topography. The resulting data set is used to describe relationships between the biomass of size fractions as expressions of the size structure of the zooplankton communities. Each of the 3 size fractions was positively related to the total zooplankton biomass (sum of fractions). The medium size fraction, which contained on average 48% of the total biomass, showed a strong linear correlation with total biomass (R2 ≈ 0.8). The medium fraction contained the older copepodite stages of Calanus species (C. finmarchicus and C. glacialis), which are dominant species interpreted to drive the overall changes in zooplankton biomass. The biomass of the small size fraction decreased with decreasing total biomass, but less so than the medium fraction. Thus, the small/medium biomass ratio increased as the total biomass decreased. This trend was most pronounced for shallow and central polygons, which coincide with the core feeding area of the large Barents Sea stock of capelin Mallotus villosus. The change in the small/medium biomass ratio is interpreted to reflect a shift from Calanus to smaller zooplankton in response to predation by capelin and other planktivorous predators, and possibly also by warming due to climate change.

Long-term variability in spawning stock age structure influences climate-recruitment link for Barents Sea cod.

Fish populations may spawn a vast number of offspring, while only a small and highly variable fraction of a new cohort survives long enough to enter into the fisheries as recruits. It is intuitive that the size and state of the spawning stock, the adult part of the fish population, is important for recruitment. Additionally, environmental conditions can greatly influence survival through vulnerable early life stages until recruitment. To understand what regulates recruitment, an essential part of fish population dynamics, it is thus necessary to explain the impact of fluctuations in both spawning stock and environment, including interactions. Here, we examine if the connection between the environment and recruitment is affected by the state of the spawning stock, including biomass, mean age and age diversity. Specifically, we re-evaluate the hypothesis stating that recruitment from a spawning stock dominated by young fish and few age classes is more vulnerable to environmental fluctuations. We expand upon earlier work on the Barents Sea stock of Atlantic cod, now with data series extended in time both backwards and forwards to cover the period 1922-2019. While our findings are correlative and cannot prove a specific cause and effect mechanism, they support earlier work and strengthen the evidence for the hypothesis above. Furthermore, this study supports that advice to fisheries management should include considerations of environmental status.

High genomic diversity in the endangered East Greenland Svalbard Barents Sea stock of bowhead whales (Balaena mysticetus)

The East Greenland-Svalbard-Barents Sea (EGSB) bowhead whale stock (Balaena mysticetus) was hunted to near extinction and remains Endangered on the International Union of Conservation of Nature Red List. The intense, temporally extensive hunting pressure may have left the population vulnerable to other perturbations, such as environmental change. However, the lack of genomic baseline data renders it difficult to evaluate the impacts of various potential stressors on this stock. Twelve EGSB bowhead whales sampled in 2017/2018 were re-sequenced and mapped to a previously published draft genome. All individuals were unrelated and void of significant signs of inbreeding, with similar observed and expected homo- and heterozygosity levels. Despite the small population size, mean autosome-wide heterozygosity was 0.00102, which is higher than that of most mammals for which comparable estimates are calculated using the same parameters, and three times higher than a conspecific individual from the Eastern-Canada-West-Greenland bowhead whale stock. Demographic history analyses indicated a continual decrease of Ne from ca. 1.5 million to ca. 250,000 years ago, followed by a slight increase until ca. 100,000 years ago, followed by a rapid decrease in Ne between 50,000 and 10,000 years ago. These estimates are lower than previously suggested based on mitochondrial DNA, but suggested demographic patterns over time are similar.

Resource-driven colonization by cod in a high Arctic food web.

Climate change is commonly associated with many species redistributions and the influence of other factors may be marginalized, especially in the rapidly warming Arctic.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.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.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.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.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.

Development of a new \u2018ultrametric\u2019 method for assessing spawning progression in female teleost serial spawners

The collection and presentation of accurate reproductive data from wild fish has historically been somewhat problematic, especially for serially spawning species. Therefore, the aim of the current study was to develop a novel method of assessing female spawning status that is robust to variation in oocyte dynamics between specimens. Atlantic cod (Barents Sea stock) were used to develop the new ‘ultrametric’ method, that is based on the progressive depletion of the vitellogenic oocyte pool relative to the rather constant previtellogenic oocyte (PVO) pool. Fish were subsequently partitioned into one of four categories that accurately reflected changes in their oocyte size frequency distribution characteristics and gonadosomatic index throughout spawning. The ultrametric method overcomes difficulties associated with presence of bimodal oocyte distributions, oocyte tails, lack of clear hiatus region, and presence of free ova, and can be implemented at a single sampling point. Much of the workflow is fully automated, and the technique may circumvent the need for histological analysis depending on the desired outcome. The ultrametric method differs from the traditional autodiametric method in that PVOs can be separated by ultrasonication and then enumerated, and ovarian homogeneity is not a mandatory requirement per se. The method is designed for determinate spawners but might be extended to include indeterminate spawners.

Sea ice, temperature, and prey effects on annual variations in mean lengths of a key Arctic fish, Boreogadus saida, in the Barents Sea

Abstract Oceanographic conditions in the Arctic are changing, with sea ice cover decreasing and sea temperatures increasing. Our understanding of the effects on marine populations in the area is, however, limited. Here, we focus on the Barents Sea stock of polar cod (Boreogadus saida). Polar cod is a key fish species for the transfer of energy from zooplankton to higher trophic levels in the Arctic food web. We analyse the relationships between 30-year data series on the length-at-age of polar cod cohorts (ages 0–4) and sea surface temperature, sea ice concentration, prey biomasses, predator indices, and length-at-age the previous year using multiple linear regression. Results for several ages showed that high length-at-age is significantly associated with low sea ice concentration and high length-at-age the previous year. Only length-at-age for age 1 shows a positive significant relationship with prey biomass. Our results suggest that retreating sea ice has positive effects on the growth of polar cod in the Barents Sea despite previous observations of a stagnating stock biomass and decreasing stock abundance. Our results contribute to identifying mechanisms by which climate variability affects the polar cod population, with implications for our understanding of how future climate change may affect Arctic ecosystems.

Formation of the Population Structure of the Atlantic Cod Gadus morhua Linnaeus, 1758 in the Quaternary Period

This paper presents a comparative analysis of the variability of genetic characteristics in samples from the largest stocks of the Atlantic cod Gadus morhua Linnaeus, 1758 in the Northeast Atlantic (NEA) and the Northwest Atlantic (NWA) waters. The results of a pairwise estimation of the genetic differentiation, performed using allozyme and microsatellite markers, detected a significant difference between the cod sample from the New England Shelf (NWA) and the cod of the Icelandic–Greenlandic and Norwegian–Barents Sea stocks. At the same time, cod samples from areas of East and West Greenland and samples from the Barents Sea, despite considerable geographic differences, demonstrated a high level of identity according to molecular genetic markers. The peculiarities of the formation of the cod population structure in the Quaternary period are considered in connection with the dynamics of global climatic and oceanological processes.

Ocean Acidification Effects on Atlantic Cod Larval Survival and Recruitment to the Fished Population.

How fisheries will be impacted by climate change is far from understood. While some fish populations may be able to escape global warming via range shifts, they cannot escape ocean acidification (OA), an inevitable consequence of the dissolution of anthropogenic carbon dioxide (CO2) emissions in marine waters. How ocean acidification affects population dynamics of commercially important fish species is critical for adapting management practices of exploited fish populations. Ocean acidification has been shown to impair fish larvae’s sensory abilities, affect the morphology of otoliths, cause tissue damage and cause behavioural changes. Here, we obtain first experimental mortality estimates for Atlantic cod larvae under OA and incorporate these effects into recruitment models. End-of-century levels of ocean acidification (~1100 μatm according to the IPCC RCP 8.5) resulted in a doubling of daily mortality rates compared to present-day CO2 concentrations during the first 25 days post hatching (dph), a critical phase for population recruitment. These results were consistent under different feeding regimes, stocking densities and in two cod populations (Western Baltic and Barents Sea stock). When mortality data were included into Ricker-type stock-recruitment models, recruitment was reduced to an average of 8 and 24% of current recruitment for the two populations, respectively. Our results highlight the importance of including vulnerable early life stages when addressing effects of climate change on fish stocks.

A review of early life history dynamics of Barents Sea cod (Gadus morhua)

Abstract The Barents Sea stock of Atlantic cod (Gadus morhua) is currently the world's largest cod stock. It is also a stock for which long time-series are available and much research has been carried out. With this review, we wish to present an overview and evaluation of the knowledge on Barents Sea cod early life dynamics. The focus is on the effects of the biotic and abiotic drivers, which jointly determine the strength of a year class. A stage-by-stage approach is employed. We summarize and assess the significance of the different processes described in the literature to be at play during each specific life stage, from spawning stock, through eggs, larvae, and pelagic juvenile, to demersal juvenile and recruitment at age 3. Also Russian work is included, some of which until now has not been available to non-Russian readers. Physical drivers examined include sea temperature, advection and dispersal, wind-induced turbulence, and light. Biotic mechanisms studied range from maternal effects and skipped spawning in the adult stock through egg quantity and quality, to prey availability for the larvae and effects of cannibalism on the juveniles. Finally, we evaluate the main hypotheses put forth by Johan Hjort a hundred years ago in the light of our synthesis of present knowledge. A main conclusion is that it is unlikely that there is any one single life stage during which recruitment with any generality is determined.

A comparison of recent changes in distribution of capelin (Mallotus villosus) in the Barents Sea, around Iceland and in the Northwest Atlantic

Capelin stocks in the Barents Sea, around Iceland and off eastern Newfoundland and Labrador have traditionally undergone similar basic migration patterns, from southern spawning sites to more northern feeding locations. In this paper, the migration and distribution patterns are reviewed for the three stocks. Special attention is paid to the relatively recent past, when Icelandic and Newfoundland/Labrador stocks exhibited changes in these patterns, while the Barents Sea stock did not undergo any significant distributional change. The changes occurred first off Newfoundland/Labrador and later in Icelandic waters. We examine the possible role of temperature and stock abundance in controlling capelin migration and distribution through comparisons of the three stocks. From the available observations, it is not possible to determine the relative impacts of temperature and abundance. However, all three areas have exhibited significant warming trend and we suggest that capelin may be reacting to large-scale ocean climate changes that have been occurring in the Arctic as a result of global warming. We speculate that such changes should appear first off Newfoundland/Labrador because this area is more heavily dominated by waters from the Arctic. In contrast, while both the Barents Sea and Icelandic areas are influenced by waters of Arctic origin, they also receive a significant input of warmer, Atlantic waters. The appearance of capelin in the Canadian Arctic is consistent with the movement towards the Arctic and/or an increase in abundance of a subarctic species previously at the margins of its historical distribution, under conditions of increasing ocean temperatures in the Arctic.

Trophic interactions affecting a key ecosystem component: a multistage analysis of the recruitment of the Barents Sea capelin (Mallotus villosus)

The Barents Sea stock of capelin ( Mallotus villosus ) has suffered three major collapses (>90% reduction) since 1985 due to recruitment failures. As capelin is a key species in the area, these population collapses have had major ecosystem consequences. By analysing data on spawner biomass and three recruitment stages (larvae, 0-group, and 1-year-olds), we suggest that much of the recruitment failures are caused by predation from herring ( Clupea harengus ) and 0-group and adult Northeast Arctic cod ( Gadus morhua ). Recruitment is furthermore positively correlated with sea temperatures in winter and spring. Harvesting of maturing capelin on their way to the spawning grounds reduced the abundance of larvae significantly, but this reduction to a large extent is compensated for later in life, as mortality is strongly density-dependent between the larval stage and age 1. Altogether, our study indicates a very high importance of trophic interactions, consistent with similar findings in other high-latitude marine ecosystems.

Seasonal distribution and dive behaviour of harp seals (Pagophilus groenlandicus) 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.

Trends in age-at-maturity and growth parameters of female Northeast Atlantic harp seals, Pagophilus groenlandicus (Erxleben, 1777)

AbstractWe analyzed and compared trends in age-at-maturity and body growth in the Greenland Sea and Barents Sea stocks of harp seals, Pagophilus groenlandicus, from the early 1960s to the early 1990s. Mean and median age at sexual maturity (MAMPM and MdAM) were estimated from Richards curves fit to age-specific proportions mature. No long-term trends were found in the Greenland Sea seals, where a common value of MAMPM (5.6 years) and MdAM (4.8 years) could be fit to samples from 1959 through 1990. There were also no significant changes in length-at-age of molting females between 1964 and 1987. For Barents Sea harp seals, MAMPM increased significantly from 5.4 years in the period 1962–1972 to 6.6 years in 1976–1985 and 8.2 years in 1988–1993, concurrently with a decline in body growth rates. Tests on MdAM also showed an increasing trend, but the grouping of samples was slightly different. Estimates of MAMPM for the Barents Sea stock were similar to previously published back-calculated values of MAM, but simulations showed that this method is sensitive to the age distribution of the sample, thus complicating comparisons between samples with different age structures. The high values of MAMPM and low growth rates in the Barents Sea stock in the late 1980s to early 1990s coincided with severe depletion of important prey species in the Barents Sea, reports of mass invasions of harp seals along the Norwegian coast and indications of reduced body condition. All these are consistent with a hypothesis of reduced per-capita resource levels within the distribution area of Barents Sea harp seals at that time, but no cause-and-effect relationship for the long-term trend in age-at-maturity can be established.

The effects of swimbladder size, condition and gonads on the acoustic target strength of mature capelin

Abstract Pre- and post-spawning capelin from the Barents Sea stock were observed in a net pen with a calibrated 38 kHz scientific split-beam echosounder. The transducer was positioned vertically or tilted. The acoustic target strength (TS) of capelin depended on swimbladder length. In female capelin within 15.5–18.0 cm length the logarithm of weight was significantly negatively related to TS. The negative effect of weight on TS could be due to a higher condition factor (assumed higher fat content), which gave the fish additional buoyancy and less need for swimbladder volume. The effect of gonad weight on TS was not significant. Comparisons of measurements made with vertical and tilted transducers demonstrated a small but significant effect of tilt angle on TS.

Covariability in early growth and year-class strength of Barents Sea cod, haddock, and herring: the environmental link

We address the hypothesis that survival of a cohort is directly related to growth rates during the pre-recruit period for marine fish. This hypothesis is widely accepted, but supporting field evidence has been elusive. Here the connection between size and year-class strength at the early stages is examined for the commercially important Barents Sea stocks of cod (Gadus morhua), haddock (Melanogrammus aeglefinus), and herring (Clupea harengus). We show that there is a close link between length and year-class strength at the 0-group stage within and among the populations studied, and that the connection also holds between length at the 0-group stage and strength of recruitment at age 3. Both length and abundance are closely related to temperature. Interannual variability in the temperature conditions is concluded to be the underlying cause of the covariability between growth and year-class strength. We hypothesize that for stocks at the high latitude end of the overall range of the species the environmental signal tends to over-ride density-dependent effects on growth. High temperature will cause a high production of prey items leading to higher growth rates and higher survival through the vulnerable larval and juvenile stages. The duration of the high-mortality and vulnerable stages is also decreased by higher temperature directly increasing the development rate. 2000 International Council for the Exploration of the Sea

Challenges for the fishing industry: The Norwegian governmental view

The paper briefly summarizes the Norwegian governmental view regarding regional and global fisheries problems. In particular, the need for international co-operation, especially in international waters and distant-waters interests, is discussed. The findings of the 1993 UN Conference on the conservation and management of straddling stocks are outlined, particularly with the example of the Barents Sea stocks.

Relationship between spawning mode and phylogeographic structure in mitochondrial DNA of North Atlantic capeiin Mallotus villosus

Capelin Mallotus villosus spawn on beaches in Alaska and British Columbia, but spawn offshore in Icelandic waters and the Barents Sea. Both modes of reproduction CO-occur In the northwest Atlantic. The Southeast Shoal population spawns on the Grand Banks 350 km to the SE of Newfoundland at the same time as other stocks, all of which are beach spawners. These observat~onsg ave rise to 2 alternative hypotheses concerning the zoogeography and evolution of life cycle in capelin. First, the Southeast Shoal population was originally a beach-spawning population during the late Wisconsinian glaciation and is ancestral to all other northwest Atlantic capelin stocks. In such a case, present-day stocks from this area would represent a monophyletic group derived from a common ancestor no more than 10000 to 12000 yr ago. The alternative hypothesis is that the 2 modes of reproduction orignginally evolved in isolatlon. Beach spawners are hypothesized to have originated in the north Pacific and recolonized Canadian Arctic waters and the northwest Atlantic following glaciation Bottom spawners originated in the North Atlantic and continued to reproduce where environmental conditions permitted. In such a case. genetic divergence among bottom-spawners and among beach-spawners from across the North Atlantic would be less than that between beach- and bottom-spawners. We tested these hypotheses by comparing mitochondrial DNA (mtDNA) restriction fragment length polymorphisms among 6 stocks of beachspawning capelin (St. Lawrence estuary; Gulf of St. Lawrence; Placentia Bay, Conception Bay and Notre Dame Bay, Newfoundland; Nain, Labrador) and 3 stocks of bottom-spawning capelin (Southeast Shoal; Iceland; Barents Sea). We observed 2 major mtDNA genotype groups separated by a mean sequence divergence of 3.42 O/O, clearly reflecting the genetic separation of the Iceland and Barents Sea stocks from the northwest Atlantlc stocks. No geographical heterogeneity in the frequency of mtDNA genotypes was observed among the northwest Atlantic sampling sites. However, differences in nucleon diversities among sites did not support the view that capelin form one large panmictic population in the northwest Atlantic. Although our results do not permit the identification of the Southeast Shoal stock as ancestral to northwest Atlantic capelin, these observations refute the hypothesis that the beach- and bottomspawning stocks evolved in isolation long before the end of the Wisconsinian glaciation.

Freezing Resistance, Temperature and Salinity Tolerance in Eggs, Larvae and Adults of Capelin, Mallotus Villosus, From Balsfjord

The capelin of lodde, Mallotus villosus Müller is a salmonoid teleost of great commercial importance, being caught in large quantities by Norway, Iceland, the U.S.S.R. and Canada. The species has a circumpolar distribution, is limited to high northern latitudes, and has been the subject of two major review articles (Templeman, 1948; Jangaard, 1974).Most Norwegian capelin spawn sublittorally in the Barents Sea, but a population living in a long fjord near Tromsö in northern Norway (Balsfjord) spawns, like the Newfoundland capelin (Jeffers, 1931, Ph.D. Thesis, University of Toronto), between the tidemarks, their sticky eggs adhering to gravel, stones and weed. The capelin from Balsfjord appear to be separate from the Barents Sea stocks, and are now believed to remain within the fjord throughout their life history (Friis-Sörensen, 1983, unpublished Cand. real, thesis, University of Tromsö).