Distribution Of Cod Research Articles

Impact of engineering measures on water exchange and water quality in semi-closed bay: A model study in Zhelin Bay

Coastal aquatic environments are increasingly polluted due to human activities such as engineering, fishing, and agriculture. Semi-enclosed bays are particularly vulnerable to this issue due to weaker tidal currents and water circulation than open coastal areas. Although engineering measures, such as tidal channel dredging, have been implemented, their impact on semi-enclosed bays remains inadequately quantified. This study aimed to investigate the water exchange and water quality of Zhelin Bay, a semi-enclosed bay, after the implementation of two engineering measures: dredging the LG (Ligang) Channel and upgrading the SBM (Sanbaimen) Gate. The finite element model TELEMAC was used to simulate hydrodynamic processes before and after these two measures. The LG Channel dredging project increased flow velocity and tracer discharge during ebbing tides. The watergate project significantly reduced the semi-exchange time of adjacent waters. The combined two measures could enhance water exchange by up to 15 %. Furthermore, the projects slightly impacted spatial distribution of COD, while TP and TN levels near the SBM Gate decreased over by 80 %. This study represents the first effort to evaluate the water exchange improvement strategy of Zhelin Bay, a semi-enclosed bay with multiple tidal channels, and provides valuable insights for future research in marine ecology protection.

Variable trends in the distribution of Atlantic cod (Gadus morhua) in the Celtic seas.

Despite decades of active fisheries management, many stocks of Atlantic cod in its southern range are in a depleted state and mortality estimates remain high. Recovery of these stocks, as defined by management areas, could be confounded by cod distributions shifting outside of these areas. Here, we assess data from internationally coordinated trawl surveys to investigate the distribution of three cod stocks in the Celtic Seas ecoregion, Irish Sea, Celtic Sea, and West of Scotland, from 1985 to 2021. We mapped cod densities, analyzed trends in mean weighted depth and bottom temperature, and calculated the center of gravity and equivalent area of the stocks. The distribution of the West of Scotland stock shifted north and east, spilling into the North Sea, while the Irish Sea and Celtic Sea stocks shifted west. Each stock showed decreasing trends in equivalent area, but there were no clear trends in the average depth occupied by the fish. There was no apparent relationship between temperature and the distribution of cod, as bottom temperature varied little from 1993 to 2021. Although Irish Sea cod showed a shift into warmer water, this was due to changes in survey distribution. The shift in distribution of the West of Scotland cod stock towards the North Sea whilst impairing local recovery provides further justification for the recent definition of its incorporation into a larger stock unit that includes the northwest of the North Sea. The Irish Sea and Celtic Sea cod stocks are neither shifting northwards, nor into deeper waters, but remained within current boundaries. This suggests that recent temperature conditions did not affect their distribution, but this may change as temperatures increase towards the limit for reproduction.

Distribution and habitat preference of polar cod (Boreogadus saida) in the Bering and Chukchi Seas inferred from species-specific detection of environmental DNA

Ongoing warming and sea-ice reductions in the Arctic can seriously impact cold-water species, such as polar cod (Boreogadus saida), necessitating biomonitoring to reveal the ecological consequences. Recent methodological advancements in environmental DNA (eDNA) techniques have increased our ability to conduct ecological monitoring at various locations, including the Arctic. This study aimed to provide an overview of the distribution of polar cod across the Bering and Chukchi Seas by employing species-specific detection of eDNA. First, we successfully developed novel species-specific qPCR assay targeting the mitochondrial D-loop region, which exclusively amplifies eDNA derived from polar cod. Subsequently, polar cod eDNA was detected using the assay from the samples that we collected latitudinally across the study area during the open-water season. Polar cod eDNA was primarily detected in the surface water from the central Chukchi Sea shelf and the northernmost observation line (75°N), which was located on the shelf slope, off the Point Barrow, and in the marginal ice zone. In contrast, only trace amounts of eDNA were detected in the Bering Sea. This pattern corresponded well with the distribution of water masses classified based on environmental conditions. The detection of eDNA in surface water was clearly limited to cold (-1 to 5°C) and low salinity (25–32) water, whereas it was detected in a higher salinity range (32–34) in the middle and bottom layers. These findings are consistent with current knowledge about the distribution and habitat of the polar cod, suggesting that eDNA can be regarded as a reliable tool to replace or supplement conventional methods. Incorporating eDNA techniques into large-scale oceanographic surveys can improve the spatial and temporal resolution of fish species detection with a reasonable sampling effort and will facilitate the continuous monitoring of Arctic ecosystems.

Cod distribution in the Barents Sea under climate changes

The aim of the paper is to assess the impact of climate changes and oceanographic conditions on the distribution of cod stocks in the Barents Sea in recent decades. The material for the study was oceanographic data obtained during surveys in the Barents Sea by PINRO and other available information on hydrometeorological conditions of the sea in 1981–2021, as well as data onRussian catches of cod in the Barents Sea based on bottom trawl fishing operations. Methods of descriptive statistics as well as comparative, correlation and regression analyses were applied. Results: The modern changes in the climate and oceanographic conditions of the Barents Sea are described. Since the early 1980s, the warming has been observed against the background of increasing storm activity, having peaked in 2016. After that, despite still warm and low-ice conditions, there has been a cooling trend in the Barents Sea. Under climate changes in recent decades, cod made increasingly long migrations north and east, spreading more widely in the feeding areas. The most informative predictors in assessing the impactof climate changes and oceanographic conditions on the distribution of cod stocks in the Barents Sea were Arctic ice extent, area-averaged temperature at a depth of 100 m, Atlantic water area in the 50–100 m layer, and parameters of the Barents Sea thermal frontal zones. Practical significance: The obtained results are useful for a better understanding of the processes occurring in the Barents Sea ecosystem and, in the future, will be used to predict cod distributions in the sea, depending on oceanographic conditions and climate changes.

The circumpolar impacts of climate change and anthropogenic stressors on Arctic cod (Boreogadus saida) and its ecosystem

Arctic cod (Boreogadus saida) is the most abundant forage fish in the Arctic Ocean. Here we review Arctic cod habitats, distribution, ecology, and physiology to assess how climate change and other anthropogenic stressors are affecting this key species. This review identifies vulnerabilities for different life stages across the entire distribution range of Arctic cod. We explore the impact of environmental (abiotic and biotic) and anthropogenic stressors on Arctic cod with a regional perspective in a scenario up to the year 2050 and identify knowledge gaps constraining predictions. Epipelagic eggs and larvae are more vulnerable to climate change and stressors than adults. Increased water temperatures, sea-ice decline, altered freshwater input, acidification, changing prey field, increased interspecific competition, new predators, and pollution are the principal stressors that will affect Arctic cod populations. Detrimental effects are likely to be greater in regions characterized by the advection of warmer Atlantic and Pacific waters. In contrast, Arctic cod may benefit from ocean warming in colder areas of the High Arctic. The risk from fisheries is moderate and primarily limited to bycatch. Overall, a decrease in suitable habitat and an associated decline in total Arctic cod biomass are predicted. In most Arctic seas, the relative abundance of Arctic cod within the fish community will likely fluctuate in accordance with cold and warm periods. A reduced abundance of Arctic cod will negatively affect the abundance, distribution, and physiological condition of certain predators, whereas some predators will successfully adapt to a more boreal diet. Regional management measures that recognize the critical role of Arctic cod are required to ensure that increased anthropogenic activities do not exacerbate the impacts of climate change on Arctic marine ecosystems. Ultimately, the mitigation of habitat loss for Arctic cod will only be achieved through a global reduction in carbon emissions.

Predatory walls may impair climate warming-associated population expansion.

Climate change has a profound impact on species distribution and abundance globally, as well as local diversity, which affects ecosystem functioning. In particular, changes in population distribution and abundance may lead to changes in trophic interactions. Although species can often shift their spatial distribution when suitable habitats are available, it has been suggested that predator presence can be a constraint on climate-related distribution shifts. We test this using two well-studied and data-rich marine environments. Focusing on a pair of sympatric fishes, Atlantic haddock Melanogrammus aeglefinus and cod Gadus morhua, we study the effect of the presence and abundance of the latter on the former distribution. We found that the distribution of cod and increased abundance may limit the expansion of haddock to new areas and could consequently buffer ecosystem changes due to climate change. Though marine species may track the rate and direction of climate shifts, our results demonstrate that the presence of predators may limit their expansion to thermally suitable habitats. By integrating climatic and ecological data at scales that can resolve predator-prey relationships, this analysis demonstrates the usefulness of considering trophic interactions to gain a more comprehensive understanding and to mitigate the effects of climate change on species distributions.

Some data on the fishery, distribution and size composition of saffron cod Eleginus gracilis Tilesius, 1810 near the Southern Kuril Islands

Data on harvesting, seasonal dynamics, structure of coastal and marine fishery, size and age composition of saffron cod catches near the Southern Kuril Islands are given. Summer–autumn distribution and size composition of saffron cod aggregations is considered based on the data of bottom trawl surveys. Distribution of saffron cod is shown almost everywhere near Kunashir Island, the Lesser Kuril Ridge and Pacific coast of Iturup Island. Harvesting is provided throughout the year, but mainly at December fishing for bottom and demersal fish species in the South Kuril Strait and May–June fishing with small fixed nets near Kunashir. Although catches of saffron cod have increased in recent years, the species is still under exploited commercially.

Polar cod (Boreogadus saida) of the Siberian Arctic: Distribution and biology

Integrated pelagic trawl surveys were conducted in the eastern sector of the Russian Arctic (Laptev Sea, Chukchi Sea and East Siberian Sea) in August–September of 2003–2018. Data were used to further describe biology and spatial distribution of Polar cod (Boreogadus saida) in this region. Polar cod in surveyed areas are characterized by similar linear size and spatial distribution. In all surveyed seas, polar cod aggregations consisted of individuals 3–29 cm in length with the age of 0+ - 6+ years. Lower growth rates of polar cod were evident in the eastern sector compared to the Kara Sea (the western sector of Russian Arctic). The lower growth rates in the eastern sector are probably due to the significant difference in environmental conditions (mainly temperature) that directly affect polar cod metabolic rates.In the Chukchi and East Siberian seas, the main concentrations were observed within the near-bottom layer, while in the Laptev Sea they were recorded throughout the water column. The abundance and biomass of polar cod in the Chukchi Sea in different years ranged from 514 million inds. and 0.83 thousand tons (2008) to 8.26 billion inds. and 117.5 thousand tones (2003). Respective indices for the Laptev Sea amounted to 233 thousand tones and 12.75 billion individuals. The abundance and biomass of the East Siberian Sea polar cod were at a relatively low levels compared to other areas in the Russian Arctic (about 0.150 thousand tons and 20 million individuals).

Annual and spatial variation in the condition and lipid storage of juvenile Chukchi Sea gadids during a recent period of environmental warming (2012 to 2019).

The Arctic is undergoing dramatic environmental change with decreasing sea-ice extent and increasing summer temperatures. The late summers of 2017 and 2019 on the eastern Chukchi Sea were anomalously warm, nearly 4 °C warmer than the previous 30-year average. Increased ocean temperatures can affect the energetics of North Pacific fish by increasing their metabolic demands and via shifting fish prey assemblages. Here we describe the total lipids as well as fatty acid (FA) trophic markers in juveniles of two Arctic gadids (polar cod, Boreogadus saida and saffron cod, Eleginus gracilis) as well as two sub-Arctic gadids (walleye pollock, Gadus chalcogrammus and Pacific cod, Gadus macrocephalus) collected on recent ecosystem surveys spanning the north Bering and Chukchi seas. Fifty percent of the variance in the lipid composition of gadids was accounted for by species-specific differences, while ecosystem measurements such as bottom temperature, large > C3 stage Calanus abundance, and surface temperature were found to independently account for 25%, 12% and 10%, respectively. Allometric relationships in lipid storage revealed that polar cod have a different lipid profile than other gadids, suggesting a species-specific life-history strategy for high lipid storage that is an adaptation to Arctic environments. Both polar cod and saffron cod had reduced lipid storage in 2017 compared to fish collected in earlier years. Polar cod in 2017 were significantly lower in total lipid, triacylglycerols (TAG), diatom- (16:1n-7/16:0) and Calanus-sourced (∑C20+ C22) FAs over the Chukchi Shelf. Juvenile gadids showed interspecific differences in the spatial distribution of high lipid individuals, with polar cod having the highest lipids in the northern ice-associated regions of the Chukchi Sea and walleye pollock in the southern Chukchi Sea. In 2019, polar cod's distribution had shifted north such that they were only abundant in the northern Chukchi Sea, where they maintained higher region-specific lipid storage than in 2017. It is concerning that reduced lipid content in polar cod was associated with elevated water temperatures, given predicted continued warming in the Chukchi Sea. Energetic changes in juvenile gadids may be associated with future increased natural mortality rates for regional populations (e.g. overwintering) and unstable foraging value for birds and mammals in the Arctic.

Removal and kinetics of basic pollutants in serially constructed wetlands

In this research, the spatial distribution and kinetic coefficient of basic pollutants in the artificial wetlands (CW) of the DACBiol-UJAT wastewater treatment plant were evaluated. It consists of a sump-settler and two identical modules with HA in series 8.3 m long, 2.5 m wide and 0.5 m long. In both trains, the first HA is subsurface flow (SSFCW) with Pontederia cordata (tule) species, the second HA is free flow (FFCW) with Thalia geniculata (popal) and the last HAFL with Sagittaria lancifolia (dovetail). The characterization of the support medium, water quality (inlet, outlet and internal points), modeling of the spatial distribution and kinetics of color degradation, turbidity and COD were carried out. Both trains operated with a 3-day HRT with a Qmed of 8.9 ± 3.4 m3/day. The quality of the inlet water of train 1 presents an average turbidity of 72.3 UTN, color of 1340.0 UC and COD with 373 mg/L, likewise at the entrance of train 2 the average turbidity is 69.6 UTN, color of 1,190.3 UC and COD with 373 mg/L. The degradation kinetic constant was estimated in train 1 at -0.52 days-1 for turbidity, -0.32 days-1 for color and -0.58 days-1 for COD, for train 2 of -0.47 days-1 for turbidity, -0.24 days-1 for color and -0.49 days-1 for COD. The removal efficiency for turbidity in train 1 was 65.6% and train 2 was 67.9%, for the apparent color in train 1 it was 48.8% and train 2 was 58.3% and in COD in train 1 it was 81% and for train 2 of 76%. The maximum permissible COD limit according to PROY-NOM-001-SEMARNAT-2017 is met, which is 150 mg/L.

Diffusion of soluble organic substrates in aerobic granular sludge: Effect of molecular weight

Aerobic granular sludge (AGS) is an advanced biofilm-based technology for wastewater treatment. Diffusion of substrates into the granules is a key aspect of this technology. Domestic wastewater contains soluble organic substrates of different sizes that could potentially diffuse into the granules. In this study, the relation between the molecular weight of a substrate and its diffusion coefficient within the granule was studied with model substrates (polyethylene glycols (PEGs) with a molecular weight between 62 and 10 000 Da). The diffusion coefficients of the model substrates within granules from a full-scale installation were measured with the ‘transient uptake of a non-reactive solute’ method. The diffusion coefficients in the granules were not significantly different from the diffusion coefficients in water, at least up to 4000 Da molecular weight. This indicates that these PEGs were not obstructed by the granule matrix. The 10 kDa PEG behaved differently from the lighter PEGs, as it could not penetrate the entire granule. Furthermore, the granule structure was characterized with Environmental Scanning Electron Microscopy (ESEM). The granules displayed an open structure with large macropores and semi-solid regions, which contained microbial cells. The diffusion results suggest that most diffusing molecules were unobstructed in the macropores and barely obstructed in the semi-solid regions. Only the diffusion of the 10 kDa PEG seemed to be hindered by the semi-solid regions, but not by the macropores. Lastly, the apparent molecular weight distribution of domestic wastewater soluble COD was determined with ultrafiltration membranes of 100, 10, and 1 kDa molecular weight cut-off. The influent fractionation revealed that a large part (61–69%) of the influent soluble COD was lighter than 1 kDa. As molecules lighter than 1 kDa diffuse easily, the majority of the influent soluble COD can be considered as diffusible COD. These findings provide new insight into the availability of influent COD for granular sludge.

Ancient DNA reveals a southern presence of the Northeast Arctic cod during the Holocene.

Climate change has been implicated in an increased number of distributional shifts of marine species during the last century. Nonetheless, it is unclear whether earlier climatic fluctuations had similar impacts. We use ancient DNA to investigate the long-term spawning distribution of the Northeast Arctic cod (skrei) which performs yearly migrations from the Barents Sea towards spawning grounds along the Norwegian coast. The distribution of these spawning grounds has shifted northwards during the last century, which is thought to be associated with food availability and warming temperatures. We genetically identify skrei specimens from Ruskeneset in west Norway, an archaeological site located south of their current spawning range. Remarkably, 14C analyses date these specimens to the late Holocene, when temperatures were warmer than present-day conditions. Our results either suggest that temperature is not the only driver influencing the spawning distribution of Atlantic cod, or could be indicative of uncertainty in palaeoclimate reconstructions in this region. Regardless, our findings highlight the utility of aDNA to reconstruct the historical distribution of economically important fish populations and reveal the complexity of long-term ecological interactions in the marine environment.

Timing and magnitude of climate-driven range shifts in transboundary fish stocks challenge their management.

Climate change is shifting the distribution of shared fish stocks between neighboring countries’ Exclusive Economic Zones (EEZs) and the high seas. The timescale of these transboundary shifts determines how climate change will affect international fisheries governance. Here, we explore this timescale by coupling a large ensemble simulation of an Earth system model under a high emission climate change scenario to a dynamic population model. We show that by 2030, 23% of transboundary stocks will have shifted and 78% of the world's EEZs will have experienced at least one shifting stock. By the end of this century, projections show a total of 45% of stocks shifting globally and 81% of EEZs waters with at least one shifting stock. The magnitude of such shifts is reflected in changes in catch proportion between EEZs sharing a transboundary stock. By 2030, global EEZs are projected to experience an average change of 59% in catch proportion of transboundary stocks. Many countries that are highly dependent on fisheries for livelihood and food security emerge as hotspots for transboundary shifts. These hotspots are characterized by early shifts in the distribution of an important number of transboundary stocks. Existing international fisheries agreements need to be assessed for their capacity to address the social–ecological implications of climate‐change‐driven transboundary shifts. Some of these agreements will need to be adjusted to limit potential conflict between the parties of interest. Meanwhile, new agreements will need to be anticipatory and consider these concerns and their associated uncertainties to be resilient to global change.

Some data about distribution of fry saffron cod <i>Eleginus gracilis</i> and walleye pollock <i>Theragra chalcogramma</i> on the west coast of Кamchatka and species composition in the catches of midwater trawl with a small-meshed insertion

Data of trawl surveys for the period 2010–2017 revealed distribution of fry saffron cod in vicinity of river outlets, while juvenile walleye pollock was dictributed either near the shore or over the deepwater area in the southern part of shelf. In 2017 aggregations of saffron cod individuals with the body length 29.4–70.5 mm (averaged 52.9 mm) were observed in the area between 52° and 53°N at the depth range 14–195 m. The aggregation densities were maximum at the plots of the highest salinity, temperature and turbidity of the water and higher chlorophyll- a concentration. Principal catches of juvenile walleye pollock were in the area between 53° and 54°N at the depth from 20 to 195 m (averaged 68.2 mm). The aggregation densities of juvenile walleye pollock were maximal at the plots of higher salinity and oxygen concentrations. The concentration of chlorophyll- a was mediate at these plots, comparing to values in the area observed. The body length of the fry varied from 44.5 to 74.6 mm (57.8 mm averaged). Besides juvenile saffron cod and walleye pollock, adult and juvenile individuals of 11 bottom and demersal-pelagic species were found in the catches of the midwater trawl.

The role of spatial distribution for growth and survival of juvenile cod Gadus morhua in the Barents Sea

Abstract While the importance of early life survival and growth variations for population dynamics is well documented, there is still a relatively limited understanding of how survival and growth is affected by the species’ spatial distribution. Using Barents Sea spatial bottom survey data (1994–2018), we study the spatiotemporal variability of the juvenile Atlantic cod (Gadus morhua) growth and survival. We used indices of the spatial distribution of juvenile cod at age-1 to study the role of distribution for the change in abundance and mean body size through their second winter of life (from age-1 to age-2). Over the 24 years analysed, we found that the location where the age-1 cod are in the Barents Sea matters for their growth and survival. We found that year-classes growing up in the western Barents Sea have higher mortality but faster growth than year-classes distributed farther east. Our results indicate that the biotic and abiotic conditions encountered at the settlement location may influence the spatial survival and growth of age-1 cod and subsequently the population dynamics. Our results underscore the importance of distribution for survival and growth early in life and by providing this essential information has implications for stock assessment and spatial fisheries management.

Tidally Induced Temporal Variations in Growth of Young‐of‐the‐Year Pacific Cod in the Yellow Sea

AbstractThe Yellow Sea is the southern limit of the distribution of Pacific cod (Gadus microcephalus), and the Yellow Sea Cold Water Mass (YSCWM) provides an appropriate over‐summering habitat for this species. Tidal current is the major component of the Yellow Sea hydrodynamic system, and its periodic variations influence the local ecological conditions and related ecosystems, particularly in the YSCWM, which is stratified in summer. To investigate the influence of tidal currents on Pacific cod growth, otolith daily increment in young‐of‐the‐year (YOY) Pacific cod in the Yellow Sea was analyzed in 2017. These data were integrated with a simulated tidal current using a multi‐scenario time‐delay analysis. This showed that: (1) the otolith daily increment exhibited a strong signal similar to the fortnightly tidal variation; (2) there was a lag of ∼8 days between the highest growth rate induced by the tidal current variation and monthly spring tide; and (3) the influence of the tidal current on otolith daily growth was stronger after migration of the cod into the YSCWM, particularly during the settlement stage. Given the stratified ocean structure during the over‐summering period, enhanced tidal mixing during the fortnightly spring tide would improve the exchange of nutrients and oxygen within the YSCWM, and lead to a delayed response of otolith growth due to feeding/digestion and subsequent growth. Therefore, variations in the tidal current have a marked positive ecological effect on the growth of YOY Pacific cod, and ultimately on the YSCWM ecosystem.

Fine-scale variability in otolith chemistry: Application to the life history analysis of Pacific cod (Gadus macrocephalus) in the Yellow Sea

The Yellow Sea is the southwestern limit of the distribution of Pacific cod (Gadus macrocephalus), a typical cold water and commercially important fish in the North Pacific. The catch of Pacific cod in the Yellow Sea has increased gradually in recent years, and has received substantial attention. To explore the use of otolith chemistry in the characterization of different spawning grounds and crucial life history events, a total of 60 specimens of Pacific cod were obtained from October 2016 to August 2017. The contents of barium (Ba) and strontium (Sr) in otoliths from the core to the edge along the ventral side were determined using laser ablation-inductively coupled plasma-mass spectrometry. Based on hierarchical cluster analysis (HCA), the specimens were assigned to two groups based on the ratio of Ba:Ca at the core. The various patterns of core-to-edge Ba:Ca inferred using a linear mixed-effect (LMM) model suggested differences in the spawning ground environments for Pacific cod in the Yellow Sea, and step changes found in the Sr:Ca ratio, and a corresponding check observed in otolith images suggest that the settlement of Pacific cod occurred approximately 80–120 days after hatching. The nature of the data prevents firm conclusions being drawn from the data available at present, but the significant differences in certain elements among individuals and the association with crucial life history events in Pacific cod are noteworthy.

Distribution of rorquals and Atlantic cod in relation to their prey in the Norwegian high Arctic

Recent warming in the Barents Sea has led to changes in the spatial distribution of both zooplankton and fish, with boreal communities expanding northwards. A similar northward expansion has been observed in several rorqual species that migrate into northern waters to take advantage of high summer productivity, hence feeding opportunities. Based on ecosystem surveys conducted during August–September in 2014–2017, we investigated the spatial associations among the three rorqual species of blue, fin, and common minke whales, the predatory fish Atlantic cod, and their main prey groups (zooplankton, 0-group fish, Atlantic cod, and capelin) in Arctic Ocean waters to the west and north of Svalbard. During the surveys, whale sightings were recorded by dedicated whale observers on the bridge of the vessel, whereas the distribution and abundance of cod and prey species were assessed using trawling and acoustic methods. Based on existing knowledge on the dive habits of these rorquals, we divided our analyses into two depth regions: the upper 200 m of the water column and waters below 200 m. Since humpback whales were absent in the area in 2016 and 2017, they were not included in the subsequent analyses of spatial association. No association or spatial overlap between fin and blue whales and any of the prey species investigated was found, while associations and overlaps were found between minke whales and zooplankton/0-group fish in the upper 200 m and between minke whales and Atlantic cod at depths below 200 m. A prey detection range of more than 10 km was suggested for minke whales in the upper water layers.

Changes in population depth distribution and oxygen stratification are involved in the current low condition of the eastern Baltic Sea cod (<i>Gadus morhua</i>)

Abstract. During the past 20 years, hypoxic areas have expanded rapidly in the Baltic Sea, which has become one of the largest marine “dead zones” in the world. At the same time, the most important commercial fish population of the region, the eastern Baltic cod, has experienced a drastic reduction in mean body condition, but the processes behind the relation between deoxygenation and condition remain elusive. Here we use extensive long-term monitoring data on cod biology and distribution as well as on hydrological variations to investigate the processes that relate deoxygenation and cod condition during the autumn season. Our results show that the depth distribution of cod has increased during the past 4 decades at the same time of the expansion, and shallowing, of waters with oxygen concentrations detrimental to cod performance. This has resulted in a progressively increasing spatial overlap between the cod population and low-oxygenated waters after the mid-1990s. This spatial overlap and the actual oxygen concentration experienced by cod therein statistically explained a large proportion of the changes in cod condition over the years. These results complement previous analyses on fish otolith microchemistry that also revealed that since the mid-1990s, cod individuals with low condition were exposed to low-oxygen waters during their life. This study helps to shed light on the processes that have led to a decline of the eastern Baltic cod body condition, which can aid the management of this population currently in distress. Further studies should focus on understanding why the cod population has moved to deeper waters in autumn and on analyzing the overlap with low-oxygen waters in other seasons to quantify the potential effects of the variations in physical properties on cod biology throughout the year.

Some peculiarities of saffron cod <i>Eleginus gracilis</i> (Tilesius) biology and distribution on South-East Kamchatka

Analysis of Danish seine catches from the coastal waters of South-East Kamchatka has revealed all seasonal distribution of saffron cod connected to the sites of the continental sandbanks influenced by aquatic vortexes with increased benthic biomass, where the catches could reach 2.5 t/fishing operation. Based on the data of the trawl and Danish seine surveying, the stock abundance, the occurence frequency, the catches of saffron cod and the contribution into the total catch in the area mentioned increased in the fall-winter period, but could not be characterized as significant. In the period of the research (2003–2018) the Danish seine catches of saffron cod varied as 1–189 t, averaged 31.8 t. The fish in the Danish seine catches had the body length from 11 to 56 cm and the body weight from 20 to 750 г. The mean length and weight of saffron cod in summer was 30.8 cm and 241.7 g and in winter – 31.2 cm and 301.6 g respectively. Occurrence of independent reproductive groups of saffron cod in the coastal bays and on the shelf of South-East Kamchatka can be suggested based on the data of stock abundance distribution, differences in the size composition and the time of maturity.