Distribution Of Cod Research Articles

Multispecies acoustic dead-zone correction and bias ratio estimates between acoustic and bottom-trawl data

Abstract In this study, we extended the original work of Kotwicki et al. (2013. Combining bottom trawl and acoustic data to model acoustic dead zone correction and bottom trawl efficiency parameters for semipelagic species. Canadian Journal of Fisheries and Aquatic Sciences 70: 208–219) to jointly estimate the acoustic dead-zone correction, the bias ratio, and the gear efficiency for multiple species by using simultaneously collected acoustic and bottom-trawl data. The model was applied to cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) in the Barents Sea and demonstrated a better or similar performance compared with a single species approach. The vertical distribution of cod and haddock was highly variable and was influenced by light level, water temperature, salinity, and depth. Temperature and sunlight were the most influential factors in this study. Increase in temperature resulted in decreasing catch and fish density in the acoustic dead zone (ADZ), while increasing sun altitude (surrogate for light level) increased the catch and fish density in the ADZ. The catch and density of haddock in the ADZ also increased at the lowest sun altitude level (shortly after midnight). Generally, the density of cod and haddock changed more rapidly in the ADZ than in the catch (from bottom to the effective fishing height) indicating the importance of modelling fish density in the ADZ. Finally, the uncorrelated variability in the annual residual variance of cod and haddock further strengthen the conclusion that species vertical distribution changes frequently and that there are probably many other unobserved environmental variables that affect them independently.

Changes in spatial and temporal variability of prey affect functional connectivity of larval and juvenile cod

Abstract Changes in structural connectivity as it can affect functional connectivity, the biological and behavioural responses of an organism, has been examined here over 2 contrasting years when the spatial distribution of larval and juvenile cod and their prey shifted from the flank to the crest on Georges Bank. New data on the gut contents of pelagic juvenile cod are compared with known prey distributions, potential growth and how climate warming can change connectivity in this region. Centropages spp. was the most important prey for pelagic juveniles, especially in June 1987 when they had high abundance on the crest and were dominant in the cod stomachs. In June 1986, copepod abundance was low where the juveniles were distributed along the flank. The potential growth of juvenile cod was greater in June 1987, consistent with the higher abundance of Centropages spp., and with higher recruitment survival, compared with June 1986. Annual changes in the spatial distribution of cod early life stages within the environment of cold or warm years can have different impacts on their growth and survival. Whereas the small copepods, Pseudocalanus spp., are primary prey for cod larvae and very abundant in cold years, larger copepods, Calanus finmarchicus and Centropages spp., are important prey for the pelagic juveniles and the latter species can have a high impact in warm years on the crest. The different spatial structure during cold or warm years provides an explanation why different year classes respond differently to environmental change. Depending on the presence or absence of specific prey, the functional connectivity response changes pathways that determine the growth and survival of early life stages and ultimately a role in recruitment.

The influence of complex structure on the spatial dynamics of Atlantic cod (Gadus morhua) in the Gulf of Maine

AbstractAtlantic cod (Gadus morhua) in the Gulf of Maine (GOM) is an iconic marine fishery stock that has experienced a substantial distributional shift since the mid-1990s. A geostatistical delta-generalized linear mixed model was utilized to hindcast yearly season-specific distributions of GOM cod. These distributions were calculated using the spring and fall bottom trawl survey data for the stock, along with cell-based bathymetry and bottom temperature data for the study area for the years 1982–2013. The centre of stock distribution (the centre of gravity), spatial extent in latitude and longitude, area occupied and median habitat temperature were estimated annually to quantify changes in the spatial dynamics of GOM cod. Time series of these distributional metrics were then used to evaluate the influences of climate change and density-dependent habitat selection on GOM cod’s distribution. Results showed that the rapid southwestward shift in the stock distribution after the late 1990s could not simply be attributed to decreasing stock abundance or warming bottom temperatures. The observed shift in cod distribution requires further investigation on whether it is possibly a result of other factors, like fluctuating productivity among subpopulations.

Spatio-temporal dynamics of a fish predator: Density-dependent and hydrographic effects on Baltic Sea cod population.

Understanding the mechanisms of spatial population dynamics is crucial for the successful management of exploited species and ecosystems. However, the underlying mechanisms of spatial distribution are generally complex due to the concurrent forcing of both density-dependent species interactions and density-independent environmental factors. Despite the high economic value and central ecological importance of cod in the Baltic Sea, the drivers of its spatio-temporal population dynamics have not been analytically investigated so far. In this paper, we used an extensive trawl survey dataset in combination with environmental data to investigate the spatial dynamics of the distribution of the Eastern Baltic cod during the past three decades using Generalized Additive Models. The results showed that adult cod distribution was mainly affected by cod population size, and to a minor degree by small-scale hydrological factors and the extent of suitable reproductive areas. As population size decreases, the cod population concentrates to the southern part of the Baltic Sea, where the preferred more marine environment conditions are encountered. Using the fitted models, we predicted the Baltic cod distribution back to the 1970s and a temporal index of cod spatial occupation was developed. Our study will contribute to the management and conservation of this important resource and of the ecosystem where it occurs, by showing the forces shaping its spatial distribution and therefore the potential response of the population to future exploitation and environmental changes.

Wind-dependent beluga whale dive behavior in Barrow Canyon, Alaska

Beluga whales (Delphinapterus leucas) are the most abundant cetacean in the Arctic. The Barrow Canyon region, Alaska, is a hotspot for Pacific Arctic belugas, likely because the oceanographic environment provides reliable foraging opportunities. Fronts are known to promote the concentration of planktonic prey; when Barrow-area winds are weak or from the west, a front associated with the Alaskan Coastal Current (ACC) intensifies. This front is weakened or disrupted when strong easterly winds slow or displace the ACC. To determine if winds influence the diving depth of belugas, we used generalized linear mixed models (GLMM) to examine how the dive behavior of animals instrumented with satellite-linked time-depth recorders varied with wind conditions. When projected along-canyon winds are from the WSW and the front associated with the ACC is enhanced, belugas tend to target shallower depths (10–100m) associated with the front. In contrast, when strong winds from the ENE displaced the ACC, belugas tended to spend more time at depths >200m where the Arctic halocline grades into relatively warmer Atlantic Water (AW). The probability of diving to >200m, the number of dives >200m, and the amount of time spent below 200m were all significantly related to along-canyon wind stress (p<0.01). From these results and known relationships between wind stress, currents and frontal structure in Barrow Canyon and the characteristic vertical distribution of Arctic cod, we infer that the probability of belugas targeting different depth regimes is based upon how wind stress affects the relative foraging opportunities between these depth regimes. Belugas are known to target AW throughout the Beaufort Sea; however, this is the first work to show that the probability of targeting the AW layer is related to wind stress.

Broadband acoustics: A viable tool for quantification of cod egg density and distribution?

Fish eggs are ichthyoplankton with a similar behavior in the water column as small zooplankton. Their specific density relative to sea water determines their vertical distribution and spread; horizontal transport of the layers will then determine their fate and geographical position during the hatching of the larvae. Acoustic techniques are routinely used to assess the quantities and distribution of fish populations. In comparison to fish, zooplankton and ichthyoplankton are very weak acoustic reflectors and this makes the application of acoustic techniques challenging. We assessed the potential feasibility of using broadband acoustics to detect cod eggs in a two stage process: first, we carried out small-scale tank experiments to measure the acoustic reflectivity of individual and grouped cod eggs over the 170 to 360 kHz frequency range. These experiments showed that natural densities of cod eggs could be detected above the background acoustic noise level. This lead to the second stage whereby we conducted a field test in an area with known concentrations of cod eggs, using a combined acoustic-optical towed and moored system. We postulate that the high spatial resolution, yet rapid wide area coverage possible with acoustics will lead to a better description of egg distributions and subsequent larval distributions and help to understand the extremely variable recruitment success in sub-arctic and arctic ecosystems.

Analysis and modelling of predation on biofilm activated sludge process: Influence on microbial distribution, sludge production and nutrient dosage

The influence of predation on the biofilm activated sludge (BAS) process is studied using a unified model that incorporates hydrolysis and predation phenomena into the two stages of the BAS system: moving bed biofilm reactor pre-treatment (bacterial-predator stage) and activated sludge (predator stage). The unified model adequately describes the experimental results obtained in a cellulose and viscose full-scale wastewater plant and has been used to evaluate the role and contribution of predator microorganisms towards removal of COD, nutrient requirements, sludge production and microbial distribution. The results indicate that predation is the main factor responsible for the reduction of both nutrient requirements and sludge production. Furthermore, increasing the sludge retention time (SRT) does not influence the total biomass content in the AS reactor of a BAS process in two different industrial wastewater treatments.

New encounters in Arctic waters: a comparison of metabolism and performance of polar cod (Boreogadus saida) and Atlantic cod (Gadus morhua) under ocean acidification and warming

Oceans are experiencing increasing acidification in parallel to a distinct warming trend in consequence of ongoing climate change. Rising seawater temperatures are mediating a northward shift in distribution of Atlantic cod (Gadus morhua), into the habitat of polar cod (Boreogadus saida), that is associated with retreating cold water masses. This study investigates the competitive strength of the co-occurring gadoids under ocean acidification and warming (OAW) scenarios. Therefore, we incubated specimens of both species in individual tanks for 4 months, under different control and projected temperatures (polar cod: 0, 3, 6, 8 °C, Atlantic cod: 3, 8, 12, 16 °C) and PCO2 conditions (390 and 1170 µatm) and monitored growth, feed consumption and standard metabolic rate. Our results revealed distinct temperature effects on both species. While hypercapnia by itself had no effect, combined drivers caused nonsignificant trends. The feed conversion efficiency of normocapnic polar cod was highest at 0 °C, while optimum growth performance was attained at 6 °C; the long-term upper thermal tolerance limit was reached at 8 °C. OAW caused only slight impairments in growth performance. Under normocapnic conditions, Atlantic cod consumed progressively increasing amounts of feed than individuals under hypercapnia despite maintaining similar growth rates during warming. The low feed conversion efficiency at 3 °C may relate to the lower thermal limit of Atlantic cod. In conclusion, Atlantic cod displayed increased performance in the warming Arctic such that the competitive strength of polar cod is expected to decrease under future OAW conditions.

Abundance and distribution of Arctic cod (Boreogadus saida) and other pelagic fishes over the U.S. Continental Shelf of the Northern Bering and Chukchi Seas

We conducted acoustic-trawl (AT) surveys of the northern Bering and Chukchi Seas during ice-free periods in 2012 and 2013. The mixed species assemblages in the study area required refinement of standard AT survey methods, and adjustment of trawl catches for the effects of trawl selectivity. Sensitivity analyses indicate that the AT abundance estimates are relatively robust to the assumptions of the analysis. These surveys indicate that midwater fishes are dominated by age-0 Arctic cod (Boreogadus saida), age-0 saffron cod (Eleginus gracilis), capelin (Mallotus villosus), and Pacific herring (Clupea pallasii). In both years, age-0 Arctic cod were distributed principally ≥69.5°N, age-0 saffron cod were abundant in coastal areas between 66.5 and 69.5°N, and Pacific herring were distributed south of 67°N. These three fishes exhibited consistent associations with temperature, salinity and bottom depth: e.g., age-0 Arctic cod were abundant at lower mean water column temperatures than saffron cod. In contrast, capelin were distributed throughout the study area, and were not consistently associated with environmental measures. There was a geographic trend in body length, with smaller Arctic cod, saffron cod and capelin in northern areas, but smaller herring in the south. Arctic cod, saffron cod, herring and capelin were all >2 times more abundant in 2013 than 2012. Sizeable populations of age-0 Arctic cod were observed in the northern Chukchi Sea, which suggests that this area is an important nursery ground. However, relatively few older Arctic cod were observed in this and other surveys of the area, which suggests that either overwinter mortality of age-0 Arctic cod is high, and/or these fish are not retained on the Chukchi shelf.

Biodegradability, toxicity, and photochemical treatment of a textile surfactant

This study investigates the biodegradability, ecotoxicity, and photochemical treatment of a segregated textile bath discharge carrying a commonly applied nonionic surfactant. The wastewater having a total COD of 1,085 mg/L is completely soluble in nature. The COD distribution of this wastewater at different molecular size cut-off levels shows a fluctuating nature. Around 40% of the COD originates from a molecular weight cut-off range less than 30 kDa fraction. The wastewater is observed to contain an initially inert soluble COD of about 20 mg/L and the residual COD level achieved after passing it from aerobic biological treatment is only 40 mg/L indicating high biodegradability. After 120 min of UV-C application similar TOC removal efficiencies of 35 and 42% are obtained at pH 4 and pH 7, respectively. Photochemical treatment does not yield significant TOC removals at pH 11. The raw discharge is highly inhibitory toward Vibrio fischeri. 60 min of UV-C application lowered ecotoxicity toward V. fischeri.

Environmental Studies on the Aqaba Gulf Coastal Waters during 2011-2013

In order to assess the quality of the Aqaba Gulf coastal waters for the sustainable use and development, and consequently for the national income, four field campaigns were annually carried out during the period from 2011-2013 to monitor hydrographic variables, i.e. temperature, salinity, pH, etc., nutrients, some heavy metals, i.e. iron, lead, copper, cadmium, chromium, etc. in addition to petroleum hydrocarbons. Except for the area of Sharm El-Sheikh Harbor, the results for the density, composition, and distributions of beach litter cleared out that the Egyptian shoreline of the Aqaba Gulf is relatively not affected by man-made litter. The area of Sharm El-Sheikh Harbor was subjected to many factors which undoubtedly affected the rate of man-made litter ac-cumulation, especially, oil contamination (oil and old or new tar) which is a good es-timator of levels of oil contamination and an effective means of evaluating the poten-tial threat of oil on coastal resources. The results of the present study cleared out also that water temperature followed seasonal changes in air temperature. Aqaba Gulf water is characterized by its high salinity and the presence of well oxygenated waters. Minor changes in the distribution of pH, BOD, DOM and COD, revealed limited effects of human impacts. Low levels of Chl-a and TSM concentrations and high transparency revealed also negligible effects for human impacts. Significantly higher sea water temperatures, TSM, DO, BOD, DOM, COD, NH4, NO2, PO4 and TP were observed in summer season compared to their corresponding values in winter season. On contrast, higher values of chlorophyll-a, NO2, TN, and SiO4 were observed in winter compared to summer season. Based on the mean annual values, Aqaba Gulf coastal waters are classified as oligotrophic to mesotrophic state. The concentrations of dissolved inorganic nitrogen forms followed the following order: NH4= NO3＞NO2. In general, the majority of TN in winter was in the form of organic-N (96.3%) compared to 91.7% in summer season. On the meantime, TP exhibited more or less comparable organic-P percentage in summer (86.2%) and winter (81.2%) seasons. Moreover, the DIN/DIP ratio showed relatively higher values in winter season (14.2:1) compared to summer season (12.4:1). This indicates high nitrogen concentrations in comparison with that of phosphorous in winter; yet it reflects the generally lower nutrient levels present in the coastal waters of the Aqaba Gulf due to the lack of significant nutrient input or oceanic upwelling. Concentrations of heavy metals were relatively low signifying that Aqaba Gulf Coastal waters are not yet seriously threatened in spite of the rapid recreational and human developments that are considered a challenge to the safety and viability of Aqaba Gulf marine environment. Furthermore, concentrations of petroleum hydrocarbons were significantly higher in summer than in winter that are mainly attributed to the increase in the levels of oil pollution that most probably re-sulted from the increase in accidental, deliberate or operational discharges and spills of oil from ships. In general, the maximum concentration was much lower than the harmful concentrations reported for seawater. A stepwise multiple linear regression, analysis of variance (ANOVA), and principle component analysis (PCA) were applied.

Evaluation of the Quality for the Egyptian Red Sea Coastal Waters during 2011-2013

To assess the quality of the Egyptian Red Sea coastal waters for the sustainable use and development, due to its importance for the national income, four field campaigns were annually carried out during the period from 2011-2013 to investigate the hydrography, nutrient salts, heavy metals and petroleum hydrocarbons. Except for the area of Bir Shalatein, the results of beach litter cleared out that the shoreline of the studied area is not affected by man-made litter. No sewage could be observed. The results of the present study showed that water temperature followed seasonal changes in air temperature. Red Sea water is more saline than adjacent Arabian Sea. DO revealed high values and presence of well oxygenated waters. Minor changes in the distribution of pH, BOD, DOM and COD, revealed that limited effects of human impacts and depend mainly on the dynamics of its water as well as on the geographical location. Low Chl-a and TSM concentrations and high transparency revealed that also the effect of human impacts is almost negligible. Significantly higher sea water temperatures, TSM, pH, DO, BOD, DOM, and COD were observed in summer season compared to their corresponding values in winter season. Dissolved inorganic nitrogen concentrations were quite low because there is little nutrient input from soil, agriculture and pollution on land. Based on the annual mean values, the pattern concentrations of dissolved inorganic nitrogen forms followed the order: NO3 > NH4 > NO2. The Red Sea coastal waters are classified as oligotrophic to mesotrophic state. A remarkable increase of PO4 concentration was observed in the middle Red Sea stations due to huge amounts of effluents enriched with phosphate from the main shipping and industry of Phosphate Companies. SiO4 displayed a large variability due to the supply of SiO4, which flows in the Red Sea through the strait of Bab El-Mandab, biological consumption, organic matter decomposition and the partial dissolution of quartz particle transported to the sea from the surrounding desert during sand storms. Concentrations of ammonia, phosphorus, total nitrogen, and total phosphorus were significantly higher in summer compared to their corresponding values in winter. In general, the majority of TN and TP in winter were in the form of organic-N (91.3%) and organic P (96.8%). The mean DIN/DIP ratio revealed high nitrogen concentrations in comparison with that of phosphorous and the surface coastal waters of the Red Sea are principally, P-limited for phytoplankton growth with higher values in winter season compared to summer season. Concentrations of heavy metals were quite low most probably due the absence of major local impacts of any land-based sources and/or any major negative impacts of coastal tourism. Regional variations were almost negligible and except for Mn, Cd, and Hg insignificant seasonal variations were observed. The present study revealed concentrations for metals in the acceptable levels. Furthermore, concentrations of petroleum hydrocarbons were significantly higher in winter than in summer which is mainly attributed to the increase in the rate of evaporation for petroleum hydrocarbons in summer. In general, the maximum concentration was much lower than the harmful concentrations reported for seawater. Correlation coefficients as well as principle component analysis (PCA) were applied.

Temperature-based targeting in a multispecies fishery under climate change

Temperature controls important physiological processes in fish and determines aspects of their niches. In an effort to inform selective fishing and spatiotemporal management in the U.S. Northeast Multispecies fishery, we used 16 years of data from the Northeast Fisheries Science Center Spring and Fall Scientific Trawl Surveys to determine if bottom temperature can be used to differentiate the distribution of Atlantic cod (Gadus morhua) from other species within the fishery management plan (FMP). We identified two separate regimes in spring temperatures and used empirical cumulative distribution functions to calculate biomass availability by temperature for each species. We applied a bagged approach to find optimum thermal threshold values that maximize the difference in cod biomass from each of the other species. For our study area, 38% to 54% of the species considered were well separated from cod by temperature in spring, whereas only 17% were separable in the fall. This study suggests that temperature targeting can be used seasonally to separate cod from many other species in the FMP including top catches and no‐retention species. The use of temperature targeting may allow fishermen to better meet multiple quotas while avoiding choke species. Our results also suggested increasing thermal overlap between cod and species inhabiting higher median temperatures (e.g., spiny dogfish, Squalus acanthias) under the current warming temperature regime. These results indicate that the ability to selectively fish in the US Northeast Multispecies fishery will become more difficult under a warming ocean.

Distribution and diet of demersal Arctic Cod, Boreogadus saida, in relation to habitat characteristics in the Canadian Beaufort Sea

Arctic Cod (Boreogadus saida) occur throughout the circumpolar north; however, their distributions at localized scales are not well understood. The seasonal habitat associations and diet preferences across life-history stages of this keystone species are also poorly known, thereby impeding effective regulatory efforts in support of conservation objectives. The distribution of Arctic Cod in the Canadian Beaufort Sea was assessed using bottom trawling in shelf and slope habitats between 20 and 1000 m depths. Highest catch biomasses occurred at 350 and 500 m depth slope stations, coinciding with >0 °C temperatures in the Pacific–Atlantic thermohalocline and Atlantic water mass. Calanus glacialis, Calanus hyperboreus, Themisto libellula, and Themisto abyssorum were identified as key prey species in the diet of Arctic Cod, comprising approximately 86 % of total biomass in guts. Hierarchical cluster analysis with a SIMPROF test identified five statistically significant (p 200 m depth) coinciding with an associated increase in fish standard length with depth. Smaller Arctic Cod fed primarily on Calanus copepods and larger Arctic Cod fed primarily on the larger Themisto species. The habitat and diet associations presented here will inform knowledge of structural and functional relationships in Arctic marine ecosystems, aid in mitigation and conservation efforts, and will enhance our ability to predict the effects of climate change on the local spatial and depth associations of this pivotal marine fish.

Impact of the Great East Japan Earthquake tsunami on growth and survival of Pacific cod (Gadus macrocephalus)

A massive tsunami hit the north Pacific coast of Japan on 11 March 2011. We evaluated the effects of the tsunami on the distribution, growth, and survival of young Pacific cod (Gadus macrocephalus) using data from benthic trawl surveys and otolith microstructure analysis. The distribution of Pacific cod juveniles differed between 2011 and 2008–2010. The 2011 year class was smaller in body size than those in 2008–2010. The majority of 2011 year-class juveniles was captured in deeper areas at a smaller body size, whereas those of previous year classes were primarily distributed in shallower water. The peak hatch of the 2011 year-class juveniles was from early to late February, which was several days later than those of the 2008 and 2010 year classes. The 2011 year class grew at the same rate as those from 2008 and 2010 until the day of the tsunami, but had a lower growth rate during the 30 days after the tsunami, resulting in a decrease in fish size at settlement. Growth of the 2011 year class was also inferior soon after the day of the tsunami in almost all groups divided by the hatching period. Growth of juveniles captured in shallower water following the tsunami in 2011 tended to be slower than that of juveniles captured in deeper water. Our results suggest that the tsunami caused a decrease in growth rates for approximately 30 days and a change in the distribution of juvenile fish relative to prior years. The change in distribution was primarily the result of mortality in shallow water caused by physical damage and (or) subsequent decreased growth. Considering these results and the finding that the recruitment level and the recruits per spawning in the 2011 year class were low, the tsunami may have affected the determination of the recruitment level.

The effect of recent warming on polar cod and beaked redfish juveniles in the Barents Sea

During 1980–2008, climatic conditions in the Barents Sea changed from cold to warm. This recent warming has led to a greater area of Atlantic water and a smaller area of Arctic water within the Barents Sea, which influences suitable habitats for arctic and boreal fish. Using extensive survey observations of temperature and the presence of juvenile polar cod (arctic) and beaked redfish (boreal), we defined a core thermal habitat for these species and studied how climate variability has influenced abundance indices, fish density and length, and geographical distribution. The distribution of polar cod has often been split into western (Svalbard) and eastern (Novaya Zemlya) components, which correspond to spawning sites. Most polar cod were found in a temperature band of 2–5.5 °C (core thermal habitat, CTH). Higher temperatures will lead to worse conditions (less CTH) for polar cod, which will result in decreased fish density and distribution for this species in the Barents Sea. Decreased ice cover provides less suitable spawning sites for polar cod, which may result in reduced abundance (0-group index). Redfish were primarily distributed in the western and central parts of the Barents Sea and were seldom observed east of 30°E. The majority of beaked redfish were found in a temperature band of a 5.5–8.5 °C. For redfish, higher temperatures resulted in better conditions (larger CTH), but redfish did not seem to utilise the increased habitat. The abundance of redfish (0-group index) seemed to depend more on exploitation than on CTH. Thus, recent warming negatively influenced polar cod recruitment due to a shrinking habitat, but it positively affected redfish recruitment.

Under-ice distribution of polar cod Boreogadus saida in the central Arctic Ocean and their association with sea-ice habitat properties

In the Arctic Ocean, sea-ice habitats are undergoing rapid environmental change. Polar cod (Boreogadus saida) is the most abundant fish known to reside under the pack-ice. The under-ice distribution, association with sea-ice habitat properties and origins of polar cod in the central Arctic Ocean, however, are largely unknown. During the RV Polarstern expedition ARK XXVII/3 in the Eurasian Basin in 2012, we used for the first time in Arctic waters a Surface and Under Ice Trawl with an integrated bio-environmental sensor array. Polar cod was ubiquitous throughout the Eurasian Basin with a median abundance of 5000 ind. km−2. The under-ice population consisted of young specimens with a total length between 52 and 140 mm, dominated by 1-year-old fish. Higher fish abundance was associated with thicker ice, higher ice coverage and lower surface salinity, or with higher densities of the ice-amphipod Apherusa glacialis. The fish were in good condition and well fed according to various indices. Back-tracking of the sea-ice indicated that sea-ice sampled in the Amundsen Basin originated from the Laptev Sea coast, while sea-ice sampled in the Nansen Basin originated from the Kara Sea. Assuming that fish were following the ice drift, this suggests that under-ice polar cod distribution in the Eurasian Basin is dependent on the coastal populations where the sea-ice originates. The omnipresence of polar cod in the Eurasian Basin, in a good body condition, suggests that the central Arctic under-ice habitats may constitute a favourable environment for this species survival, a potential vector of genetic exchange and a recruitment source for coastal populations around the Arctic Ocean.

Occurrence of a gelatinous predator (Cyanea capillata) may affect the distribution of Boreogadus saida, a key Arctic prey fish species

Although the Arctic cod (Boreogadus saida) has a pan-Arctic distribution, little is known about its occurrence in near-shore waters where this species is the principal prey for seabirds, marine mammals and other fish. Published research describes the scyphomedusa Cyanea capillata as an Arctic cod predator, and this paper presents observations from long-term investigations using active hydroacoustics that suggest the Arctic cod avoided C. capillata in two small bays of Cornwallis Island (Canadian High Arctic archipelago). Distribution patterns in echograms suggested that features such as boundary layer fronts restricted jellyfish movements and Arctic cod were often abundant on the side of fronts where C. capillata were absent. Thus, habitat partitioning allowed Arctic cod to share habitat with its predator, albeit exceptions to this sharing occurred when jellyfish abundance was high and Arctic cod were displaced. Thus, if a warmer Arctic triggers an increase in C. capillata abundance, it is possible that small-scale aspects of Arctic cod distribution could be affected. This in turn could have significant ripple effects within the Arctic food web, an additional and previously unrecognized consequence of climate change.

Reply to 'Sources of uncertainties in cod distribution models'

Ingvaldsen et al. comment on our study assessing global fish interchanges between the North Atlantic and Pacific oceans for more than 500 species during the entire 21st century. They propose that discrepancies between our model projections and observed data for cod in the Barents Sea are the result of the choice of Atmosphere-Ocean General Circulation Models (AOGCMs). We address this assertion here, re-running the cod model with additional observation data from the Barents Sea1, 3, and show that the lack of open-access, archived data for the Barents Sea was the primary cause of local prediction mismatch. This finding recalls the importance of systematic deposit of biodiversity data in global databases

Sources of uncertainties in cod distribution models

Sources of uncertainties in cod distribution models