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The Pacific cod (Gadus macrocephalus) fishery recently collapsed in the Gulf of Alaska after a series of marine heat-waves that began in 2014. To gauge the likelihood of population recovery following these extreme warming events, we investigate potential thermal stress on age-0 cohorts through a comprehensive analysis of juvenile cod abundance, condition, growth, and survival data collected from 15 years of beach seine surveys. Abundance was strongly negatively related to ocean temperature during the egg and larval phase (winter-spring), but age-0 cod were larger in the early summer following warm winter-spring temperatures. Body condition indices suggest that warm summers may improve energetic reserves prior to the first winter; however, there was no summer temperature effect on post-settlement growth or survival. Spatial differences in abundance, condition, or growth were not detected, and density-dependent effects were either weak or positive. While the positive effects of increased summer temperatures on juvenile condition may benefit overwintering survival, they cannot compensate for high pre-settlement mortality from warming winter-spring temperatures. We conclude the critical thermal bottleneck for juvenile abundance occurs pre-settlement. La peche a la morue du Pacifique (Gadus macrocephalus) s'est recemment effondree dans le golfe d'Alaska apres une serie de vagues de chaleur marines qui ont debute en 2014. Afin d'evaluer la probabilite d'un retablissement de la population apres ces episodes de rechauffement extreme, nous examinons le stress thermique potentiel sur des cohortes de moins de 1 an par une analyse exhaustive de donnees sur l'abondance, l'embonpoint, la croissance et la survie de morues juveniles obtenues de 15 annees de leves a la seine de rivage. L'abondance presente une forte relation negative avec la temperature de l'ocean durant la phase des oeufs et des larves (hiver-printemps), mais les morues de moins de 1 an sont plus grandes au debut d'etes suivant des temperatures hivernales-printanieres elevees. Les indices d'embonpoint indiqueraient que des etes chauds pourraient ameliorer les reserves d'energie avant le premier hiver; aucun effet de la temperature estivale sur la croissance ou la survie apres l'etablissement n'est cependant releve. Aucune variation spatiale de l'abondance, de l'embonpoint ou de la croissance n'est detectee, et les effets dependants de la densite sont faibles ou positifs. Si les effets positifs de temperatures estivales plus elevees sur l'embonpoint des juveniles pourraient ameliorer la survie hiemale, ils ne peuvent pallier la mortalite elevee qui precede l'etablissement, causee par la hausse des temperatures hivernales-printanieres. Nous concluons que le goulot d'etranglement thermique critique pour l'abondance des morues juveniles se produit avant l'etablissement. [Traduit par la Redaction], Introduction The recent collapse of the Pacific cod (Gadus macrocephalus) stock in the Gulf of Alaska has been directly linked to warming ocean temperatures in 2014-2016 (Barbeaux et al. 2020b) [...]

In subarctic Newfoundland, age-0 Atlantic cod (Gadus morhua) settle into coastal habitats in several summerautumn pulses, resulting in broad length-frequency distributions before winter. Low winter temperatures and potential decreases in food availability pose challenges for young-of-year fish. To examine how size variation affects overwintering success under contrasting food scenarios, we conducted 114-day laboratory feeding trials at ambient overwinter sea temperatures, using demersal age-0 cod collected from Newman Sound, Newfoundland. We reared two size classes of juvenile cod under four daily ration levels (starvation, low, medium, high). We used Fulton's K condition factor to interpret effects of food availability and fish size on survival over winter. We showed that small amounts of consumed food (80% until Day 74. Therefore, we expect higher survival of earlier settlers and increased size-selective mortality in age-0 cod during either unproductive or protracted winters, when food abundance is often low. Dans les regions subarctiques de Terre-Neuve, des morues (Gadus morhua) de moins d'un an s'etablissent dans des habitats cotiers en plusieurs vagues durant l'ete et l'automne, ce qui produit des distributions etalees de la frequence des longueurs avant l'hiver. Les basses temperatures hivernales et des diminutions potentielles de la disponibilite de nourriture presentent des defis pour les jeunes de l'annee. Afin d'examiner l'incidence des variations de la taille sur la survie hiemale pour differents scenarios de disponibilite de nourriture, nous avons mene des essais d'alimentation en laboratoire sur 114 jours aux temperatures de la mer hivernales ambiantes, en utilisant des morues demersales de moins d'un an prelevees dans la baie de Newman, a Terre-Neuve. Nous avons eleve des morues juveniles de deux classes de taille en leur donnant l'une ou l'autre de quatre rations quotidiennes differentes (privation, faible, moyenne et grande). Nous avons utilise le facteur d'embonpoint de Fulton, K, pour interpreter les effets de la disponibilite de nourriture et de la taille des poissons sur la survie hiemale. Nous avons demontre que de petites quantites de nourriture consommee (, Introduction Juvenile fish experience high natural mortality in their first year of life (Sogard 1997; Kristiansen et al. 2000). Despite multiple studies of mortality in juvenile Atlantic cod (Gadus morhua) [...]

Warming temperatures in the Gulf of Alaska have been linked to recruitment failure in Pacific cod (Gadus macrocephalus), but the mechanisms and timing of mortality events for juveniles are unclear. To date, limited research has focused on overwintering success, and the knowledge of juvenile ecology and physiology is based entirely on summer observations. Here, we investigate the changes in body condition, diet composition, and tissue-specific fatty acid (FA) storage for age-0 Pacific cod in Kodiak, Alaska, from February to December during 2018 and 2020. We observed protracted nearshore residency from June to December. Cod body condition (K dry) and predicted weight at length were lowest in October, November, and December. Although not different interannually, diet composition varied seasonally, which corresponded to an increase in cod length. A range of condition metrics (HSIwet, FA concentration in liver tissue, and the % of whole body FAs stored in the liver) began to increase in September. Cod prioritized growth during the summer, while in the autumn and pre-winter they allocated more energy into lipid storage. We conclude that seasonal changes in tissue-specific FA storage and pre-winter fish conditions are important factors to consider for understanding overwintering potential of juvenile Pacific cod. [ABSTRACT FROM AUTHOR]

Pacific cod (Gadus macrocephalus) stocks in the Gulf of Alaska experienced steep, unexpected declines following an unprecedented 3-year marine heatwave (i.e., 'warm blob') from 2014 to 2016. We contend that stock reproductive potential was reduced during this period, evidenced by a combination of new laboratory data demonstrating narrow thermal hatch success (3-6 [degrees]C), mechanistic-based models of spawning habitat, and correlations with prerecruit time series. With the exception of single-year El Nino events (1998, 2003), the recent 3-year heatwave (2014-2016) and return to similar conditions in 2019 were potentially the most negative impacts on spawning habitat for Pacific cod in the available time series (1994-2019). Continued warming will likely reduce the duration and spatial extent of Pacific cod spawning in the Gulf of Alaska. Les stocks de morue du Pacifique (Gadus macrocephalus) dans le golfe d'Alaska ont connu des declins abrupts et inattendus a la suite d'une vague de chaleur marine (ou <>) sans precedent d'une duree de 3 ans, de 2014 a 2016. Nous soutenons que le potentiel de reproduction des stocks a diminue durant cette periode, comme en fait foi une combinaison de nouvelles donnees de laboratoire qui demontrent une etroite fourchette de temperatures associee au succes d'eclosion (de 3 a 6 [degrees]C), des modeles mecanistes des habitats de frai et des correlations avec des series chronologiques pre-recrutement. A l'exception d'episodes El Nino d'une seule annee (1998, 2003), la vague de chaleur recente de trois ans (2014-2016) et le retour a des conditions semblables en 2019 constituent possiblement les impacts les plus importants sur les habitats de frai pour la morue du Pacifique dans les series chronologiques disponibles (1994-2019). La poursuite du rechauffement reduira vraisemblablenient la duree et l'etendue spatiale du frai des monies du Pacifique dans le golfe d'Alaska. [Traduit par la Redaction], Introduction From late 2013 to 2016, the Gulf of Alaska (GOA) experienced 3-4 [degrees]C above average sea surface temperature anomalies (relative to 1980-2010) associated with a reduction in winter heat [...]

The rapid decline in Pacific cod (Gadus macrocephalus, Gadidae) biomass following multiple Gulf of Alaska marine heatwaves (2014–2016 and 2019) may be one of the most dramatic documented changes in a sustainably managed marine fishery. As such, fisheries managers are exploring new recruitment paradigms for Pacific cod under novel environmental conditions. In this review, we address the challenges of managing and forecasting Pacific cod populations in the Eastern Pacific where thermal habitats for early life stages are undergoing varying rates of change across space and time. We use observational data to examine changes in distribution, abundance and demographics of the population from 1993 to 2020, and model contemporary and future changes of thermal habitat for both spawning success and age‐0 juvenile growth potential. Results indicate that reduced spawning habitat and early life stage abundance may be a precursor to regional population decline, but the recent apparent increases in size‐at‐age of pre‐recruits will have unknown impacts on future recruitment in these regions. We contend that continued monitoring of early life stages will be necessary to track changes in phenology and growth that likely determine size‐at‐age and the survival trajectories of year classes into the adult population. These include complex size‐ and temperature‐dependent energetics spanning seasonal habitats through the first winter. Climate‐ready management of Pacific cod will, therefore, require new process investigations beyond single‐season surveys focused on one‐life stage. [ABSTRACT FROM AUTHOR]

Coastal seine surveys contain some of the only direct measures of age-0 abundance for Atlantic cod (Gadus morhua) and Pacific cod (Gadus macrocephalus), yet their utility in forecasting future year-class strength has not been evaluated among regions. We analyzed coastal time series from the Gulf of Alaska, Newfoundland, and Norway to test the hypothesis that recruitment signals are stronger when assessed under thermal conditions that provide high juvenile growth potential. Weaker recruitment signals were associated with low growth potential from cold winters (Newfoundland) and recent warmer summers (Norway). We conclude that temperature-dependent growth strongly influences the utility of coastal surveys in recruitment forecasting among regions. Temporal changes in growth potential (e.g., via climate change) will likely affect recruitment signals by way of changes in juvenile mortality or spatial shifts to more favorable thermal habitats. Si les releves cotiers a la senne constituent un des rares moyens d'obtenir des mesures directes de l'abondance de la classe d'age 0 pour la morue franche (Gadus morhua) et la morue du Pacifique (Gadus macrocephalus), leur utilite pour predire la force future des classes d'age n'a pas ete evaluee entre differentes regions. Nous avons analyse des series chronologiques cotieres pour le golfe de l'Alaska, Terre-Neuve et la Norvege afin de verifier l'hypothese selon laquelle les signaux de recrutement sont plus forts quand ils sont evalues pour des conditions thermiques qui assurent aux juveniles un potentiel de croissance eleve. Des signaux de recrutement plus faibles etaient associes a un faible potentiel de croissance decoulant d'hivers froids (Terre-Neuve) et d'etes plus chauds recents (Norvege). Nous en concluons que la croissance dependante de la temperature a une forte incidence sur l'utilite des releves cotiers pour predire le recrutement entre regions. Des variations temporelles du potentiel de croissance (p. ex., decoulant de changements climatiques) auront vraisemblablement une incidence sur les signaux de recrutement, par l'entremise de modifications de la mortalite des juveniles ou de deplacements dans l'espace vers des niches thermiques plus favorables. [Traduit par la Redaction], Introduction Marine teleost fishes of the Family Gadidae comprise some the largest and most valuable wild fisheries in the world (e.g., Atlantic cod, Gadus morhua; walleye pollock, Gadus chalcogrammus) and [...]

Abstract--Northern rock sole (Lepidopsetta polyxystra) is a commercially important flatfish in Alaska and was recently classified as a distinct species from southern rock sole (L. bilineata). Taxonomic and vital rate [...]

The match-mismatch hypothesis (MMH) predicts that marine fish larvae will have their highest rate of growth and survival when they overlap with their prey. However, Pacific cod (Gadus macrocephalus) and other gadids continue to thrive in the Bering Sea despite delayed prey production resulting from warming and loss of sea ice. In this study, we examined how temperature mediates growth and survival of Pacific cod larvae under varying match-mismatch conditions. Cod larvae were reared at two temperatures (3°C and 8°C) and exposed to one of five different food treatments over a 6week period: (i) high food (HF) (match); (ii) low food (LF); (iii) HF then LF (HF-LF); and (IV) LF then HF (LF-HF) (2-4 mismatch); and (v) no food (NF) (complete mismatch). Results showed that cold environments allow Pacific cod larvae to bridge gaps in prey availability (i.e., timing and magnitude), but negatively impact survival over longer periods. Under warmer conditions, mismatches in prey significantly impacted growth and survival. However, both yolk reserves and compensatory growth mechanisms reduced the severity of mismatches occurring in the first 3 weeks of development. Our results demonstrate a clear need to incorporate the direct effects of temperature on fish larvae in food limitation models. L'hypothese de l'appariement-meisappariement (MMH, match-mismatch hypothesis) predit que les larves de poissons marins possedent une croissance et une survie maximales lorsqu'il y un chevauchement entre elles et leurs proies. Cependant, la morue du Pacifique (Gadus macrocephalus) et d'autres gadides continuent de prosperer dans la mer de Beering malgre la production retardeee de proies causee par le rechauffement et la perte de glace de mer. Nous examinons dans notre etude comment la temperature controle la croissance et la survie de la morue du Pacifique sous diverses conditions d'appariement-mesappariement. Nous avons eleve des larves de morue a deux temperatures (3°C et 8°C) et les avons exposees a l'un de cinq traitements alimentaires durant une periode de 6 semaines: (i) nourriture abondante (HF) (appariement), (ii) nourriture restreinte (LF), (iii) HF puis LF (HF-LF), (iv) LF puis HF (LF-HF) (2-4 mesappariements) et (v) absence de nourriture (NF) (meesappariement complet). Nos resultats montrent que les environnements froids permettent aux larves de morues du Pacifique de surmonter des interruptions dans la disponibilite des proies (c'est-a-dire le moment et l'intensite de la penurie); mais cela affecte negativement la survie sur des peeriodes plus longues. Dans des conditions plus chaudes, les mesappariements des proies affectent significativement la croissance et la survie. Cependant, les reserves de vitellus et les mecanismes compensatoires de la croissance reduisent tous deux la seveerite; des mesappariements qui se produisent durant les trois premieres semaines du developpement. Nos resultats demontrent clairement la necessite d'incorporer les effets directs de la temperature sur les larves de poissons dans les modeles de restriction de nourriture. [Traduit par la Redaction], Introduction Climate-driven impacts on ocean environments are changing the seasonal distribution and timing of zooplankton production worldwide (Richardson 2008). Marine fish larvae, which depend on zooplankton for food, are highly [...]

Abstract--Pacific cod (Gadus macrocephalus) is an important component of fisheries and food webs in the North Pacific Ocean and Bering Sea. However, vital rates of early life stages of this [...]

In subarctic Newfoundland, age-0 juvenile Atlantic cod (Gadus morhua) settle into coastal habitats in several summer and fall pulses, yielding a broad length-frequency distribution prior to their first winter. The first winter is often associated with physiological and metabolic stress and has been considered a critical survival period determining cohort strength. We evaluated size-structured overwinter mortality and movement using mark–recapture and condition metrics by marking 226 cod in two batches one week apart, in October 2016. We estimated fall and overwinter mortality, and documented movement of fish recaptured in May 2017 using Cormack–Jolly–Seber models. We recaptured 30 marked juveniles. High fall mortality characterized late settling cohorts relative to earlier settling cohorts (16.6%∙d−1 vs. 4.5–7.7% ∙d−1). Overwinter mortality was unexpectedly low (0.0052 and 0.0022% ∙d−1). Individual condition (Fulton's K) of juvenile cod remained high throughout winter across all size groups. We expected higher mortality of juvenile cod and broad dispersal of juveniles over winter (32 weeks). In contrast, our results indicated low mortality and high site-fidelity in their first winter. This study indicates the period leading up to winter is important for survival, suggesting winter is not a survival bottleneck and may even provide a refuge compared to the rest of the year. [ABSTRACT FROM AUTHOR]

Abstract: The success of marine protected areas (MPAs) as fisheries management tools in tropical latitudes has generated interest in their applicability and potential elsewhere. Here we suggest that dispersal and [...]

Arctic cod (Boreogadus saida) is an ecologically significant species that is uniquely adapted to occupy ice edges, but warming and loss of sea ice are hypothesized to favour more facultative gadids, such as walleye pollock (Gadus chalcogrammus). To test this hypothesis, we experimentally measured the growth and survival of Arctic cod and walleye pollock at two larval stages across a range of temperature and food conditions in the laboratory. Results indicated early and late-stage Arctic cod larvae have a competitive growth and survival advantage over walleye pollock at low temperatures. However, these advantages are lost under warmer, food-productive conditions where walleye pollock larvae survived and experienced accelerated growth rates. Growth models developed from this study emphasize the need to account for both species- and stage-specific differences in the thermal response of closely related marine fish larvae. More broadly, these new vital rate data provide a mechanistic framework to forecast spatial-temporal shifts of gadids at the Arctic-boreal interface resulting from climatic warming and altered productivity regimes. [ABSTRACT FROM AUTHOR]

Changes in Arctic fish assemblages resulting from climate change will likely be determined by the differential thermal response of key species during their early life history. In this study, we incubated multiple batches of eggs and larvae of two ecologically important gadids co-occurring at the Pacific–Arctic interface, Arctic cod (Boreogadus saida) and walleye pollock (Gadus chalcogrammus). Fertilized egg batches (n  = 11 Arctic cod; n  = 6 walleye pollock) were collected in the late winter/early spring from laboratory broodstock held under simulated seasonal environmental conditions. Image and lipid analyses indicated that Arctic cod eggs and larvae were ∼25–35% larger than walleye pollock and had nearly 3–6× more energetic reserves. Two batches of eggs from each species were incubated in replicated containers (n  = 3/batch/temperature) at −0.4, 1.2, 2.5, 3.8, 5.0, 9.0, and 12.0°C for Arctic cod and −0.8, 0.3, 2.2, 4.5, 9.0, and 12.0°C for walleye pollock. Both species had very similar low thermal tolerance, but Arctic cod were much more sensitive to higher thermal stress in terms of hatch success and size-at-hatch. For example, Arctic cod hatch success declined precipitously at temperatures above 3.5°C yet remained above 50% in walleye pollock at 9°C. Arctic cod also had significantly longer development times, such that embryos could survive for ∼4 months at temperatures <0°C from the time of spawning to first-feeding. Collectively, these results indicate Arctic cod have a much smaller thermal window for survival, but can survive for longer periods in the absence of food than walleye pollock at cold temperatures. These temperature-dependent rates will be useful in the development of population forecasts and biophysical transport models for these species in the northern Bering, Chukchi, and Beaufort seas. [ABSTRACT FROM AUTHOR]

The Arctic marine environment is rapidly changing with rising sea surface temperatures, declining sea ice habitat and projected increases in boreal species invasions. The success of resident Arctic fish will depend on both their thermal tolerance and their ability to cope with changing trophic interactions. Larval fish energetic condition is closely associated with mortality rates and therefore provides an indicator of overall well-being or fitness. In this study, we experimentally determined larval morphometric and lipid-based condition in an Arctic gadid (Arctic cod, Boreogadus saida) and a boreal gadid (walleye pollock, Gadus chalcogrammus) in response to different temperatures and food rations. Our results suggest that larval condition is highly sensitive to both factors but varies in a species- and ontogenetic-dependent manner. Results indicated that condition metrics based on length–weight relationships were not as sensitive as those based on lipid storage. Further, condition metrics changed with ontogeny and were best used within a developmental stage rather than across developmental stages. As expected, larval condition in first-feeding Arctic cod was higher at colder temperatures (2–5°C) than in the boreal gadid (5–12°C). However, at more developed larval stages the peak condition for Arctic cod was at warmer temperatures (7°C), while walleye pollock had the same thermal optimum as during earlier stages. Arctic cod were more sensitive to food ration at first feeding than walleye pollock, however; at later larval stages both species had a negative condition response to low food ration, especially at elevated temperatures (5 vs. 7°C). The lower thermal tolerance of Arctic cod, coupled with a higher sensitivity to food availability indicates that Arctic cod are particularly vulnerable to on-going environmental change. Arctic cod is a lipid-rich keystone species and therefore a reduction in their energetic condition during summer has the potential to affect the health of higher trophic levels throughout the Alaskan Arctic. [ABSTRACT FROM AUTHOR]

Forecasting climate-driven ecosystem impacts in the Arctic is currently limited by the availability of biological data on key species. In this study, we carried out a series of temperature-dependent growth experiments on juvenile Arctic cod (Boreogadus saida), an important for-age fish species at the Atlantic- and Pacific-Arctic interface. Multi-week growth experiments were conducted across a range of temperatures (-1 to16 °C), ages (age-0 and age-1) and sizes (45-125mm SL) to determine best-fit temperature-dependent growth models. Temperature-dependent growth models considered were 1) a single model using pooled fish of mixed ages and sizes (Model A), separate models for age 0 and age 1 fish (Model B0 and B1), and a Lorentzian three-dimensional model incorporating size but not age (Model C). All juvenile Arctic cod demonstrated relatively high growth at cold temperatures (<2 °C; ~0.75-1.00% d-1) with a deceleration in growth rate above 6.4 °C and 9.0 °C for age-1 and age-0 cod respectively. As such, Model A did not explain as much temperature-dependent growth (R² = 51%) as Model B0 (R² = 98%) and Model B1 (R² = 93%). Model C did reasonably well in explaining growth over a continuous range of sizes (R² = 71%) but explained less variance than either of the age models (Model B). These results stress the importance of considering the early ontogenetic effects on temperature-dependent growth variation in juvenile Arctic cod when projecting climate-driven changes in Arctic marine ecosystems. [ABSTRACT FROM AUTHOR]

Multiple aspects of climate change are expected to co-occur such that ocean acidification will take place in conjunction with warming and a range of trophic changes. Previous studies have demonstrated that nutritional condition plays a significant role in the responses of invertebrates to ocean acidification, but similar studies have yet to be conducted with marine fishes. In this study, we examined the potential interactive effects of elevated CO2 levels and nutritional stress on the growth and development of northern rock sole ( Lepidopsetta polyxystra). Separate experiments examined the effects of these two environmental stressors during the pre-flexion (3-31 days) and post-flexion (31-87 days) larval stages. In both stages, the larval feeding regime has a much larger impact on growth rates than did the CO2 level, and there was no observed interaction between stressors. By 31 days post-hatch, larvae in the high-feeding treatment were 84.2% heavier than the fish in the low-feeding treatments, but there was no significant effect of CO2 level on body size or condition. While overall growth rates were faster during the pre-flexion stage, the effects of food limitation were greater for post-flexion larvae undergoing metamorphosis, with the high-feeding treatment fish being 3.3 times as heavy as fish in the low-feeding treatments. These results have important implications for understanding the impacts of the multi-faceted nature of climate change on population productivity of commercial fish species in the North Pacific. [ABSTRACT FROM AUTHOR]

Age-0 juveniles may be the earliest, reliable indicators of recruitment into commercial marine fisheries, but independent fisheries assessments are usually conducted on older life stages in adult habitats. We used an 8 year juvenile gadid survey along the coast of Kodiak, Alaska to examine annual abundance, growth and mortality in age-0 Pacific cod (Gadus macrocephalus), with comparisons to saffron cod (Eleginus gracilis) and walleye pollock (Gadus chalcogrammus) where possible. Annual abundance of age-0 fish was positively correlated among all three species, suggesting Pacific gadids respond similarly to processes controlling pre-settlement survival and/or delivery to coastal nurseries. In Pacific cod, June temperature was positively correlated with size-at-settlement but post-settlement growth was density- rather than temperature-dependent. Age-0 abundance indices for Pacific and saffron cod predicted the number of age-1 fish the following year (i.e. positive "recruitment signals"), but only in the larger nursery (Anton Larsen Bay) where age-1 gadids were more likely to remain resident after their first year. Recruitment signals for Pacific cod improved with later estimates of age-0 abundance, likely because of high mortality following settlement in July. In contrast, very fewage-0 and age-1 walleye pollock were caught across the entire time-series of the survey. Collectively, these data suggest that nearshore surveys may be a tractable means of examining early life history processes and assessing year-class strength in juvenile Pacific and saffron cod, but have relatively low value in understanding the population dynamics of walleye pollock. [ABSTRACT FROM AUTHOR]

The Bering Sea and Gulf of Alaska support a number of commercially important flatfish fisheries. These high latitude ecosystems are predicted to be most immediately impacted by ongoing ocean acidification, but the range of responses by commercial fishery species has yet to be fully explored. In this study, we examined the growth responses of northern rock sole (Lepidopsetta polyxystra) eggs and larvae across a range of CO2 levels (ambient to 1500 µatm) to evaluate the potential sensitivity to ocean acidification. Laboratory-spawned eggs and larvae were reared at 8°C in a flow-through culture system in which CO2 levels were maintained via computer-controlled injection of CO2 into a seawater conditioning tank. Overall, we observed only minor effects of elevated CO2 level on sizes of northern rock sole larvae. Size at hatch differed among offspring from four different females, but there was no significant effect of CO2 level on egg survival or size at hatch. In three separate larval growth trials, there was little effect of CO2 level on growth rates through the first 28 d post-hatch (DPH). However, in the one trial extended to 60 DPH, fish reared at the highest CO2 level had lower condition factors after 28 DPH, suggesting that larvae undergoing metamorphosis may be more sensitive to environmental hypercapnia than earlier pre-flexion stages. These results suggest that while early life stages of northern rock sole are less sensitive to ocean acidification than previously examined flatfish, they may be more sensitive to elevated CO2 levels than a previously studied gadid with a similar geographic range. [ABSTRACT FROM AUTHOR]

Northern rock sole ( Lepidopsetta polyxystra) is a commercially important fish in the North Pacific and a focal species in understanding larval transport to nursery grounds in the Bering Sea. However, the temperature-dependent vital rates and settlement dynamics for this species have not been described in detail. We reared northern rock sole larvae in the laboratory to measure growth, condition, development and settlement parameters across four temperatures (2, 4, 7 and 10°C). Both length and mass-measured growth rates increased with temperature and were best described by non-linear regression. Residuals of the length-mass relationships were positively related to temperature, indicating larval condition also increased with temperature. Larval development and settlement were largely size dependent, resulting in reduced larval stage duration and earlier settlement at higher temperatures owing to more rapid growth at elevated temperatures. However, larvae at colder temperatures were less developed at a given size, but more likely to settle at smaller sizes than larvae reared in warmer conditions. These temperature-response parameters can be used to refine current and future transport models for northern rock sole larvae under changing environmental conditions in the North Pacific. [ABSTRACT FROM AUTHOR]

The utility of the fatty acid biomarker (FAB) approach in fisheries ecology is limited by our understanding of how biotic and abiotic factors determine dietary markers in fish tissues. An 8-week laboratory experiment was conducted on two species of juvenile gadids (Pacific cod, Gadus macrocephalus and walleye Pollock, Theragra chalcogramma) reared at 3°C or 9°C and fed a diet enriched with either oils of marine origin or terrestrial plant origin. Non-linear models were fitted to investigate how tissue type and temperature mediated the proportion of FABs in fish. Across temperatures, fatty acid (FA) profiles were similar for both species of gadids. FAs also showed high temporal sensitivity across temperatures, and were evident in fish after only one week of feeding. Pacific cod held at 9°C and fed a terrestrial plant oil (TPO) enriched diet had significantly higher C18 polyunsaturated FAs (PUFAs) in their liver than cod held at 3°C after one week, but this temperature effect diminished as tissues reached equilibrium with their diet. C18 PUFAs were significantly higher in liver than in muscle. Differential proportions of C18 PUFAs among tissues provide temporal patterns that may help with disentangling the timing of offshore–inshore nursery migrations in juvenile fish. Calibration coefficients were determined to explain the relationship between FAs in the diet and FAs in fish tissues. These coefficients will support future development of quantitative estimates of diet in juvenile low-fat fish. [ABSTRACT FROM AUTHOR]

The role of behavior, especially vertical migration, is recognized as a critical component of realistic models of larval fish dispersion. Unfortunately, our understanding of these behaviors lags well behind our ability to construct three-dimensional flow-field models. Previous field studies of vertical behavior of larval Pacific cod ( Gadus macrocephalus) were limited to small, preflexion stages (≤11 mm SL) in a narrow range of thermal conditions. To develop a more complete picture of larval behavior, we examined the effects of ontogeny, temperature, and light on vertical responses of larval Pacific cod in experimental columns. While eggs were strictly demersal, yolk-sac larvae displayed a strong surface orientation as early as 1 day post hatch (∼ 5 mm SL). Consistent with field observations, small preflexion larvae (<10 mm SL) showed no response to varying light levels. However, there was a direct effect of temperature on larval behavior: Pacific cod larvae exhibited a stronger surface orientation at 4°C than at 8°C. The behavior of larger, postflexion larvae (>15 mm SL) in experimental columns was consistent with a diel vertical migration and independent of water temperature: fish were more widely distributed in the column, and median positions were consistently deeper at higher light levels. These laboratory observations are combined with observations from discrete-depth (MOCNESS) sampling in the Gulf of Alaska to characterize the vertical distribution of larval Pacific cod and contrast ontogenetic patterns with walleye pollock ( Theragra chalcogramma). The vertical movements of larval Pacific cod described here will be applied in the development of dispersal projections from Gulf of Alaska spawning grounds. [ABSTRACT FROM AUTHOR]

Abstract: Habitat use of age-0 and age-1 juvenile Pacific cod (Gadus macrocephalus) was examined in coastal regions in Kodiak Alaska over daily, seasonal and annual scales. Catch data indicated highly variable recruitment to nursery areas, but a strong separation of distribution by depth among age groups. Age-0 cod were most abundant in the shallows (<3m) whereas age-1 cod were typically found in depths (9.0–13.5m). In comparison, age-1 saffron cod (Eleginus gracilis), another highly abundant gadid in the region, were found in shallower depths where age-0 cod often resided. Age-1 cod Pacific cod made diel lateral movements, moving into shallow regions at night where they co-occurred with age-0 cod to a greater extent. Laboratory light-gradient experiments indicated that age-0 cod tolerated intense lighting (~20–80µE m−2 s−1) typical of shallow water regions whereas larger age-1 Pacific cod strongly avoid bright light given the choice. However, while diet data indicate age-1 cod of both species are moderately piscivorous (3% saffron cod; 16% Pacific cod), we found no direct evidence of predation on smaller conspecific cod, possibly due to the low densities of age-0 cod in the year of the diet study. Together, these data suggest that coastal regions continue to serve a nursery function beyond the 1st year of development for juvenile Pacific gadids, and that small-scale temporal and depth partitioning in these regions is a mechanism by which varying cod species and age classes co-occur. [Copyright &y& Elsevier]

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 (∑C 20 + C 22) 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. • Sub-Arctic gadids surveyed over the Chukchi shelf were in low energetic condition. • Species-specific differences accounted for most of the variance in gadid lipids. • Age-0 polar cod had a unique lipid storage strategy compared to other gadids in the region. • Elevated lipids in polar cod were associated with increased storage of diatom- and Calanus -sourced fatty acids. • Temperature, salinity and large Calanus abundance explained 21% of the variation in polar cod lipid storage. [ABSTRACT FROM AUTHOR]

In the Arctic, winter warming and loss of sea ice pose largely unknown risks to keystone species and the marine ecosystem that they support. Young-of-the-year juvenile polar cod, Boreogadus saida , are an energy-rich forage fish that accumulate high levels of lipid in the summer but retain a relatively small body size during the winter. To address winter bioenergetics and survival, we held age-0 juveniles under simulated winter conditions (food deprived, 24-hr darkness) at a range of four constant temperatures (−1, 1, 3, 5 °C). Our goals were to 1) determine how age-0 polar cod utilize lipid energy in muscle and liver across variable temperatures and durations of food deprivation, 2) understand temperature- and size-dependent impacts on survival and 3) provide energy loss models using multiple condition metrics that are commonly used in fisheries science (lipids, morphometric ratios, body weight). These data have relevance to projecting winter outcomes for polar cod sampled pre-winter, when fish are more easily sampled in the field. As expected, in the absence of food, juvenile polar cod better conserved lipids and survived longer at colder temperatures. There was no negative impact of cold extremes on this pattern; for example, 50% mortality was at 170 days when polar cod were held at −1 °C, compared to only 94 days when they were held at 5 °C. During the first 28 days of simulated winter, polar cod preferentially catabolized triacylglycerols from muscle tissue, then depleted this storage lipid class in their muscle and liver until starvation. Mortality occurred when whole-body lipid concentrations fell below 12.4 mg g−1 wet weight. Temperature-dependent declines in morphometric condition (hepatosomatic index and Fulton's K) and lipid content were parameterized and developed into temperature-dependent condition loss models. Applying a laboratory-based lipid loss model to field-collected polar cod demonstrated that winter survival is highly sensitive to small changes in temperature between −1 and 1 °C when fish are in good condition at the end of the preceding summer. Alternatively, fish in poor summer condition cannot survive winter relying exclusively on stored energy reserves, and will be required to forage throughout the winter. Collectively, these results suggest that lipid-based indices offer a sensitive means of predicting overwintering success for polar cod experiencing climate-driven changes in summer and winter habitats in the Arctic. • Both larger body sizes and colder culture temperatures significantly prolonged polar cod winter survival duration. • During simulated Arctic winters, polar cod lost lipid mass at twice the rate that they lost wet mass. • Mortality of polar cod occurs at a Fulton's K of 0.44, a HSI of 0.67, and lipids per WWT of 12.4 mg g−1. • Lower lipid storage in field-collected fish following a warm summer may indicate subsequent poor winter survival. [ABSTRACT FROM AUTHOR]

Abstract: We have observed inter-annual variability in the depth distribution of juvenile northern rock sole Lepidopsetta polyxystra on their nursery grounds around Kodiak Island, Alaska. This study evaluates whether this variability is a response to inter-annual changes in the availability of habitat created by polychaete tubes; principally Sabellides sibirica. We suspect that worm tubes constitute an alternative refuge and/or feeding habitat for juvenile flatfish. Accordingly, we hypothesized that during years of low worm abundance, fish would concentrate in the shallows (<10m depth) where they find refuge from predation, but would move to greater depths (>15m, where the worms occur) during years when the worms were abundant. Using data on worm abundance and fish density over 5yr, we tested this hypothesis at 2 Kodiak nursery embayments. Whether worms were abundant in a given year or embayment had no influence on overall fish abundance, however, worm abundance did influence juvenile flatfish depth distributions. At Holiday Beach, where worms tended to be scarce, fish were typically concentrated in shallow water. However, during the 1year when worms were abundant, fish were concentrated in deeper water. At Pillar Creek Cove, where worms are more regularly found, fish tended to concentrate in deeper water, the exception being the one year when worms were nearly absent. Regression analysis for both sites and all years indicated that the percent of fish occupying shallow water (<10m) decreased with increasing worm abundance. When worms were prevalent, fish were most commonly found on bottom with sparse to moderate worm cover, but avoided bottom where the worms were so dense as to form a ‘turf’. These results demonstrate that the geographic and inter-annual variation in worm tube abundance has significant influence over the distribution of juvenile northern rock sole. [Copyright &y& Elsevier]