The US Chukchi Sea consists of the waters off the northwest of Alaska and is a naturally dynamic ice-driven ecosystem. The impacts from climate change are affecting the Arctic marine ecosystem as well as the coastal communities that rely on healthy marine ecosystems. In anticipation of increased ecosystem monitoring in the area, there is an opportunity to evaluate improved sampling designs for future ecological monitoring of the Chukchi Sea, an area that is sampled less comprehensively compared to other regions in Alaska. This analysis focused on standardized NOAA-NMFS-AFSC bottom trawl surveys (otter and beam trawls) and three types of survey designs: simple random, stratified random, and systematic. First, spatiotemporal distributions for 18 representative demersal fish and invertebrate taxa were fitted using standardized catch and effort data. We then simulated spatiotemporal taxon densities to replicate the three survey design types to evaluate design-based estimates of abundance and precision across a range of sampling effort. Modest increases in precision were gained from stratifying the design when compared to a simple random design with either similar or lower uncertainty and bias of the precision estimates. There were often strong tradeoffs between the precision and bias of the systematic estimates of abundance (and associated variance) across species and gear type. The stratified random design provided the most consistent, reliable, and precise estimates of abundance indices and is likely to be the most robust to changes in the survey design. This analysis provides a template for changing bottom trawl survey designs in the Chukchi Sea and potentially other survey regions in Alaska going forward and will be important when integrating new survey objectives that are more ecosystem-focused.

Many fish species have moved poleward with ocean warming, and species distribution shifts can occur because of adult fish movement, or juveniles can recruit to new areas. In the Bering Sea, recent studies document a dramatic northward shift in the distribution of Gadus macrocephalus (Pacific cod in English and tikhookeanskaya treska in Russian) during a period of ocean warming, but it is unknown whether the current northward distribution shift continues into the Chukchi Sea. Here, we use catch data from multiple gear types to present larval, age-0, and older Pacific cod distributions from before (2010 and 2012) and during (2017, 2018, and 2019) recent Chukchi Sea warming events. We also report on the habitat, diet, and condition of age-0 Pacific cod, which were present in the eastern Chukchi Sea in recent warm years (2017 and 2019), but were absent in a cold year (2012). We hypothesize that age-0 recruitment to the eastern Chukchi Sea is associated with recent warm temperatures and increased northward transport through the Bering Strait in the spring. Age-0 fish were present in both benthic and pelagic habitats and diets reflected prey resources at these capture locations. Age-1 Pacific cod were observed in the western Chukchi Sea in 2018 and 2019, indicating possible overwinter survival of age-0 fish, although there was little evidence that they survive and/or remain in the Chukchi Sea to age-2. Observed low lipid accumulation in age-0 Pacific cod from the Chukchi Sea suggests juvenile overwinter mortality may be relatively high compared to more boreal regions (e.g. Gulf of Alaska). Adult Pacific cod were also observed in the Chukchi Sea during 2018 and 2019. Although densities in the western Chukchi Sea were very low compared to the Bering Sea, the adults are the first known (to us) records from the Chukchi Sea. The increased presence of multiple age-classes of Pacific cod in the Chukchi Sea suggests poleward shifts in both nursery areas and adult summer habitat beyond the Bering Sea, but the quantity and quality (e.g. summer productivity and overwintering potential) of these habitats will require continued surveys.

Sand lance or sand eels ( Ammodytes spp. ) are small planktivorous forage fishes that play an integral role in pelagic ecosystems in the Northern Hemisphere. Arctic sand lance ( Ammodytes hexapterus ) is prevalent in the North Pacific in the Sea of Okhotsk, northern Bering Sea, Chukchi Sea, and Beaufort Sea. Few studies have focused on this species despite its critical position in energy transfer and trophic food webs. Recent surveys in the Chukchi Sea and Beaufort Sea have noted an increase in the prevalence of this species in concert with reduced ice extent. Sand lance are unique among forage species in having close associations with specific water column conditions as well as a reliance on specific sand substrates for burrowing. Life history and habitat dependency is influenced by both dynamic oceanographic conditions and static benthic substrata. This taxa, therefore, provides a unique opportunity to examine the potential expansion of a boreal species into Arctic marine habitats. We use comprehensive surveys conducted throughout the Chukchi Sea shelf over multiple years to evaluate spatial distribution and abundance relative to oceanographic variables in the water column and sediment composition on the seafloor. We applied logistic regression and generalized additive models to investigate presence and relative abundance of Arctic sand lance and to evaluate spatial distribution, as a function of oceanographic and benthic environmental variables. Spatial distribution shifted considerably between years in response to environmental conditions. Arctic sand lance presence was influenced by surface water mass and positively associated with Alaskan Coastal Water. Relative abundance was positively associated with high surface temperature, low surface salinity, and coarser substrates. Evidence is mounting that the distributions of many boreal species are expanding on the margins of the Arctic. Our research reports increased aggregations of Arctic sand lance in the Chukchi Sea and at the highest latitude on record for this species, at the shelf break of the Arctic Basin.

Groundfish species in the Bering Sea are undergoing pronounced changes in spatial distribution and abundance due to warming ocean temperatures. The main drivers of interannual variability in this ecosystem are the alternating warm and cold thermal stanzas. Yellowfin sole (Limanda aspera; YFS) and northern rock sole (Lepidopsetta polyxystra; NRS) are commercially-valuable flatfishes in the Bering Sea and are among the most dominant groundfish species there in numbers and biomass. We examined the variability in the spatial distribution and abundance of juvenile NRS and YFS in relation to the ice and temperature conditions associated with warm-cold thermal shifts from 1982 to 2017. The goal was to assess the implications of the fluctuating thermal environment for Bering Sea flatfish production. We found ice cover and bottom temperature indices in the preceding 1 to 3 years to be the best predictors of NRS juvenile distribution. In contrast, these indices were not significantly correlated with YFS juvenile distribution, which could be an artifact of their relatively low availability to sampling. A warm stanza, as the Bering Sea is currently in, is expected to favor high numbers of NRS juveniles and the northward expansion of their distribution.

Northern rock sole recruitment in the eastern Bering Sea has been hypothesized to (a) depend on wind-driven surface currents linking spawning and nursery areas, (b) be density-dependent, and (c) be negatively impacted by cold bottom temperatures over a large nursery area during the first summer of life. A suite of models was developed to test these hypotheses. Data included 32 years of recruitment and spawning biomass estimates derived from a stock assessment model and wind and temperature indices customized to the environmental exposure of age-0 northern rock sole in the eastern Bering Sea. The predictive ability of the models was evaluated, and the models were used to forecast recruitment to age-4 for recent year classes which are poorly retained by the standard multi-species bottom trawl survey gear. Models which included wind and temperature indices performed better than a naive forecast based on the running mean. The best-performing model was a categorical model with wind and temperature thresholds, which explained 49% of the variation in recruitment. Ricker models performed more poorly than models without a spawning biomass term, providing no evidence that recruitment is related to stock size. The models forecast higher recruitment for the most recent year classes (2015-2018) than for prior year classes with observed poor recruitment (2006-2013). These environment-based recruitment forecasts may improve recruitment estimates for the most recent year classes and facilitate study of the effects of future climate change on northern rock sole population dynamics.

Describing essential habitat is an important step toward understanding and conserving harvested species in ecosystem-based fishery management. Using data from fishery-independent ichthyoplankton, groundfish surveys, and commercial fisheries observer data, we utilized species distribution modeling techniques to predict habitat-based spatial distributions of federally managed species in Alaska. The distribution and abundance maps were used to refine existing essential fish habitat descriptions for the region. In particular, we used maximum entropy and generalized additive modeling to delineate distribution and abundance of early (egg, larval, and pelagic juvenile) and later (settled juvenile and adult) life history stages of groundfishes and crabs across multiple seasons in three large marine ecosystems (Gulf of Alaska, eastern Bering Sea, and Aleutian Islands) and the northern Bering Sea. We present a case study, featuring Kamchatka flounder (Atheresthes evermanni), from the eastern and northern Bering Sea to represent the >400 habitat-based distribution maps generated for more than 80 unique species–region–season–life-stage combinations. The results of these studies will be used to redescribe essential habitat of federally managed fishes and crabs in Alaska.

In recent decades, Arrowtooth Flounder (Atheresthes stomias) has been the most abundant groundfish in the Gulf of Alaska and an apex predator with trophic links to many pelagic and benthic species. Its abundance and trophic status implies that a small change in survival may result in substantial uncertainty in the ecosystem, with potentially large effects across multiple species. A synthesis of Arrowtooth Flounder ecology in the Gulf of Alaska was undertaken to determine exposure to the environment during different life history stages, and to develop hypotheses regarding population response to environmental forcing. Historical data sets were used to identify mechanisms of interaction with the pelagic environment during the egg and larval phase, assess habitat utilization and trophic interactions from early settlement through adult life, and evaluate sensitivity and potential response of the population to climate-induced variability in the Gulf of Alaska ecosystem. Modeling approaches include Individual-Based Modeling of the planktonic drift phase from spawning to settlement, Generalized Additive Modeling to examine the effects of location, bottom temperature, and depth on the distribution and density of different size categories of fish, and Habitat Suitability Modeling which integrates presence-absence and environmental data to develop predictive maps of suitable habitat for early juveniles, late juveniles, and adults. A strategy of high endurance characterizes the early ontogeny phase. Spawning and hatching occur during winter in deep water where predation risk is relatively low, and cold temperatures along with intrinsically low metabolic rates ensure extended availability of yolk reserves, lowering the risk of larval starvation in a food-poor environment. Larval duration and drift is protracted, contributing to widespread delivery of larvae to coastal, continental shelf and slope waters throughout the Gulf of Alaska, as well as expected transportation into the Bering Sea through the Aleutian Island Passes. Connectivity between spawning and settlement areas is less directed and juveniles are more ubiquitous across depths than previously understood. Juvenile and adult Arrowtooth Flounder are habitat and prey generalists, with some ontogenetic shifts apparent. Based on this comprehensive ecological synthesis, a preliminary climate-related vulnerability assessment indicates low risk, high resilience overall for this species in the Gulf of Alaska. However, some stage-specific sensitivity is hypothesized primarily relating to the potential for exacerbated temporal mis-match between early larvae and suitable zooplankton prey with increased temperatures. Density-dependent effects during the juvenile to adult stage may constrain further increases in Arrowtooth Flounder biomass in the Gulf of Alaska. This comprehensive ecological approach to assessing environmental sensitivities across life history stages for a commercially and ecologically important fish species has substantial merit for furthering the ecosystem approach to fisheries management, especially in marine ecosystems where there are robust sampling programs across trophic levels.

In 2014, the Bering Sea shifted back to warmer ocean temperatures (+2 oC above average), bringing concern for the potential for a new warm stanza and broad biological and ecological cascading effects. In 2015 and 2016 dedicated surveys were executed to study the progression of ocean heating and ecosystem response. We describe ecosystem response to multiple, consecutive years of ocean warming and offer perspective on the broader impacts. Ecosystem changes observed include reduced spring phytoplankton biomass over the southeast Bering Sea shelf relative to the north, lower abundances of large-bodied crustacean zooplankton taxa, and degraded feeding and body condition of age-0 walleye pollock. This suggests poor ecosystem conditions for young pollock production and the risk of significant decline in the number of pollock available to the pollock fishery in 2-3 years. However, we also noted that high quality prey, large copepods and euphausiids, and lower temperatures in the north may have provided a refuge from poor conditions over the southern shelf, potentially buffering the impact of a sequential-year warm stanza on the Bering Sea pollock population. We offer the hypothesis that juvenile (age-0, age-1) pollock may buffer deleterious warm stanza effects by either utilizing high productivity waters associated with the strong, northerly Cold Pool, as a refuge from the warm, low production areas of the southern shelf, or by exploiting alternative prey over the southern shelf. We show that in 2015, the ocean waters influenced by spring sea ice (the Cold Pool) supported robust phytoplankton biomass (spring) comprised of centric diatom chains, a crustacean copepod community comprised of large-bodied taxa (spring, summer), and a large aggregation of midwater fishes, potentially young pollock. In this manner, the Cold Pool may have acted as a trophic refuge in that year. The few age-0 pollock occurring over the southeast shelf consumed high numbers of euphausiids which may have provided a high quality alternate prey. In 2016 a retracted Cold Pool precluded significant refuging in the north, though pollock foraging on available euphausiids over the southern shelf may have mitigated the effect of warm waters and reduced large availability of large copepods. This work presents the hypothesis that, in the short term, juvenile pollock can mitigate the drastic impacts of sustained warming. This short-term buffering, combined with recent observations (2017) of renewed sea ice presence over southeast Bering Sea shelf and a potential return to average or at least cooler ecosystem conditions, suggests that recent warm year stanza (2014-2016) effects to the pollock population and fishery may be mitigated.

ABSTRACT Temperature strongly influences the growth of crustaceans and directly controls distribution. Despite being commercially important, there are no data on temperature effects upon growth in newly settled southern Tanner crab Chionoecetes bairdi. Recently settled crabs were reared through three consecutive molts, both individually and in groups, at four different temperatures; 2°C, 6°C, 9°C, and 12°C. Survival was higher for crabs grown individually (92%) than those in groups (66%) owing to cannibalism, particularly during molting. There were no temperature effects on survival. Growth rate (mm/day) increased linearly with temperature, a product of exponential decline in intermolt period with increasing temperature and amolt increment that was relatively constant across temperatures. Growth rate, intermolt period, and growth increment did not differ between crabs grown singly or in groups. These data suggest that growth in 0 y aged southern Tanner crabs is strongly inhibited at temperatures less than 2°C. A small mesh beam trawl survey was conducted in the southeast Bering Sea during September 2012, a year when a cold pool (<2°C bottom water) dominated the middle shelf. Southern Tanner crab less than 12 mm (carapace width) were concentrated just north of the Alaska Peninsula and along the outer shelf where bottom water was more than 2°C, but largely absent from the middle shelf. In contrast, adult southern Tanner crabs were widely distributed across the middle and outer shelves. It was hypothesized that early juvenile southern Tanner growth is inhibited by cold pool conditions, prolonging vulnerability to predation, and reducing survival. In the future, if sea temperature rises and sea ice is diminished, less frequent and less intense cold pool events may lead to increased persistence of thermally suitable juvenile southern Tanner crab habitat across the southeastern Bering Sea middle shelf region.

Annual spatial distribution and relative abundance of age-2 and age-3 northern rock sole (Lepidopsetta polyxystra) and summer bottom temperatures were analysed using data from eastern Bering Sea summer trawl surveys from 1982 through 2012. Previously observed differences in age-0 northern rock sole distribution persisted until age-2 and age-3. Latitudinal distributions of age-2 and age-3 fish were correlated most strongly with summer bottom temperatures 2 and 3 years prior to the survey year, during the time that the fish would have been age-0. Thus, temperature during the age-0 year may affect spatial distribution for the first few years of life. Distribution of age-2 and age-3 fish shifted northwards 2 years after the beginning of a warming trend from 1999 to 2003, and shifted southwards 2 years after a cooling trend from 2004 through 2010. Northerly distributions were correlated with high abundances. Density dependence was ruled out as a reason for northward shifts in distribution given a lack of correlation between latitudinal distributions and the annual abundances within the southern part of the distribution. We propose that the large northern nursery area produces large cohorts of northern rock sole, and that bottom temperatures in the age-0 year affect use of the northern nursery area.

Shallow, subtidal waters of coastal embayments are the primary nursery habitats of juvenile Pacific cod through much of their range. However, the importance of these habitats to the Bering Sea population is poorly understood as the Bering Sea offers relatively little of this habitat. In this study, we examined the use of demersal and pelagic habitats in the southeast Bering Sea by age-0 Pacific cod. In 4 years of demersal beam trawling on the shelf at depths of 20-146 m, fish were most abundant along the Alaska Peninsula (AKP) at depths to 50 m. In addition, 1 year of spatially intensive beam trawl sampling was conducted at depths of 5-30 m in a nearshore focal area along the central AKP. In this survey, age-0 cod were more abundant along the open coastline than they were in two coastal embayments, counter to patterns observed in the Gulf of Alaska. Demersal sampling in 2012 was conducted synoptically with surveys of surface and subsurface waters over the continental shelf. Age-0 cod were captured in pelagic waters over the middle and outer shelf, with maximum catches occurring over depths of 60-80 m. The similar size distributions of fish in coastal-demersal and shelf-surface habitats and the proximity of concentrations in the two habitat types suggests that habitat use in the Bering Sea occurs along a gradient from coastal to pelagic. While capture efficiencies may differ among trawl types, trawl-based estimates of age-0 cod density in demersal waters along the AKP was 10 times that observed in the highest density pelagic-shelf habitats, demonstrating the importance of coastal nursery habitats in this population. Despite representing a much smaller habitat area, the coastal waters along the AKP appear an important nursery area and support a significant fraction of the age-0 Pacific cod in the Bering Sea.

Age-0 and age-1 northern rock sole were collected over large-scale areas of the eastern Bering Sea in the summers of 2003, 2008 and 2010. Age-0 presence was poorly predicted by a published resource selection model developed for the Gulf of Alaska, and the failure of that model may have been caused by oceanographic features in the eastern Bering Sea. Where a front (inner front) separated the well-mixed coastal domain from the stratified middle domain, age-0 fish were less abundant and occurred at fewer stations in the nearshore, thermally mixed coastal domain than expected by the Gulf of Alaska model. In contrast, where the inner front was not established, age-0 fish were present in the highest densities in nearshore and thermally mixed waters. North of Unimak Island, the same hydrographic pattern that inhibits the formation of the inner front also likely transports larvae near shore. Age-1 densities were highest in the coastal domain, and age-0 length decreased with distance from shore, suggesting northern rock sole move shoreward after settlement. Juvenile northern rock sole were abundant in a nursery area between Nunivak Island and Cape Newenham in a warm period (2003), but were almost completely absent in cold periods (2008 and 2010), leading to the hypothesis that climate variability limits the utility of this nursery area during cold periods.

Connectivity between spawning and potential nursery areas of northern rock sole, Lepidopsetta polyxystra, in the eastern Bering Sea was examined using an individual-based biophysical-coupled model. Presumed spawning areas were identified using historical field-collected ichthyoplankton data, and nursery habitats were characterized based on previously described settlement areas. Simulated larvae were released from spawning areas near the Pribilof Islands, south of the Pribilof Islands along the outer continental shelf, on the north side of the Alaska Peninsula, and in the Gulf of Alaska south of Unimak Island. Simulated larvae were transported along two general pathways: 1) northwards along the outer continental shelf from Unimak Island towards the Pribilof Islands and further north offshore of mainland Alaska, and 2) eastward along the Alaska Peninsula. At the end of the 2-month simulation, drift pathways placed pre-settlement stage larvae offshore of known nursery areas of older juveniles near mainland Alaska, consistent with a hypothesis that initial settlement may be followed by substantial post-settlement redistribution.

Abstract We investigated the spatial distribution of Atka mackerel Pleurogrammus monopterygius with respect to maturity stage at Seguam Pass in the Aleutian Islands as well as their spawning locations within trawlable areas at three small-scale (10–20-nautical mile or 18.5–37.0-km) sites. Histological methods were used to determine maturity stages, and male maturity stages are described for the first time. Spawning areas were identified by catch per unit effort of demersal egg masses. At Seguam Pass, spawning was concentrated in the area adjacent to the east side of Amlia Island and between Amlia and Seguam islands. The observed spatial segregation of Atka mackerel by sex and maturity stage appears to interact with the boundaries of the trawl exclusion zone (TEZ) at Seguam Pass. During spawning, mature fish aggregate inside the closed area and immature fish and nonreproductively active mature males aggregate outside the TEZ in the area open to fishing. This increased the commercial catch selectivity of nonreproductively active males and immature fish during the September fishery. The spawning areas observed at Seguam and Tanaga passes and near Amchitka Island were within TEZs, which may serve as de facto marine protected areas for spawning Atka mackerel.

Trade-offs in energy allocation between growth and reproduction can result in variations in reproductive potential in fish with differing growth patterns. Spawning biomass is often used as a proxy for reproductive potential on the assumption that fecundity is directly proportional to body weight. We examined variations in the reproductive potential of Atka mackerel Pleurogrammus monopterygius by studying the effect of differential growth and condition patterns on fecundity, atresia, and egg energy. Fecundity and egg energy were determined for fish from two geographic areas, Seguam Pass and Amchitka Island, Alaska, and compared with those of fish held in captivity. These Atka mackerel showed distinct differences in growth and condition, with weight at length and length at age being the highest among captive fish, intermediate among fish from Seguam Pass, and lowest among fish from Amchitka Island. Realized fecundity showed that on average captive fish spawned seven batches, fish from Seguam Pass six batches, and fish from Amchitka Island five batches. For wild fish, potential and realized fecundity at length or age was significantly higher at Seguam Pass than at Amchitka Island, whereas the fecundity-at-weight relationship did not differ by area, suggesting that weight is a better predictor of fecundity than length or age. Atresia and batch fecundity by length or weight did not differ by area, suggesting that the variation in fecundity is better explained by the variation in batch number than by batch size. Oocyte dry weight was higher for captive fish than for wild fish, whereas batch order did not significantly affect oocyte dry weight. Increased potential fecundity, realized fecundity, and oocyte quality in Atka mackerel females were strongly related to body size, indicating that growth differences and maternal feeding success impact the fecundity and oocyte quality of Atka mackerel. Therefore, changes in growth and condition patterns need to be taken into account to accurately estimate the reproductive potential of this species.

Abstract The behavioral ecology of seasonal and ephemeral variations in color patterns of Atka mackerel Pleurogrammus monopterygius was investigated and is discussed relative to alternative mating tactics, reproductive condition, social status, and predation risk. Breeding males underwent a conspicuous seasonal color change during the mating and brooding period, resulting in one of two nuptial phenotypes. Type I males held and defended territories inside nesting colonies and had a uniform yellow coloration with a golden hue across the head and dorsum. Type II males were nonterritorial, hovered above the nesting colony, and attempted periodic forays into the nesting colony; their color was plain yellow with irregular dark blotches across the head and dorsum and a light patch on the nape. When displaced from their nests, type I males also showed dark blotches and a light patch on the nape, but these characteristics were less pronounced than those in type II males. The color of females and nonbreeding males was indistinguishable and exhibited little seasonal variation; however, females close to spawning showed an ephemeral darkening of the body with white spots and patches along the dorsum. Nonbreeding males and females undergoing diel migrations also showed the same pattern across the dorsum but without darkening. The overall mean ratio of males to females in the trawl-sampled population was 1.22:1.00, of which 44.9% were females, 40% were nonbreeding males, 9.8% were intermediate males (with characteristics intermediate between those of breeding and nonbreeding males), and 5.3% were breeding males. The proportion of breeding males was 12 times higher inside nesting colonies than outside the colonies. The mean fork length (FL) of intermediate and breeding males was larger (by ≤1 cm) than that of females or nonbreeding males. Understanding the behavioral ecology of Atka mackerel is important to the development of a comprehensive ecological index for monitoring and assessing the reproductive health of Atka mackerel stocks.

Spatial and temporal variability in maturity at length and maturity at age was examined for female Atka mackerel Pleurogrammus monopterygius in the eastern and central Aleutian Islands of Alaska. Postovulatory follicles (POFs) were evaluated as a potential character to distinguish spent mature fish from immature fish. We estimated that POFs remain in the ovaries for approximately 1 month after spawning, therefore allowing us to include some collections of recently spent fish for maturity estimates. Maturity-atlength data varied significantly among geographic areas and years, whereas maturity-at-age data failed to indicate differences. These results are probably attributable to different growth rates between the areas. An age-based model was developed to account for differences in year-class strengths, and modeling results showed that length at maturity could vary by as much as 4 cm over time because of variable recruitment. Maturity in Atka mackerel seems to be determined more by age than by somatic body size.

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 (similar to 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 degrees C than at 8 degrees 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.