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.

1. Taxa can expand beyond historical scientific survey footprints and into new areas with different survey protocols as they move to track their preferred climate. In global groundfish fisheries, for example, scientists estimate population dynamics within the spatial extent of a fishery-independent survey using an index known as a design-based estimator. Observed changes in species distribution in recent years suggest that some groundfish are moving beyond the spatial extent of single surveys. We must intercalibrate disparate data that cover a larger spatial extent to maintain our ability to accurately index populations as their availability to historical surveys changes. 2. We combine US and Russian data from the northern, eastern and western Bering Sea to understand the proportion of fish biomass within the extent of the eastern survey ('availability'). Surveys are within close proximity to each other, but with different sampling protocols (hence catch a different proportion of local densities, termed 'sampling efficiency ratio'). We use Alaska pollock Gadus chalcogrammus, Pacific cod Gadus macrocephalus and Alaska plaice Pleuronectes quadrituberculatus as case studies to calculate survey efficiency ratios and two area-swept estimators, termed local and conventional, to summarize groundfish biomass over various spatial scales across the Bering Sea. 3. We estimated variation in spatial availability of transboundary stocks to the eastern Bering Sea (EBS) survey. In 2017, the most recent available year of survey coverage that included all three Bering Sea regions, estimated availability in the EBS of pollock biomass was similar to 33%, cod biomass was similar to 27% and plaice biomass was similar to 26%, down from similar to 58%, similar to 71% and similar to 30%, respectively, in 2010. 4. Synthesis and applications. This is the first study to provide an empirical way to combine Russian and US data in the Bering Sea to assess changes in the availability of groundfish biomass, which, in turn, will alter the interpretations and values of population indices used in regional management. We recommend leveraging this approach using existing global fishery-independent datasets that span different spatiotemporal footprints to monitor transboundary stocks, and as a template to initiate international cooperation on the assessment of spatial availability of stocks common to multiple countries.

Estimating fish condition, the relative weight of an individual fish given its body length, is a convenient way to relate the physiological health and energetic status of fishes to their productivity. Despite evidence of density-dependence effects on condition in some species, previous research has not jointly estimated synchronous changes in condition and density operating at fine spatial scales (a few km). Therefore, we developed a spatio-temporal modeling approach that simultaneously estimates correlated variation in density (measured as numbers per area) and condition. We applied our approach to 6 eastern Bering Sea (EBS) groundfish species (4 flatfishes and 2 gadoids) for the period 1992-2016, and estimated correlations in spatial variation (unmeasured variation that is stable over time) and spatio-temporal variation (unmeasured variation that changes between years). Spatial variation in density had a strong significant negative association with spatial variation in condition for 3 flatfishes and a positive association for one gadoid. Spatio-temporal variation in density had a significant association with spatio-temporal variation in condition for one flatfish (negative) and one gadoid (positive). Moreover, for the 6 study species, bottom temperature was identified as an important predictor of both density and condition. The increasing trend in bottom temperatures between 1992 and 2016 was accompanied by an overall increase in the abundance-weighted condition of 5 species. We conclude that forecasts of changes in weight-at-age within some EBS groundfish assessments will require an understanding of both density-dependence and bottom temperature effects on fish condition to better prepare for future climate and exploitation changes.

Life-history traits of individuals in marine populations exhibit large sources of variability. In marine fish, variation of individual size at a given age has three main components: (1) spatial, correlated with the location in which individuals are caught, (2) temporal, correlated with the time when individuals are caught, and (3) generational, correlated with the year of birth of the examined individuals. These variations, if present, have practical implications for individual fitness as well as for sampling, survey design, and population assessment. Disentangling these variations and understanding their sources is hard, given the potentially correlated nature of their effects on individual traits. This study examines the size-at-age relationship of the Bering Sea Pacific cod, an economically and ecologically important groundfish. We used extensive records spanning 1994 to 2016 (inclusive) of 25,213 observations of both environmental variables and catch, lengths, and ages. We found that the average size of individuals of the same age could differ up to 7 cm. Notably, we found that the cohort composition of the sampled population explained >75% of the year effect and that individuals caught in the northwest and shallower portion of the sampling area were on average 5 cm smaller than individuals caught in the southern and deeper portion. We further found that northwest movement of young cod (age 1-5) as a result of warming places individuals in areas where we predict them to have smaller size at age. Smaller and less conditioned individuals are less fecund and may not be able to perform long migrations to return to their distant spawning grounds. Both the spatial distribution and water temperature experienced by Pacific cod in the Bering Sea are changing, and this study provides a mechanism for how these changes affect Pacific cod life-history traits and individual fitness.

The climate regime in the eastern Bering Sea has recently been dominated by a pattern of multi-year stanzas, in which several successive years of minimal sea-ice formation and warm summer temperatures (e.g., 2002–2005, 2014–2017) alternate with several years of relatively extensive sea-ice formation and cold summer temperatures (e.g., 2006–2013). This emerging climate pattern may be forcing long-term changes in the spatial distributions of the Bering Sea’s marine fauna. The National Marine Fisheries Service’s Alaska Fisheries Science Center recently conducted two bottom trawl surveys covering the entire Bering Sea shelf from the Alaska Peninsula to the Bering Strait. The first, in the summer of 2010, was conducted during a cold year when the majority of the continental shelf was covered by a pool of cold (< 2 °C) water. The second, in the summer of 2017, was during a warmer year with water temperatures above the long-term survey mean. These two surveys recorded significantly different spatial distributions for populations of several commercially important fish species, including walleye pollock (Gadus chalcogrammus), Pacific cod (Gadus macrocephalus), and several flatfish species, as well as jellyfishes. Population shifts included latitudinal displacement as well as variable recruitment success. The large-scale distributional shifts reported here for high-biomass species raise questions about long-term ecosystem impacts, and highlight the need for continued monitoring. They also raise questions about our management strategies for these and other species in Alaska’s large marine ecosystems.

Scientific surveys are widely used for stock assessment, but the estimated population parameters are based on the size-at-age relationship and age structure derived from a small subsample of the catch that is aged. This calls for an assessment of subsampling strategies, especially when population's life history traits are spatially structured. In the Eastern Bering Sea, Pacific cod (Gadus macrocephalus) size and age are spatially structured, with younger and smaller individuals being more abundant at shallower depths. We conducted parallel subsamplings during Pacific cod surveys to compare two contrasting subsampling strategies: length-stratified and random. Geographical heterogeneity of Pacific cod length resulted in divergent estimates of ages between subsampling strategies. When this spatial variability was taken into account to estimate population parameters, random strategy provided more accurate mean and modal size-at-age and estimated age structure. Bias in the length-stratified subsampling arises from the poor efficacy in capturing the geographical patterns of size observed in the population. However, combining age data samples from multiple years helps to minimize the divergences between the two strategies.

Availability of yellowfin sole Limanda aspera to the National Marine Fisheries Service eastern Bering Sea trawl survey, rather than trawl sampling efficiency, is proposed as the primary reason for relatively high annual variability of biomass estimates in this region, including most recently, a 48% increase from 2015 to 2016. The main hypothesis presented here is that temperature-mediated differences in the timing of spring-summer spawning migrations to unavailable nearshore spawning grounds, affect survey biomass estimates. Colder bottom temperatures delay both migrations and spawning, causing higher proportions of mature individuals to reside in the unavailable nearshore grounds at the time of annual survey (June–July). Indicators of this scenario include decreases of mature fish proportions and decreases in mean overall fish lengths during colder years when biomass was less than expected. Further evidence includes differences in spatial distribution between warm and cold years, and spatial shifts away from nearshore areas between early June and July–August sampling during which catch per unit effort (CPUE) increased and proportion of females increased. That neither of these spatial shifts nor temperature-CPUE relationships occurred for northern rock sole Lepidopsetta polyxystra, a species of similar morphology and abundance, and overlapping spatial distribution, suggests that temperature-mediated trawl sampling efficiency was not a major contributing factor for yellowfin sole. We have also found a positive relationship between survey biomass estimates and survey start times, reinforcing that availability is a function of timing. The addition of survey start time to the catchability (q) parameter within the current stock assessment model significantly improved model fits to abundance data.

Ocean currents, water masses, and seasonal sea ice formation contribute to determining relationships among the biota of the Bering and Chukchi seas. The Bering Sea communicates with the Chukchi Sea via northward advection of water, nutrients, organic matter, and plankton through Bering Strait. We used data from concurrent surveys of zooplankton, pelagic fishes and jellyfish, epibenthic fishes and invertebrates, and seabirds to identify faunal distribution patterns and environmental factors that are related to these faunal distributions within the US portions of the Chukchi Sea shelf and Bering Sea shelf north of Nunivak Island. Regional differences in late summer (August-September) distributions of biota largely reflected the underlying hydrography. Depth, temperature, salinity, stratification, and chlorophyll a, but less so sediment-related or nutrient-related factors, were related to the distributions of the assemblages (zooplanlcton: depth, salinity, stratification; pelagic fishes and jellyfish: depth, stratification, chlorophyll a; epibenthic fishes and invertebrates: depth, temperature, salinity; seabirds: temperature, salinity, stratification). These six environmental factors that most influenced distributions of zooplankton, pelagic fishes/jellyfish, epibenthic fishes and invertebrate, and seabird assemblages likely can be simplified to three factors reflecting bottom depth, water mass, and their stratification and productivity (which are tightly linked in the study region). The assemblages were principally structured from nearshore to offshore and from south to north. The nearshore to offshore contrast usually was stronger in the south, where the enormous discharge of the Yukon River is more apparent and extends farther offshore, influencing zooplankton, pelagic fish/jellyfish, and seabird assemblages. Some assemblages overlapped spatially (e.g., seabird and zooplankton), indicating shared influential environmental factors or trophic linkages among assemblages. The gradients in assemblage composition were gradual for epibenthic taxa, abrupt for zooplankton taxa, and intermediate for pelagic fish/jellyfish and seabird taxa, implying that zooplankton assemblage structure is most strongly tied to water mass, epibenthic least, with the other two taxa intermediates. Three communities (i.e., cross-assemblage groupings) emerged based on maps of ordination axes and core use areas by taxa; one associated with Alaska Coastal Water (warmer, fresher, nutrient depauperate), second associated with Chirikov Basin and the southern Chukchi Sea (colder, saltier, nutrient rich), and third associated with the northern Chukchi shelf (colder and saltier but not as nutrient rich). Gradients in species composition occurred both within and between these communities. The Chirikov Basin/southern Chukchi Sea community was characterized by distinct zooplankton and seabird taxa, but was not strongly associated with distinct pelagic or epibenthic fish and invertebrate taxa. Although comprehensive data were only available for a single year and annual variation may affect the generality of our results, our comprehensive ecosystem survey approach yielded new insights into the ecological relationships (specifically, gradients in assemblage composition and identification of communities) of this Arctic region. Published by Elsevier Ltd.

Selectivity studies have found applications in a wide range of topics within fishery science, such as fishery management, stock assessment, and ecological process studies. However, obtaining selectivity functions can often be a difficult and costly endeavor. Because of this difficulty, many studies are limited to the comparison of catch from two fishing gears, where relative differences in catch efficiency are often presented in the form of catch comparison rate or catch ratio. Studies of these rates are well known to often improve commercial fisheries, which benefit from highly selective gears. However, utility of these statistics for the purposes of fisheries surveys and stock assessment is not very well understood. In this study we adapted methods previously used for catch ratio to obtain length-dependent selectivity ratio function for two survey gears. Selectivity ratio can be obtained when area-swept or volume-swept fish density estimates are available from both gears. In other cases it is possible to obtain relative selectivity ratio. We present a general approach to obtain selectivity ratio in survey gear comparison studies and model it using three alternative techniques (linear and smooth mixed effect, and beta-regression). We use crossvalidation to choose between alternative models. We present examples of practical application of selectivity ratio with three case studies: a comparison of fine-and large mesh bottom trawls used in Arctic surveys, a study testing an assumption of non-selectivity of the Nephrops bottom trawl for snow crab in the Bering Sea, and a comparison of two survey midwater trawls for pollock in the Bering Sea. We show that selectivity ratio statistics can be used as a generalization of selectivity studies, where one gear is non-selective, as well as in catch comparison studies where selectivity of both gears is unknown.

This study uses a 30-year time series of standardized bottom trawl survey data (1982–2011) from the eastern Bering Sea shelf to model patterns of summer spatial distribution for various bottom fishes and crabs in response to changes in the areal extent of the cold pool, time lag between surveys, and fluctuations in population abundance. This investigation is the first to include data for the 2006–2010 cold period and to use between-year comparisons of local and shelf-wide spatial indices to test specific responses to three different isothermal boundaries within the cold pool. Distributional shifts in population varied considerably among species and directional vectors for some species were greater in magnitude to the east or west than to the north or south; however, in general, eastern Bering Sea shelf populations shifted southward in response to the increasing cold pool size, and after accounting for differences in temperature and population abundance, there was still a temporal northward shift in populations over the last three decades despite the recent cooling trend. Model results for local and shelf-wide indices showed that survey time lag and cold pool extent had a greater effect on spatial distribution than population abundance, suggesting that density-independent mechanisms play a major role in shaping distribution patterns on the eastern Bering Sea shelf. The area enclosed by the 1°C isotherm most commonly affects both local and shelf-wide spatial indices suggesting that 1°C is a more important boundary for describing temperature preferences of eastern Bering Sea bottom fishes and crabs than is the 2°C isotherm used for designating the physical boundary for the cold pool.

Examination of invertebrate vouchers collected during bottom trawl surveys conducted by the National Oceanic and Atmospheric Administration's National Marine Fisheries Service revealed northern range extensions of three decapod species. Two of these species are caridean shrimps (Lebbeus washingtonianus (M. J. Rathbun, 1902) (Hippolytidae) and Systellaspis braueri paucispinosa Crosnier, 1987 (Oplophoridae)) and represent the first records in Alaska. The lithodid crab Neolithodes diomedeae (J. E. Benedict, 1895) was found off California, the first record in U.S. waters, extending its known range by some 2000 km. Biological notes for these three species are provided and morphometric measurements are listed for Neolithodes diomedeae.

Latitudinal species diversity gradients are well known in both terrestrial and aquatic ecosystems throughout the world. However, trends in relative abundance and other shifts in community structure with latitude, which can be more sensitive to environmental shifts such as climate change, have received less attention. Here we investigate latitudinal trends in the seafloor community of the eastern Bering Sea using catches of fishes and epibenthic invertebrates in bottom trawl surveys conducted from 1982 to 2010. Our results indicate that the overall biomass of the epibenthic community declines with increasing latitude in the eastern Bering Sea. This latitudinal trend is primarily driven by declining fish catches in the northern Bering Sea, which in turn reflects changes in the structure of the fish community. The fish fauna in northern latitudes is increasingly dominated by gadids, though the species composition of the gadid fauna also changes with latitude, with smaller species becoming more common in the north. The biomass of the invertebrate megafauna remains relatively consistent throughout the eastern Bering Sea, but invertebrates make up a larger proportion of the catch in bottom trawls conducted at higher latitudes. The epibenthic invertebrate megafauna in the eastern Bering Sea is composed primarily of sea stars (Asteriidae) and oregoniid crabs (Chionoecetes and Hyas), though no clear latitudinal trends in the invertebrate community are evident. Limited trawl data from the eastern Chukchi Sea indicate that the fish community farther north is even more heavily dominated by gadids, and the epibenthic invertebrate community is dominated by asteriid sea stars. Temperature data from bottom trawl surveys in the southeastern Bering Sea over the past decade indicate that there was a distinct temperature shift around 2005, and the relatively warm years of 2001–2005 were followed by five relatively cold years. This shift in the summer temperature regime of the Bering Sea has resulted in lower fish catches, particularly in the “cold pool” region (58–61°N), and a higher proportion of epibenthic invertebrates in the bottom trawl catches of the past 5 years.

Ocean currents, water masses, and seasonal sea ice formation determine linkages among and barriers between the biotas of the Bering, Chukchi, and Beaufort Seas. The Bering Sea communicates with the Chukchi and Beaufort Seas via northward advection of water, nutrients, and plankton through Bering Strait. However, continuity of the ocean's physical properties is modulated by regional differences in heat, salt, and sea ice budgets, in particular, along the meridional gradient. Using summer density data from zooplankton, fish (bottom and surface trawl), and seabird surveys, we define three biogeographic provinces: the Eastern Bering Shelf Province (the eastern Bering Sea shelf south of Saint Lawrence Island), the Chirikov-Chukchi Province (the eastern Bering Sea shelf north of Saint Lawrence Island [Chirikov Basin] and Chukchi Sea), and the Beaufort Sea Province. Regional differences in summer distributions of biota largely reflect the underlying oceanography. Climate warming will reduce the duration and possibly the extent of seasonal ice cover in the Eastern Bering Shelf Province, but this warming may not lead to increased abundance of some subarctic species because seasonal ice cover and cold (< 2 degrees C) bottom waters on the Bering shelf form a barrier to the northward migration of subarctic bottom fish species typical of the southeastern Bering Sea. While Arctic species that are dependent upon the summer extent of sea ice face an uncertain future, other Arctic species' resilience to a changing climate will be derived from waters that continue to freeze each winter.

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.

Understanding the spatial and bathymetric extent of the reproductive habitat of Atka mackerel Pleurogrammus monopterygius is basic and fundamental information for managing and conserving the species. From 1998 to 2004, scuba diving and in situ and towed underwater cameras were used to document reproductive behavior of Atka mackerel and to map the geographic and depth ranges of their spawning and nesting habitat in Alaska. This study extended the geographic range of nesting sites from the Kamchatka Peninsula to the Gulf of Alaska, and extended the lower depth limit from 32 to 144 m. Male Atka mackerel guarding egg masses were observed during October—indicating that the duration of the nesting period in Alaska is more protracted than in the western Pacific. Results from this study also suggest that nearshore nesting sites constitute only a fraction of the nesting habitat and that there is no concerted nearshore spawning migration for Atka mackerel in Alaska. Nesting sites were widespread across the continental shelf and found over a much broader depth range than in the western Pacific. Nesting habitat was invariably associated with rocky substrates and water currents; however, smaller-scale geomorphic and oceanographic features as well as physical properties of the rocky substrate were variable between different island groups and nesting sites. Water temperatures for nesting sites ranged from 3.9°C to 10.5°C. Water temperatures within nesting sites varied little and did not appear to be limiting the upper or lower depth boundaries of nesting. Results from dive transects showed significantly fewer egg masses above 20 m water depth. Other possible factors limiting the upper or lower depth limit of nesting sites are discussed. Authors: robert r. LAuth, scott W. Mcentire, and hAroLd h. Zenger Jr. are with the National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Alaska Fisheries Science Center, Resource Assessment Conservation Engineering Division, 7600 Sand Point Way NE, Seattle, WA 98115. Email: Bob.Lauth@noaa.gov Acknowledgements: This project was funded by grants from the North Pacific Research Board (Number F0417) and the National Marine Fisheries Service Essential Fish Habitat fund. The authors thank the skippers and crew from all the chartered vessels. The principal author would especially like to thank Kevin Bell, Jeff Williams, and Vernon Byrd of U.S. Fish and Wildlife Service for their encouragement, the vessel time aboard the R/V Tîglâx, and sharing their enthusiasm and knowledge of the Aleutian Islands. Also extremely accommodating were Phil Deng, Tim Meintz, and the rest of the crew aboard the F/V Seafisher. Many thanks to all at the Alaska Fisheries Science Center who were very generous and supportive with their time and resources, including Lowell Fritz, Russ Nelson, Gary Stauffer, Susanne McDermott, Libby Logerwell, Kim Rand, Dan Cooper, Frank Wood, John Lowell, and Tim Clancy. Those who reviewed this manuscript and made many valuable suggestions for improving it include Dave Somerton, Libby Logerwell, Susanne McDermott, Sandra Lowe, Gary Duker, Jim Lee, Douglas Eggers, and two anonymous reviewers. INtRoDuctIoN Atka mackerel Pleurogrammus monopterygius is a gregarious, semi-pelagic and semi-demersal hexagrammid that is distributed in the continental shelf regions across the North Pacific Ocean and Bering Sea from Asia to North America. On the Asian side their distribution extends from the Kurile Islands to the Gulf of Anadyrskiy (Rutenberg 1962). From Kamchatka, they extend eastward through the Komandorskiye (Rutenberg 1962) and Aleutian Islands (Zenger 2004), north to the Pribilof Islands in the eastern Bering Sea (Acuna and Kotwicki 2004), and eastward through the Gulf of Alaska to southeast Alaska (Lowe et al. 2005). Their center of abundance is in the central and western Aleutian archipelago where a directed commercial trawl fishery operates (Lowe et al. 2004). Atka mackerel is a key prey item for marine fishes, birds, and mammals, including the endangered Steller sea lion (Murie 1959, Kenyon 1965, Merrick et al. 1997, Yang 1999, Sinclair and Zeppelin 2002, Dragoo et al. 2004). Atka mackerel are obligate demersal spawners. Females lay adhesive eggs on rocky substrate and males guard the nests to protect eggs against predation and cannibalism (Zolotov 1993). Locations of spawning and nesting grounds within Alaska are unknown. The only published account of an Atka mackerel spawning site in Alaska is by Turner (1886). Turner (1886) noted that spawning Atka mackerel, when observed from the water’s surface, appeared to form several strata with the least mature fish in the top layer and spawning

The timing and duration of the reproductive cycle of Atka mackerel (Pleurogrammus monopterygius) was validated by using observations from time-lapse video and data from archival tags, and the start, peak, and end of spawning and hatching were determined from an incubation model with aged egg samples and empirical incubation times ranging from 44 days at a water temperature of 9.85 degrees C to 100 days at 3.89 degrees C. From June to July, males ceased diel vertical movements, aggregated in nesting colonies, and established territories. Spawning began in late July, ended in mid-October, and peaked in early September. The male egg-brooding period that followed continued from late November to mid-January and duration was highly dependent on embryonic development as affected by ambient water temperature. Males exhibited brooding behavior for protracted periods at water depths from 23 to 117 in where average daily water temperatures ranged from 4.0 degrees to 6.2 degrees C. Knowledge about the timing of the reproductive cycle provides a framework for conserving Atka mackerel populations and investigating the physical and biological processes influencing recruitment.

Atka mackerel (Pleurogrammus monopterygius) is hexagrammid fish that inhabits the temperate and subarctic North Pacific Ocean and neighboring seas (Fig. 1). This highly abundant fish is a critically important prey species (Sinclair and Zeppelin, 2002; Zenger, 2004) that supports a directed commercial trawl fishery (Lowe et al., 2006). Atka mackerel is a demersal spawner and males provide parental care to eggs (Zolotov, 1993). During breeding periods, sexually mature males aggregate on the bottom at nesting sites where they establish territories (Lauth et al., in press). Sexually mature females periodically visit male nesting territories from July to October to spawn batches of demersal egg masses (McDermott and Lowe, 1997; McDermott et al., 2007). Individual nests may consist of multiple egg masses deposited by different females, and males defend nesting territories for a protracted period lasting from the time territories are being established until all eggs within the territory are completely hatched (Lauth et al., 2007). Knowledge about the timing of the reproductive cycle and the use of spawning habitat are important for understanding population structure and the dynamics of stock recruitment, which in turn are important factors in the management of Atka mackerel populations.

Data from a video camera sled and research survey trawl were used to estimate size-specific trawl selectivity for Sebastolobus spp. Sizes from the camera sled video were extracted using an oblique grid plane and image analysis software. Thornyhead mean densities were 3–5 times higher with the camera sled than the survey trawl. Experimental selectivity patterns failed to conform to traditional parametric selectivity functions so a new non-parametric model was derived. The estimates of catchability for 20–25cm thornyheads were 0.25–0.75. Catchability estimates for thornyheads larger than 30cm were much lower (<0.10). A reason for low catchability at larger sizes remains unclear but may be a size-dependent interaction with the trawl, an artifact caused by low sample size of large fish in the study area, an unresolved bias in the video measurement system, or any combination of these factors.

Numerous trawl hauls, made during the triennial bottom trawl surveys (1977-1998) conducted by the National Marine Fisheries Service off the U.S. West Coast, had unusually small catch rates of benthic fish and invertebrates (cpue(B)), probably because the trawl failed to contact the seabed (off-bottom). Technological advances in the equipment used to monitor trawl performance since 1986 have increased our ability to recognize off-bottom tows, and cpue(B) has risen. As direct trawl performance measurements were not available in earlier surveys, a minimum cpueB derived from the survey with the best monitoring of bottom contact of the time-series (1998) was used as a criterion to eliminate trawls with poor bottom contact from earlier surveys. The truncated data sets produce significantly larger biomass indices, especially in 1980, with increases of 43, 45, and 56% for Dover sole, petrale sole, and Pacific sanddab, respectively. The analysis suggests that changes in cpue(B) over the time-series may be related more to changing survey fishing methods than to changes in abundance. Other bottom trawl surveys, which have also added trawl monitoring equipment during their time-series, may have experienced similar changes in trawl performance. Published by Elsevier Ltd on behalf of International Council for the Exploration of the Sea.