Pacific Halibut Research Articles

The use of artificial illumination to reduce Pacific halibut (Hippoglossus stenolepis) bycatch for a high-rise bottom trawl in the U.S. West Coast groundfish fishery

In the last decade, there has been increasing interest in the use of artificial illumination as a bycatch reduction device. In the U.S. West Coast groundfish bottom trawl fishery, research has found that the addition of green light-emitting diode (LED) lights to the upper bridles of low-rise cutback trawls significantly reduced Pacific halibut (Hippoglossus stenolepis) bycatch. Recent regulation changes in this fishery now permit high-rise trawls, a gear configuration with a higher vertical opening, in areas where only low-rise trawls were previously permitted. This study conducted catch comparison and catch ratio analyses to determine if catches of Pacific halibut and three commercially important groundfishes (e.g., petrale sole [Eopsetta jordani], Dover sole [Microstomus pacificus], and sablefish [Anoplopoma fimbria]) differ between illuminated and non-illuminated tows for a high-rise bottom trawl. Illuminated tows caught fewer individuals than the non-illuminated tows across all species, including Pacific halibut; however, the difference in catch efficiency was not significant. Total catch volume did have a significant positive effect on levels of glucose and lactate for Pacific halibut. However, no statistically significant differences between illuminated and non-illuminated tows were exhibited across all of the physiological parameters assessed. The results from our study provide valuable information to fishers and managers that can be used for future decision-making and identifying research priorities.

Biological characteristics of the Pacific halibut in the Bering Sea and off the eastern coast of Kamchatka in spring 2022

The purpose of the work: to give a biological characteristic of the Pacific halibut in different areas of the Bering Sea and off the eastern coast of Kamchatka and to analyze its size age composition in catches of trawl fishing. Methods used: the material was collected during the expedition research, a complete biological analysis of the Pacific halibut was carried out, the otoliths were selected to determine the age. The age was determined in accordance with the guidelines developed specifically for the Pacific halibut in the International Commission on Pacific Halibut (International Pacific Halibut Commission, IPHC).Novelty: the elements of novelty are generalized and analyzed data on Pacific halibut. The material collected in the waters of Eastern Kamchatka and the Bering Sea in 2022. Pacific halibut from trawl catches have been studied, its biological characteristics have been considered, the size-age composition and growth rate have been analyzed.Practical significance: the results of these studies can be used in forecasting the stocks of Pacific halibut in this area.Result: data on catches of Pacific halibut in different regions of the North West Pacific Ocean, the size and age composition, a comparative characteristic of males and females by growth rate, the age composition of fish by maturity stages.

Environmental conditions on the Pacific halibut fishing grounds obtained from a decade of coastwide oceanographic monitoring, and the potential application of these data in stock analyses

Context Establishing baseline environmental characteristics of demersal fish habitat is essential to understanding future distribution changes and to identifying shorter-term anomalies that may affect fish density during monitoring efforts. Aims Our aim was to synthesise environmental data to provide near-bottom oceanographic baseline information on the Pacific halibut fishing grounds, to establish geographic groupings that may be used as co-variates in fish-density modelling and to identify temporal trends in the data. Methods Water-column profiler data were collected from 2009 to 2018 along the North American continental shelf, during summer fishery surveys focused on Pacific halibut. Key results In addition to establishing baseline information on the fishing grounds, this analysis illustrated that environmental variables can be grouped geographically into four regions that correspond to the four biological regions established by the International Pacific Halibut Commission. A spatio-temporal modelling approach is presented as an example of how to describe the relationship between environmental data and Pacific halibut distribution. Conclusions This study has highlighted the efficacy of environmental data in analysing fish distribution and density changes. Implications Oceanographic monitoring provides the ability to detect annual anomalies such as seasonal hypoxic zones that may affect fish density and to establish baseline information for future research.

Relating capture and physiological conditions to viability and survival of Pacific halibut discarded from commercial longline gear

Mortality of fish discarded from commercial fisheries is often estimated by using viability keys to parse fish into different categories with unique estimated survival outcomes. In this study we examined the relationships of environmental and physiological parameters and viability classifications of Pacific halibut (Hippoglossus stenolepis) released from commercial longline gear using three distinct hook-release methods. Our results indicate that the hook release method used strongly influenced viability, with careful shake and gangion cut resulting in minimal injuries leading to discarded fish characterized as having excellent viability. In contrast hook stripping resulted in most released fish having moderate or poor viability. Physiological parameters (lactate, hematocrit) were strong indicators of Pacific halibut assigned to the dead viability category. In addition to being captured at greater depth and lower sea bottom temperature, dead categorized fish were associated with high plasma lactate levels and lower hematocrit, attributed to sand flea intrusion. Reducing the use of hook strippers and limiting soak times in areas of known sand flea activity are likely to improve viability outcomes of Pacific halibut released from commercial longline gear. Discard survival and understanding of the factors affecting it remain critical to successful long-term management of Pacific halibut, as sustainable fishing levels are adjusted yearly for estimated mortality consistent with recent fishing discards.

Endocrine patterns associated with ovarian development in female Pacific halibut (Hippoglossus stenolepis)

The Pacific halibut (Hippoglossus stenolepis) is a large migratory demersal flatfish species that occupies a top trophic role in the North Pacific Ocean and Bering Sea ecosystems, where it also supports various fisheries. As a first attempt to characterize the endocrine mechanisms driving sexual maturation in this important species, we collected pituitary, ovarian and blood samples from Pacific halibut females captured in the wild that were classified histologically into various female developmental stages. We conducted gene expression analyses of gonadotropin beta subunits in the pituitary and observed that mRNA expression levels of fshb gradually increased throughout vitellogenesis, remained elevated until before ovulation and declined after spawning. In contrast, the mRNA expression levels of lhb markedly increased during oocyte maturation and remained elevated until after spawning. Ovarian mRNA expression levels of the gonadotropin receptor genes fshr and lhr peaked during oocyte maturation and before spawning, respectively, immediately following the developmental stage at which pituitary fshb and lhb mRNA expression first reached maximum levels. The ovarian gene expression patterns of steroidogenic enzyme genes cyp19a1 and hsd20b2 paralleled those of fshr and lhr, respectively. Testosterone and 17β-estradiol (E2) plasma levels increased concomitantly with fshr and cyp19a1 mRNA expression levels, and vitellogenin plasma levels increased throughout vitellogenesis and reached maximum levels prior to spawning. These results are consistent with the notion that in female Pacific halibut, as in other teleosts, vitellogenesis and oocyte maturation and ovulation are likely under the control of pituitary gonadotropic hormones Fsh and Lh, respectively.

Assembly of the poorly differentiated Verasper variegatus W chromosome by different sequencing technologies

The assembly of W and Y chromosomes poses significant challenges in vertebrate genome sequencing and assembly. Here, we successfully assembled the W chromosome of Verasper variegatus with a length of 20.48 Mb by combining population and PacBio HiFi sequencing data. It was identified as a young sex chromosome and showed signs of expansion in repetitive sequences. The major component of the expansion was Ty3/Gypsy. The ancestral Osteichthyes karyotype consists of 24 protochromosomes. The sex chromosomes in four Pleuronectiformes species derived from a pair of homologous protochromosomes resulting from a whole-genome duplication event in teleost fish, yet with different sex-determination systems. V. variegatus and Cynoglossus semilaevis adhere to the ZZ/ZW system, while Hippoglossus stenolepis and H. hippoglossus follow the XX/XY system. Interestingly, V. variegatus and H. hippoglossus derived from one protochromosome, while C. semilaevis and H. stenolepis derived from another protochromosome. Our study provides valuable insights into the evolution of sex chromosomes in flatfish and sheds light on the important role of whole-genome duplication in shaping the evolution of sex chromosomes.

Spatial restrictions hinder avoidance of choke species in an Indigenous rights‐based fishery

Abstract Nutrient‐rich waters along the Pacific coast of North America support diverse fish communities that have helped sustain coastal peoples for millennia. Five Nuu‐chah‐nulth First Nations on the west coast of what is now known as Vancouver Island, Canada, hold constitutional Indigenous rights to conduct a multispecies community fishery, which includes Pacific Halibut (Hippoglossus stenolepis). A 2009 court decision defined the extent of these Indigenous rights to be within 9 nm of the coast, thereby not fully recognizing the knowledge and authority of the traditional leadership and raising concerns about the potential for an increase in rockfish bycatch. Yelloweye Rockfish (Sebastes ruberrimus) are a potential ‘choke’ species for this fishery because the two species occupy similar depth ranges. A choke species is one that is caught incidentally while targeting other species and, if caught in excess of its quota limits, can trigger a halt to fishing on the target species. Guided by the insights of local Indigenous peoples and using both fishery‐independent survey and commercial longline catch data, we investigated the effects of fishing depth and spatial restriction on the relative catch weights of these two species using spatiotemporal models. We find evidence that a confined fishing area can limit opportunities for avoiding choke species. Specifically, fishing at depths deeper than 175 m, which occur outside the court defined area (CDA), would provide more opportunities for catching halibut while avoiding Yelloweye Rockfish than are currently available within the CDA. This Indigenous‐informed, analytical approach to a management problem is just one example of how Western scientists can engage in coproduction of knowledge with Indigenous peoples to transition from the ‘status quo’ towards a practice of ‘Two‐Eyed Seeing’ that more effectively balances Indigenous rights and species conservation. Policy implications: Our study highlights (1) the importance of considering choke species distributions and opportunities for their avoidance when implementing spatial harvest restrictions and (2) how related analytical and management decisions can benefit from being guided by the advice of Indigenous knowledge holders. Read the free Plain Language Summary for this article on the Journal blog.

Functional characterization of obscure puffer ToNK-lysin: A novel immunomodulator possessing anti-bacterial and anti-inflammatory properties

NK-lysins are one of the most abundant antimicrobial peptides produced by cytotoxic T lymphocytes (CTLs) and natural killer cells (NKs), and identified as a new class of intrinsically disordered proteins, playing critical roles in the cell-mediated cytotoxicity response, as well as immunomodulatory and antimicrobial activities upon a significant range of pathogens. In the present study, an NK-lysin was identified from Obscure puffer Takifugu obscurus (ToNK-lysin). The open reading frame of ToNK-lysin sequence spans 423 bp, encoding a peptide with 140 amino acids which shares a moderate residue identity (18%–60%) with NK-lysin of mammals and other teleost species. Phylogenetic analysis revealed that ToNK-lysin was most closely related to NK-lysins from the Pleuronectiformes (Bastard halibut Paralichthys olivaceus and Pacific halibut Hippoglossus stenolepis). Comprehensive computational analysis revealed that ToNK-lysin have substantial level of intrinsic disorder, which might be contribute to its multifunction. The transcripts of the ToNK-lysin were detected in multiple examined tissues and most abundant in gills. After bacterial and Poly I:C challenge, the transcriptional levels of ToNK-lysin were significantly up-regulated in the head kidney, liver and spleen at different time points. The recombinant ToNK-lysin showed significant antibacterial activity against Vibrio harveyi and Escherichia coli, and the ToNK-lysin treatment not only reduced the bacterial loads in liver and head kidney, but also alleviated the pathogen-mediated upregulation of immune-related genes. In addition, the co-incubation with rToNK-lysin protein remarkably degraded bacterial genomic DNA, suggesting the potential mechanism of ToNK-lysin against microbes. These results suggest that ToNK-lysin possess antibacterial and immunoregulatory function both in vivo and in vitro, which may allow it a potential applicability to the aquaculture industry.

Examining the evolution of access to Alaska's halibut IFQ fishery

In fishing communities, livelihoods and well-being depend on sustaining access to key fisheries through changes in natural resource management. In Alaska, the rationalization of the commercial fishery for Pacific halibut (Hippoglossus stenolepis) in 1995 led to the consolidation of the halibut fleet. The high cost of halibut catch shares have since become a crucial barrier to prospective entrants, especially small-scale operations with few options for portfolio diversification. However, quantitative approaches to understanding that barrier face an information gap: datasets on harvest and catch share ownership in fisheries lack common identifiers for individuals. We match individuals across harvest and catch share ownership data from 1991 through 2019, enabling a detailed examination of entrants and non-entrants – those who acquire or do not acquire halibut catch shares over the time series. We compare fisheries portfolios in terms of participation and earnings through duration, dissimilarity, and network analyses. Differences over time and between entrants and non-entrants emerge across analyses. For both groups, cohorts of participants shrink and real individual earnings increase over the time series. However, entrants' cohorts have decreased further relative to historical participation, while entrants' real earnings and fisheries portfolio compositions have diverged from those of non-entrants. Our results reveal broad differences in Alaska fisheries participants' access to a critical fishery, underscoring the role of catch shares in shaping fishing communities’ opportunities and resilience in the face of social and environmental change.

Testing of hook sizes and appendages to reduce yelloweye rockfish bycatch in a Pacific halibut longline fishery

In Pacific halibut (Hippoglossus stenolepis) longline fisheries in the eastern North Pacific Ocean bycatch of yelloweye rockfish (Sebastes ruberrimus) is a concern as their stock status along the U.S. West Coast is “rebuilding” from being “overfished”, the southeast Alaska stock has shown a ∼60% decline since at least 1994 and through 2015 where it stabilized, and the Canadian stock has been recently declared “threatened”. In this study, we evaluated how size 16/0 and 18/0 circle hooks affect the catch efficiency of Pacific halibut and yelloweye rockfish. Further, we examined the catch efficiency of these hooks modified with a 3.1 mm stainless-steel wire appendage extending 7.6 cm from their shank at either a 45° or 90° angle. We estimated hooking location probabilities for Pacific halibut and yelloweye rockfish for the hooks tested, and tested for a difference in the time of capture between Pacific halibut and yelloweye rockfish. Results showed that hook size did not significantly affect the catch efficiency of Pacific halibut or yelloweye rockfish. However, hooks with a 45° appendage angle caught significantly fewer yelloweye rockfish than hooks without an appendage, irrespective of hook size. Appendage angle did not affect the catch efficiency of Pacific halibut. For both Pacific halibut and yelloweye rockfish, the most frequent hooking location was hook through cheek, both with and without an appendage. Time of capture of Pacific halibut and yelloweye rockfish did not differ over the duration of a set; however, the majority (75%) of individuals were caught within 2.5 h of gear deployment. Results from our study suggest that hook appendages could have potential use in reducing catch rates on yelloweye rockfish in Pacific halibut longline fisheries, which could lead to increased fishing opportunities, more efficient Pacific halibut fisheries and less effect of fluctuations in the more productive Pacific halibut stock on fisheries that may be constrained by yelloweye rockfish.

Drivers and Potential Consequences of Observed Extreme Hypoxia Along the Canadian Pacific Continental Shelf

AbstractBottom waters of the northeast Pacific continental shelf naturally experience localized hypoxic conditions, with significant influences on food webs and biogeochemical cycling. In August 2021, extreme hypoxia was detected from several measurement platforms along the southern British Columbia continental shelf, with oxygen concentration <60 μmol kg−1, and a difference from the seasonal climatology of more than 2 standard deviations. Early and intense remote upwelling and local density shifts were associated with an anomalously strong spring phytoplankton bloom, which likely stimulated localized respiration of subsurface organic matter. This event was concurrent with unsuitable habitat for Pacific halibut and calcite and aragonite undersaturation throughout most of the water column. The drivers of this extreme low oxygen event could be enhanced under future climate change, with potentially significant impacts on marine ecology and biogeochemistry.

The Phenomenon of the Glycocalyx Layer Loss in the Acanthocephalan <i>Corynosoma strumosum</i> (Rudolphi, 1802) Lühe, 1904 (Acanthocephala: Polymorphidae) in Paratenic Hosts in Nature and in Experiment

The phenomenon of renewal of the surface of the tegument in the acanthocephalan Corynosoma strumosum, which parasitizes paratenic hosts of different species, has been discovered and confirmed for the first time by electron microscopy. Under conditions of experimental invasion of Middendorffʼs eelpout Hadropareia middendorffii (Schmidt, 1904) (Zoarcidae), the separation of the glycocalyx from the tegument was observed on the 3rd day of the experiment. In acanthocephalans found in fish, dissected on the 14th and subsequent days of the experiment, and in fish infected in nature, the tegument was covered with the same thick layer of glycocalyx, formed apparently anew. A similar phenomenon was observed in two individuals of Corynosoma strumosum that invaded the halibut Hippoglossus stenolepis (Schmidt, 1904) (Pleuronectidae) in nature, however, their tegument lost the outer membrane along with a thick layer of glycocalyx. In other acanthoce-phalans that invaded this species of halibut, the tegument was covered with a thin layer of glycocalyx. It is assumed that the described phenomena are a morphological manifestation of the parasite’s defense against the host’s immune response.

Response of Pacific halibut (Hippoglossus stenolepis) to future climate scenarios in the Northeast Pacific Ocean

Pacific halibut (Hippoglossus stenolepis) are a large-bodied species of flatfish that are important culturally, economically, and as a key predator in marine systems in the USA and Canada. The species has a wide distribution, and complex life history including large-scale migrations to spawn and feed, making it potentially susceptible to climate change impacts. We examined the potential changes in halibut distribution and relative abundance that may arise from changing temperature and dissolved oxygen concentrations using species distribution models (SDMs) and future climate scenarios downscaled by two regional ocean models. SDMs were fit with both environmental variables (depth, near-bottom temperature and dissolved oxygen concentration) and spatial random field components (representing unknown habitat-related variables). The best-fitting models, trained on data from 2009 to 2013, were able to account for 33 % and 53 % of the variation in small and large halibut catch-per-unit-effort in the annual set-line survey data from 2014 to 2020. The results suggest that the response of Pacific halibut to climate change in British Columbia and Washington waters is likely to depend on changes in dissolved oxygen concentration. Pacific halibut appear sensitive to changes in dissolved oxygen, yet relatively tolerant to increases in temperature. Projections for 2046–2065 period suggest that future decreases in near-bottom dissolved oxygen in shallow waters that small halibut inhabit are likely to result in moderate decreases in relative abundance. Projected changes in relative abundance are less certain for large Pacific halibut, due to disagreement in the regional ocean models near-bottom dissolved oxygen projections at mid depths (300–600 m) where large Pacific halibut are more common. Overall, the relative abundance of Pacific halibut is expected to decrease in most areas of British Columbia. Future management strategies will need to account for the projected changes in distribution and abundance and their uncertainty.

Impact of warming and deoxygenation on the habitat distribution of Pacific halibut in the Northeast Pacific

AbstractOcean warming and deoxygenation are already modifying the habitats of many aerobic organisms. Benthic habitat in the Northeast Pacific is sensitive to deoxygenation, as low oxygen concentrations occur naturally in continental shelf bottom waters. Here, we examine the potential impacts of deoxygenation and ocean warming on the habitat distribution of Pacific halibut (Hippoglossus stenolepis), one of the most commercially important groundfish in North America. We combine fisheries‐independent Pacific halibut survey data (1998–2020) with oceanographic measurements and a regional ocean circulation model to investigate current and future (end of 21st century) influences of deoxygenation and warming on optimal Pacific halibut habitat. We use the observations and model output to derive a metabolic index of Pacific halibut‐specific suitable habitat. Our results show high Pacific halibut counts in regions where the metabolic index is greatest and demonstrate that interannual variability in Pacific halibut abundance is coherent with the Pacific Decadal Oscillation. Working with model projections, we examine potential future changes in suitable Pacific halibut habitat by the end of the century under a high carbon dioxide emissions scenario. These projections indicate that suitable Pacific halibut habitat may largely disappear off the coast of Washington state, retreating approximately 5° latitude northward. In bottom waters along coastal British Columbia and Alaska continental shelf, Pacific halibut habitat is projected to decrease by about 50%. Such habitat changes may potentially drive a northward shift in Pacific halibut, with significant implications for commercial fisheries.

A Test of Deriving Sex‐Composition Data for the Directed Pacific Halibut Fishery via At‐Sea Marking

AbstractSensitivity analyses have identified uncertainty regarding sex ratios within commercial landings of Pacific Halibut Hippoglossus stenolepis as an influential source of variance within annual stock assessments for this species in U.S. and Canadian waters. Sex composition of dockside landings cannot be directly observed because all retained fish must be eviscerated at sea, and sex cannot be visually determined in the absence of the gonads. In the current study, a marking program was evaluated in which sex‐specific marks were applied by fishers to their retained catch, the mark was recorded during dockside monitoring, and the accuracy of the recorded sexes was validated using genetic techniques. The chosen marks (two vertical cuts in the dorsal fin for females and a single cut in the operculum of males) were considered by fishers to be easy to apply during at‐sea processing and produced sex‐ratio estimates that were equivalent to genetic results for 65% of sampled landings. However, vessel‐ and region‐specific accuracy was variable. Additional incentives to encourage vessels to participate in the program, continued outreach, or potentially a regulatory requirement to mark fish would be required to produce sufficient data to satisfy stock assessment needs, and ongoing validation would likely need to accompany such a program to ensure consistent and acceptable data quality.

Assessing cross-regional flows of economic benefits: A case study of Pacific halibut commercial fishing in Alaska

The economic effects of changes to harvest levels can be far-reaching. Fisheries management policies that alter catch limits have a direct impact on harvesters, but at the same time, there is a ripple effect through the economy. To bring a better understanding of the magnitude of this multiplier effect, this paper applies multiregional economic impact assessment to the case study of Pacific halibut commercial fishing in Alaska.The results suggest that the revenue generated by Pacific halibut at the harvest stage accounts for only a fraction of economic activity that would be forgone if the resource was not available to fishermen. However, the adoption of a multiregional approach to the economic impact assessment highlights that a myriad of economic benefits in this case are realized far from where the resource is harvested. This results from strong economic ties the Alaskan fishing activities have to the economy of the rest of the United States, be it through trade, nonresident workforce or out-of-state investment in production factors such as fishing quotas.

Shifting fish distributions impact predation intensity in a sub‐Arctic ecosystem

An abundance of studies in marine systems have documented species range shifts in response to climate change, and many more have used species distribution models to project species ranges under future conditions. However, there is increasing interest in moving beyond a single‐species focus to understand how species redistribution alters ecosystem dynamics via changes in trophic interactions. We employed spatiotemporal models to characterize decadal‐scale changes in spatial overlap between the distribution of juvenile walleye pollock Gadus chalcogrammus and the distributions of four of its groundfish predators: arrowtooth flounder Atheresthes stomias, Pacific cod Gadus macrocephalus, Pacific halibut Hippoglossus stenolepis and adult walleye pollock. These fishes represent ecologically and commercially important species in a rapidly changing sub‐Arctic ecosystem, the eastern Bering Sea, Alaska, USA. We then examined whether changes in spatial overlap corresponded to changes in predation, using spatiotemporal models of predator stomach contents. We found marked shifts in spatial overlap between juvenile pollock and two predators (arrowtooth flounder and Pacific halibut) over 34 years, with changes in overlap corresponding to increases in population‐scale predation pressure. By contrast, we did not find clear relationships between spatial overlap and predation for Pacific cod and adult pollock, the two predators for which juvenile pollock constitute a much smaller diet proportion. Our findings highlight the complexity of predicting predation dynamics for generalist marine species and suggest a need for better process‐based methods for understanding the potential future ecological impacts of coupled species range shifts. However, simple metrics of spatial overlap between relatively specialized predators and their prey offer promise as a means to integrate predictions from species distribution models into ecosystem‐based fisheries management.

Dispersal and seasonal movements of Pacific halibut (Hippoglossus stenolepis) in the eastern Bering Sea and Aleutian Islands, as inferred from satellite-transmitting archival tags

BackgroundUnderstanding connectivity is critical to the management of exploited fish stocks, but migratory dynamics of Pacific halibut (Hippoglossus stenolepis) in the Bering Sea and Aleutian Islands region are not well-understood. In the current study, 145 Pacific halibut ≥ 82 cm fork length were tagged with Pop-up Archival Transmitting (PAT) tags to evaluate interannual dispersal, seasonal migration, and depth-specific habitat use.ResultsEndpoint locations obtained after 1 year at liberty (n = 79), fishery recoveries after 2–3 years at liberty (n = 5), and at-liberty geopositions based on light data (n = 5313 estimates from 109 fish) indicated geographically distinct movement patterns: Pacific halibut tagged in the Western and Central Aleutian Islands remained within the island groups in which the fish had been tagged; fish in the eastern Bering Sea remained in that ocean basin, moving among International Pacific Halibut Commission (IPHC) regulatory areas and into Russian waters; those tagged south of Unimak Pass in IPHC Regulatory Area 4A displayed the greatest amount of emigration, dispersing eastward both seasonally and interannually to as far south as Washington State. Analysis of daily maximum depth and temperature data from 113 individuals demonstrated group-level variation in summer temperatures experienced by the fish and in the timing, duration, and synchrony of movement to deep-water wintering grounds.ConclusionsDepth-specific habitat use was suggestive of regionally explicit migratory contingents, while interannual dispersal patterns were consistent with the existence of multiple functional spawning units. The results may guide future research to examine cross-basin connectivity in the Northern Bering Sea and provide inputs for numerical modelling of individual movements, larval advection, and recruitment analyses.

Generation of a chromosome-level genome assembly for Pacific halibut (Hippoglossus stenolepis) and characterization of its sex-determining genomic region.

The Pacific halibut (Hippoglossus stenolepis) is a key species in the North Pacific Ocean and Bering Sea ecosystems, where it also supports important fisheries. However, the lack of genomic resources limits our understanding of evolutionary, environmental and anthropogenic forces affecting key life history characteristics of Pacific halibut and prevents the application of genomic tools in fisheries management and conservation efforts. In the present study, we report on the first generation of a high‐quality chromosome‐level assembly of the Pacific halibut genome, with an estimated size of 602 Mb, 24 chromosome‐length scaffolds that contain 99.8% of the assembly and a N50 scaffold length of 27.3 Mb. In the first application of this important resource, we conducted genome‐wide analyses of sex‐specific genetic variation by pool sequencing and characterized a potential sex‐determining region in chromosome 9 with a high density of female‐specific SNPs. Within this region, we identified the bmpr1ba gene as a potential candidate for master sex‐determining (MSD) gene. bmpr1ba is a member of the TGF‐β family that in teleosts has provided the largest number of MSD genes, including a paralogue of this gene in Atlantic herring. The genome assembly constitutes an essential resource for future studies on Pacific halibut population structure and dynamics, evolutionary history and responses to environmental and anthropogenic influences. Furthermore, the genomic location of the sex‐determining region in Pacific halibut has been identified and a putative candidate MSD gene has been proposed, providing further support for the rapid evolution of sex‐determining mechanisms in teleost fish.

High prevalence of Ichthyophonus sp. infections in Northeast Atlantic mackerel (Scomber scombrus).

High prevalence of Ichthyophonus sp. infections in Northeast Atlantic mackerel (Scomber scombrus).