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Le flétan atlantique (Hippoglossus hippoglossus) est le poisson de fond possédant la plus haute valeur commerciale par unité de poids dans le golfe du Saint-Laurent (GSL). Il constitue ainsi une ressource importante sur le plan socio-économique au Québec et dans les provinces de l’Atlantique. Les débarquements de flétan atlantique y sont en constante augmentation depuis les années 2000, après un effondrement du stock au début des années 1950 qui a été suivi d’une longue période de faible abondance. En 2019, les débarquements (plus de 1 383 tonnes) étaient les plus importants des 60 dernières années. À la suite de ce retour marqué, de meilleures connaissances sur l’écologie larvaire et l’utilisation de l’habitat tout au long de l’ontogénie sont essentielles pour assurer une gestion durable du stock. Nous présentons l’historique de l’exploitation du stock de flétan atlantique du GSL et l’état actuel des connaissances de l’espèce dans cette unité de gestion. Nous abordons également les défis auxquels les gestionnaires font face en vue d’assurer une exploitation durable, et les projets de recherche en cours visant à combler les lacunes dans nos connaissances. Finalement, nous abordons les enjeux spécifiques à l’instauration d’une pêche récréative hivernale durable pour le flétan atlantique dans le fjord du Saguenay., Atlantic halibut (Hippoglossus hippoglossus) is the groundfish with the highest commercial value per unit weight and is thus an important socio-economic resource in Québec and the Atlantic provinces (Canada). Landings of Atlantic halibut from the Gulf of St. Lawrence have been steadily increasing since the early 2000s, after a stock collapse in the early 1950s that was followed by a long period of low abundance. In 2019, landings exceeded 1,383 tons, the highest in the last 60 years. Following this strong return, improved knowledge of larval ecology and habitat use throughout its ontogeny is essential for sustainable management of the stock. This paper presents a history of the Atlantic halibut harvest from the Gulf of St. Lawrence and the current state of knowledge concerning this management unit of the species. It also outlines the challenges managers face in ensuring a sustainable fishery and the research currently underway to fill key knowledge gaps. Finally, the specific issues involved in establishing a sustainable recreational Atlantic halibut ice fishery in the Saguenay Fjord are discussed.

This study provides evidence of two subpopulations of Atlantic halibut (Hippoglossus hippoglossus) in the southern Gulf of St. Lawrence. The migrations of 20 Atlantic halibut captured in the coastal waters of Prince Edward Island, Canada, were evaluated using pop-up satellite archival tags (PSAT). Geolocation data showed that Atlantic halibut migrated north to the Laurentian Channel via distinct eastern or western routes. Floy tagging and recapture (recapture of 18.4%) showed that less than 7.0% of Atlantic halibut exhibited dispersive behaviour outside of their annual migratory route. Overwintering occurred on the slope and in the deepest waters of the Laurentian Channel. Atlantic halibut in the deepest waters of the Laurentian Channel exhibited rapid, ∼100 m rises, presumed to be associated with spawning from January to March. The eastern and western migratory cohorts exhibited this behaviour ∼350 km apart, suggesting reproductive isolation as the basis of subpopulations. The results of this study indicate a need to reconsider the management of Gulf of St. Lawrence Atlantic halibut as one continuous population.

Presence of coastal aquaculture activities in marine landscapes is growing with impacts on the wild fish that share these habitats. However, it is difficult to disentangle subsequent ecological interactions between these activities and marine fish communities. We evaluated the impact of both salmon and halibut farms on mackerel (Scomber scombrus) and whiting (Merlangius merlangus) sampled near sea cages using condition indices and fatty acid (FA) biomarkers. Results of the stomach content analysis indicated that mackerel and whiting consumed waste feed which was also reflected in their modified FA profiles. Both mackerel and whiting had elevated levels of FAs that are of vegetable oils origin. The use of vegetable oils as replacement for marine oils is a lot more common in salmon farming than halibut farming. Additionally, the overall effects of the two fish farms were more pronounced in whiting than in mackerel sampled near the sea cages. By allowing discrimination between sources of trophic interactions, this method could lead to more informed decisions in managing different farming activities.

The Aquareovirus (AQRV) member Atlantic halibut reovirus (AHRV) is associated with severe liver pathology and high fry mortality and constitutes a significant problem for Atlantic halibut (Hippoglossus hippoglossus) production in Norway. Halibut is a batch spawner and it has been suspected that AHRV may be transmitted via eggs since outbreaks of disease arise in successive batches of fry originating from the same group of brood fish. In this study, we present the complete genome of AHRV representing the first complete AQRV genome sequence from a marine cold-water fish species in the North Atlantic. The terminal 5′- and 3′-ends of the segments have the canonical conserved nucleotides 5′-GUUUUAU------UCAUC-3′. The 13 putative proteins encoded in the 11 AHRV genome segments share the highest amino acid identity with members of species AQRV A and B. Phylogenetic analysis of the most conserved proteins (VP1, VP2, VP3 and VP5) groups AHRV in a major clade together with the same two species. However, the differences in host and environment, and the amino acid sequence identity of the RdRp (~80%) compared to either AQRV A or B, suggest that this virus could possibly represent a novel species within the genus. Furthermore, we show that even though AHRV RNA cannot be detected by real time RT PCR in the first egg batches from asymptomatic Atlantic halibut brood fish, the viral RNA load is not only detectable, but appears to increase in successive spawning batches. publishedVersion

Gonadotropins (GtHs) play a pivotal role in regulating the reproductive axis and puberty. In this study, full-length sequences coding for common glycoprotein α subunit (CGα) and luteinizing hormone β (LHβ) were isolated from female turbot (Scophthalmus maximus) pituitary by homology cloning and a strategy based on rapid amplification of cDNA end-polymerase chain reaction. Results showed that the two cDNAs consisted of 669 and 660 nucleotides encoding 129 and 139 amino acids, respectively. CGα and LHβ manifested typical characteristics of glycoprotein hormones, high homologies with the corresponding sequences of available teleosts, and high homology with that of Hippoglossus hippoglossus. CGα, FSHβ, and LHβ mRNAs were abundant in the pituitary, but less expressed in extra-pituitary tissues. The cgα, fshβ, and lhβ were detected at 1-day post-hatching (dph) and peaked simultaneously at early-metamorphosis (22 dph). cgα and fshβ mRNA levels were significantly increased at pre-metamorphosis, peaked in early metamorphosis, and then gradually decreased until metamorphosis was completed. Conversely, lhβ mRNA levels gradually decreased at pre-metamorphosis, dramatically peaked at early metamorphosis, and then decreased during metamorphosis. In addition, the mRNA levels of cgα were significantly higher than those of fshβ and lhβ during turbot larval metamorphic development, whereas no significant difference was found between fshβ and lhβ. These results suggested (i) an early activation of the GtHs system after hatching, which was the highest expression at early metamorphosis, and (ii) FSHβ and LHβ were together involved in the establishment of the reproductive axis during larval development in turbot. These findings contribute to further understanding the potential roles of GtHs during fish larval development.

A novel 27 kDa ladder-lectin-like protein, showing a multimeric structure under non-reducing conditions, was isolated from halibut serum by binding to N-acetyl glucosamine. Mass-spectrometry analysis did not show significant homology with known proteins. Specific antibodies were produced and used in immunohistochemistry on tissue sections of early halibut ontogeny from 119 until 1050 °d post hatching. A strong positive response was detected in the mucosal cells of the skin, gills and gut, indicating a role in the mucosal immune defence at these sites. Further immunopositivity was detected in liver, myeloma of kidney and the brain at different developmental stages but predominant expression was found in mucosal surfaces at later stages of development tested (1050 °d). It is still uncertain whether this ladder-like lectin forms part of the complement pathway, as a lectin or ficolin, or if it belongs to galectins. A strong detection in mucosal surfaces on skin, gills and gut, show similar patterns of expression as both mucosal lectins and galectins in other fish. Detection in neuronal tissue may indicate putative roles in tissue remodelling of brain and in ongoing neurogenesis in the fish eye.

Atlantic halibut is a cold-water pleuronectiform of the North Atlantic with a high market price and a small wild stock, therefore it has a large potential in farming. However, the halibut is difficult to grow and although work aimed at domestication has been going on since the late 1980ies, there are still only a few commercial farms. The purpose of the present study was to establish a time-point for early weaning of Atlantic halibut larvae, using feed intake as a proxy. First, three commercial diets (Otohime, Gemma Micro and AgloNorse) were compared at 28 days post first-feeding (dpff), with respect to feed intake during 5 days after weaning, using four replicate tanks. The number of larvae per tank out of 28 which had filled guts on the different diets, was 26.7 ± 2.3 (Otohime), 12.0 ± 0.6 (Gemma Micro) and 14.7 ± 1.2 (AgloNorse), respectively. Otohime was chosen for an experiment where feed intake for 5 days after weaning was monitored, starting at 15, 22 and 28 dpff. There was a gradual increase in success with larval age. At 15 dpff the larvae did not eat and there was an almost 100% mortality, at 22 dpff, 12 ± 1 out of 17 larvae survived and 7.3 ± 1.5 had filled guts. At 28 dpff, 14 ± 3 out of 17 larvae survived and all of them had filled guts. The development seems to correlate with the development of larval type digestion. Based on feed intake, successful weaning can be achieved at 28 dpff, however, further studies are needed to determine growth, survival and juvenile quality of Atlantic halibut larvae that have been weaned early on formulated diets.

The utilization of first-order information about seafloor morphology, derived from multibeam sonar data, has become common in the investigation of deep-sea benthic habitats. When combined with comp...

To further understand the complexity of developmental toxicity of dispersed oil and importance of exposure timing on fish early life stages, Atlantic halibut (Hippoglossus hippoglossus) were exposed to environmentally relevant concentrations through two embryonic developmental windows: the first period occurred during the epiboly process (named as “early embryonic exposure”) and the second period overlapped the ontogenesis and cardiogenesis processes (named as “late embryonic exposure”). Following 72 hour oil exposure, embryos were transferred to clean seawater and a toxicity screening was performed in the yolk-sac larvae until first-feeding stages (56 days). The current study demonstrated that the exposure timing is essential for the development of toxic effects of crude oil in Atlantic halibut. Neither embryonic exposures (early or late) showed notable acute toxicity during exposure, yet both showed global latent teratogenic effects during yolk sac stages. Fish exposed during organogenesis (late) displayed stronger and more severe toxic effects than fish exposed during epiboly process (early), including reduced condition, severe craniofacial deformities and cardiovascular disruptions. The uptake level of polycyclic aromatic hydrocarbons into larval tissue and metabolic activity were greater following the late embryonic exposure and remained high during the depuration period at the highest exposure concentration. Overall, the long yolk sac stage development timing of Atlantic halibut makes this species a good candidate for evaluation of embryonic crude oil toxicity and its mechanisms.

Fish communities in the shallow waters (9–20 m depth) of outer Malangen (69o N), Northern Norway was studied to investigate effects of season, bottom substrate associations and population dynamic parameters on the fish assemblages. In four fieldtrips during one year at four locations, fish were sampled by trammel nets and bottom substrate composition was monitored by underwater video. A total of 16 species of fish were caught, including four species from Pleuronectidae and six from the Gadidae family. Cod (Gadus morhua Linnaeus, 1758) was the most numerous species followed by plaice (Pleuronectes platessa Linnaeus, 1758), lemon sole (Microstomus kitt Walbaum, 1792) and halibut (Hippoglossus hippoglossus Linnaeus, 1758). Cod dominated the catches in all months except March, in which plaice and other flatfishes were most dominant. On a community scale, fish species richness was highest in October and lowest in March. Furthermore, the species richness and diversity were generally higher at locations with greater bottom substrate evenness. Cod showed a positive association with large macroalgae coverage and were less numerous at locations dominated by sand and pebble. Plaice and common dab (Limanda limanda Linnaeus, 1758) showed positive associations with sand and pebble coverage, while lemon sole was more associated with maerl beds, cobble and low algae coverage. Only halibut younger than 8 years of age were observed, and the sampling areas are suggested to be nursery area for this species. Von Bertalanffy growth functions were estimated for cod, plaice and lemon sole, and the growth coefficient K of cod and plaice were higher than for populations from adjacent fjords or the North Sea populations. Total mortality rates of cod and plaice were lower than for areas further south, and this may contribute to high and relative stable biomasses in the shallow Malangen area.

Atlantic Halibut (Hippoglossus hippoglossus) has a X/Y genetic sex determination system, but the sex determining factor is not known. We produced a high-quality genome assembly and identified parts of chromosome 13 as the Y chromosome due to sequence divergence between sexes and segregation of sex genotypes in pedigrees. Linkage analysis revealed that all chromosomes exhibit heterochiasmy, i.e. male- and female restricted meiotic recombination intervals (MRR/FRR). We show that FRR/MRR intervals differ in nucleotide diversity and repeat class content and that this is true also for other Pleuronectidae species. We further show that remnants of a Gypsy-like transposable element insertion on chr13 promotes early male specific expression of gonadal somatic cell derived factor (gsdf). Less than 4 MYA, this male-determining element evolved on an autosomal FRR segment featuring pre-existing male meiotic recombination barriers, thereby creating a Y chromosome. We propose that heterochiasmy may facilitate the evolution of genetic sex determination systems.

Different incubation techniques for yolk sac larvae of Atlantic halibut (Hippoglossus hippoglossus L.) were compared and evaluated on the basis of survival and amount of normally developed larvae. The three investigated incubator concepts were; 3-litre glass-bowls maintained with stagnant water, 100-litre flat bottom PVC-tanks run with slow water exchange, and 350–1450 litre flow through silos with up-streaming water. The survival at the end of the yolk sac stage in 3-litre bowls was 30–90%, and increased reproducibility was obtained by addition of 25 ppm antibiotics to the water at start of the incubation. In the PVC tanks the survival was lower (20–55%). The flow through silo-incubators with up-wards water flow gave survival of 30–70%, and exhibited the highest reproducibility in survival for replicates. The fraction of normally developed larvae ready for first feeding varied both with larval batches and incubator-type. For all batches of eggs, lowest amount of normal larvae were obtained in the 100 litre PVC-tanks. In the bowls and the silos the fraction of normal larvae were 55–75% in the investigated batches. The most dominating deformity was jaw deformity (gaping), and lowest relative fraction of gaping was obtained in the bowls and in silos in larval batch 2. An overall evaluation of the incubator potential of efficient large scale production of yolk sac larvae ready for first feeding suggests that the silo-incubators have the highest potential.

Control of appetite and feed intake in fish larvae are still largely unexplored. Two of the key players in controlling vertebrate's feed intake are cholecystokinin (CCK) and peptide YY (PYY). Here we investigated the mRNA expression of pyy, cck and cck receptors (cckr) in the brain (head) and gut of Atlantic halibut larvae in response to three consecutive meals. We used Artemia nauplii cysts that are commonly ingested by halibut larvae when present as inert feed, and three water-soluble extracts as attractants to stimulate appetite. Cyst intake was not affected by the use of attractants and overall ingestion rate was low. Differences in mRNA expression of cck and pyy were observed between the halibut larvae that had eaten and those that had not despite readily available feed (cysts), supporting that mechanisms for control of feed intake are at least partly functional. All genes analysed were present in the brain and gut, however the different expression profiles between paralogues suggest potential divergent functions. In the gut, cck2 and pyyb mRNA expression was significantly higher in the larvae that ate cysts compared to larvae that decided to not eat, indicating that these genes play a satiety function in the halibut larvae similar to the general vertebrate scheme. However, cck2, cck2r1, and pyy mRNA expression in the brain were lower in the fed-filled larvae group compared to larvae before eating, which contrasts with the presumable anorectic function of these genes. Further research is required to fully evaluate how PYY and CCK affect the feeding biology in halibut larvae, contributing to formulate inert diets that can stimulate appetite and feed intake.

Changes in the genetic mechanisms that control sexual determination have occurred independently across the tree of life, and with exceptional frequency in teleost fishes. To investigate the genomic changes underlying the evolution of sexual determination, we sequenced a chromosome-level genome, multi-tissue transcriptomes, and reduced representation population data for the Atlantic halibut (Hippoglossus hippoglossus), which has an XY/XX sex determination mechanism and has recently diverged (0.9-3.8 MYA) from the Pacific halibut (Hippoglossus stenolepis), which has a ZZ/ZW system. We used frequency and coverage-based population approaches to identify a putative sex-determining factor, GSDF. We characterized regions with elevated heterozygosity and linkage disequilibrium indicating suppression of recombination across a nascent sex chromosome. We detected testis-specific expression of GSDF, the sequence of which is highly conserved across flatfishes. Based on evidence from genome-wide association, coverage, linkage disequilibrium, testis and brain transcriptomes, and sequence conservation with other flatfishes, we propose a mechanism for the recent evolution of an XY sex-determination mechanism in Atlantic halibut. Changes to the ancestral sex-determining gene DMRT1 in regulating the downstream gene GSDF likely coincided with GSDF, or a proximal regulatory element of it, becoming the primary sex-determining factor. Our results suggest changes to a small number of elements can have drastic repercussions for the genomic substrate available to sex-specific evolutionary forces, providing insight into how certain elements repeatedly evolve to control sex across taxa. Our chromosome-level assembly, multi-tissue transcriptomes, and population genomic data provide a valuable resource and understanding of the evolution of sexual systems in fishes.

Viral nervous necrosis (VNN) as a worldwide and disastrous disease could be classified as one of the most important diseases in marine fish, which occurred often in fry and juvenile fishes with high economic losses. Sometimes, it was occurred in adult fish, such as golden grey mullet in Caspian Sea. In last decade, VNN was reported in all continents except Africa; but recently, some official outbreaks were reported in wild groupers in North beaches of some African countries such as, Tunisia, Algeria, Libya, and Senegal. This infectious viral disease can be spread in marine waters or freshwater of tropical, moderate, and cold areas in fishes that can live in warm or cold waters, and also in ornamental and marine fish. So far, morbidity and sensitivity of more than 50 fish species of wild marine or reared fishes have been reported to VNN virus since 1986. Furthermore, according to official reports of OIE, VNN was recognized as most important fish health challenge in Mediterranean region and inhibitor factor in development of East-South Asian aquaculture. Regarding to epidemiological studies, first outbreak of VNN was occurred in wild red spots groupers (Epinephelus akaara) in Japan, and mortality rate reached 80% in 1991. Same mortality was reported in marine sea bass (Dicentrarchus labrax) in Greece, and mortality rate was recorded between 11% and 60% in 1997, but mortality rate in cage-reared halibut fry (Hippoglossus hippoglossus) was recorded in Norway about 100% in summer of 1997. Also, the sensitivity of Salmonidae family was confirmed to barfin flounder nervous necrosis virus serotype in Norway. It would be concluded that VNN could be considered main reason of morbidity and mortality of some important marine fish in the world. So, all aspects on VNN as most important catastrophic emerging viral disease in marine aquaculture would be discussed.

This report documents the gross pathological appearance, histopathology and ultra-structural studies of the ocular abnormalities previously reported in farmed Atlantic halibut Hippoglossus hippoglossus). 37% of eyes investigated were normal on gross and histopathological examination while 32% showed formation of an intraocular cyst in the choroid resulting in anterior movement of the retina. 27% showed scleral ectasia in which the posterior portion of the globe was enlarged, either generally or in a focal ectatic focus. In a small number of eyes the cystic posterior segment lesions contained fibrous and inflammatory tissue but most affected eyes showed cystic structures filled with gas or fluid. Although morphology was somewhat compromised in the majority of samples, histopathological and electron microscopic ultrastructural examination showed an endothelial lining to these posterior segment cysts suggesting that they have developed following gas bubble formation in vessels of the choroidal rete. This finding concurs with our previous research showing increased intraocular oxygen concentrations in the eyes of affected fish and suggests that the pathology seen here is predominantly associated with the formation of oxygen bubbles in the countercurrent multiplier system of the choroidal rete.

Understanding and identifying the genetic mechanisms responsible for sex-determination are important for species management, particularly in exploited fishes where sex biased harvest could have implications on population dynamics and long-term persistence. The Pacific halibut (Hippoglossus stenolepis) supports important fisheries in the North Pacific Ocean. The proportion of each sex in the annual harvest is currently estimated using growth curves, but genetic techniques may provide a more accurate method. We used restriction-site associated DNA (RAD) sequencing to identify RAD-tags that were linked to genetic sex, based on differentiation (FST) between the sexes. Identified RAD-tags were aligned to the Atlantic halibut (Hippoglossus hippoglossus) linkage map, the turbot (Scophthalmus maximus) genome, and the half-smooth tongue sole (Cynoglossus semilaevis) genome to identify genomic regions that may be involved in sex determination. In total, 56 RAD-tags (70 single nucleotide polymorphisms) were linked to sex, and 3 RAD-tags were identified in only females. Sex-linked loci aligned to 3 linkage groups in the Atlantic halibut (LG07: 7 loci, LG15: 1 locus, and LG24: 1 locus), 3 chromosomes in the turbot (LG12: 13 loci, LG01: 1 locus, and LG05: 1 locus), and 1 chromosome in the half-smooth tongue sole (ChrZ: 9 loci). Results add support to the hypothesis that Pacific halibut genetic sex is determined in a ZW system. Two sex-linked loci were further developed into sex identification assays, and their efficacy was tested on individuals that had been morphologically sexed. The accuracy of each assay on its own was 97.5% compared to morphological sex.

The in vivo ability of Atlantic halibut (Hippoglossus hippoglossus) larvae to incorporate, de-acylate and re-acylate [1-14C]fatty acids (FA) into lipid classes and to elongate and desaturate those substrates was elucidated. To this purpose, 30 days post-first-feeding (30 dpff) larvae were incubated in the presence of either free [1-14C]-FAs (18:2n-6, 18:3n-3, 20:4n-6, 20:5n-3 and 22:6n-3), [1-14C]18:1n-9 bound to mono- (MAG) and triacylglycerols (TAG), or [1-14C]20:4n-6 bound to phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Halibut larvae can efficiently incorporate dietary FAs through MAG, while FA incorporation when provided as free FAs and especially bound to TAG or phospholipids seems to be very low. These results suggest that lipid digestion might be a limiting factor in the FA absorption and incorporation by 30 dpff Atlantic halibut. The present study also shows that esterification into phospholipids is a strong metabolic fate for dietary long-chain polyunsaturated fatty acids (LC-PUFAs), and the capacity of 30 dpff Atlantic halibut larvae to metabolize dietary FAs through elongation/desaturation processes. Nonetheless, neither EPA nor DHA were detected from [1-14C]ALA, indicating a reduced capacity to biosynthesize these LC-PUFAs and the necessity for them to be adequately supplied through the diet for proper larval development.

Background The effects of temporal data resolution on the interpretation of fish behaviours are questions fundamental to research programs using electronic tags with finite data storage and data transmission capacities. However, understanding these effects requires sub-sampling high-resolution data at multiple temporal resolutions. In pop-up satellite archival tags (PSATs), data transmissions via satellite are limited, so temporal resolution is a decreasing function of the deployment duration. Physical recovery of PSATs overcomes this limitation, providing data on temperature, depth, etc., at rates at least 15–30 times greater than transmitted data. Results Using PSATs physically recovered from Atlantic halibut (Hippoglossus hippoglossus), we evaluated the effects of data resolution (2, 30, 60 min) on interpreting vertical movements including rapid ascents during the expected spawning season. Putative ‘spawning rises’ of 6–20 min were only present within highest temporal resolution data. Vertical movement rates during spawning rises identified with high-resolution data exceeded rates in all other periods; such movement rates were virtually absent in all other tagged fish within the spawning period. Mean maximum ascent rates were 13 and 23 times higher when the sampling interval was 2 versus 30 or 60 min, respectively. Conclusions We illustrate the limitations of using satellite-transmitted data to quantify vertical movement rates and detect potential spawning events in marine teleosts. Such low temporal resolution data suggest spurious results by masking important behaviours that archived data reveal as occurring at high frequencies. We highlight new technologies that facilitate PSAT recoveries at sea to overcome these limitations and facilitate analyses of high-frequency archived data.

Juvenile Atlantic halibut (~100 mg, Hippoglossus hippoglossus) were exposed to Vibrio proteolyticus, a Vibrio spp. isolate, Photobacterium damselae ssp. damselae and five different isolates of Aeromonas salmonicida ssp. achromogenes via an hour-long bath immersion to ascertain their variation in pathogenicity to this fish species. Results were analysed using Kaplan-Meier survival analysis. Analysis of the data from challenges using A. salmonicida ssp. achromogenes revealed three survival values of zero and a spread of values from 0 to 28.43. Challenges using a Vibrio spp isolate, V. proteolyticus and P. damselae resulted in Kaplan-Meier survival estimates of 31.21, 50.41 and 57.21, respectively. As all bacterial species tested could induce juvenile halibut mortalities, they must all be considered as potential pathogens. However, the degree of pathogenicity of A. salmonicida is isolate dependent.

Characterizing migratory behaviours contributes to the sustainable management of marine fishes by resolving stock structure and identifying the timing and locations of events within fish life cycles. The migratory behaviour of Atlantic halibut (Hippoglossus hippoglossus) in the Gulf of St. Lawrence (GSL), Canada was characterized over an annual cycle using pop-up satellite archival tags (n = 15). Daily probability density functions of individual halibut positions were estimated using a geolocation model specifically developed to track demersal fish species in the GSL. Reconstructed migration routes (n = 8) revealed that Atlantic halibut displayed seasonal migrations, moving from deeper offshore waters in the winter to shallower nearshore waters in the summer. Variability in migratory behaviours was observed among individuals tagged at the same location and time. One individual resided year round in the vicinity of the tagging site, three individuals displayed homing behaviour, and four individuals did not return to the tagging site. The identification of presumed spawning rises for two individuals suggested that spawning of Atlantic halibut occurred in the GSL. Although based on a limited number of individuals, these results suggest that Atlantic halibut in the GSL forms a philopatric population, supporting the current separate management of this stock from the adjacent Scotian Shelf and southern Grand Banks stock.

Flatfish have a high market acceptance thus representing a profitable aquaculture production. The main farmed species is the turbot (Scophthalmus maximus) followed by Japanese flounder (Paralichthys olivaceous) and tongue sole (Cynoglossus semilaevis), but other species like Atlantic halibut (Hippoglossus hippoglossus), Senegalese sole (Solea senegalensis) and common sole (Solea solea) also register an important production and are very promising for farming. Important genomic resources are available for most of these species including whole genome sequencing projects, genetic maps and transcriptomes. In this work, we integrate all available genomic information of these species within a common framework, taking as reference the whole assembled genomes of turbot and tongue sole (>210× coverage). New insights related to the genetic basis of productive traits and new data useful to understand the evolutionary origin and diversification of this group were obtained. Despite a general 1:1 chromosome syntenic relationship between species, the comparison of turbot and tongue sole genomes showed huge intrachromosomic reorganizations. The integration of available mapping information supported specific chromosome fusions along flatfish evolution and facilitated the comparison between species of previously reported genetic associations for productive traits. When comparing transcriptomic resources of the six species, a common set of ~2500 othologues and ~150 common miRNAs were identified, and specific sets of putative missing genes were detected in flatfish transcriptomes, likely reflecting their evolutionary diversification.

Vibrionaceae infections are a major obstacle for marine larviculture; however, little is known about virulence differences of Vibrio strains. The virulence of Vibrio strains, mostly isolated from vibriosis outbreaks in farmed fish, was tested in larval challenge trials with cod (Gadus morhua), turbot (Scophthalmus maximus) and halibut (Hippoglossus hippoglossus) using a multiwell dish assays with single-egg/larvae cultures. The strains differed significantly in virulence as some caused a high mortality of larva reaching 100% mortality after a few days, while others had no or only marginal effects on survival. Some Vibrio strains were pathogenic in all of the larva species, while some caused disease only in one of the species. Twenty-nine of the Vibrio anguillarum strains increased the mortality of larvae from at least one fish species; however, pathogenicity of the strains differed markedly. Other Vibrio species had no or less pronounced effects on larval mortalities. Iron uptake has been related to V. anguillarum virulence; however, the presence or absence of the plasmid pJM1 encoding anguibactin did not correlate with virulence. The genomes of V. anguillarum were compared (D. Castillo, P.W. D'Alvise, M. Middelboe & L. Gram, unpublished data) and most of the high-virulent strains had acquired virulence genes from other pathogenic Vibrio.