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In this study Atlantic halibut (Hippoglossus hippoglossus L.) larvae (0.12 +/- 0.04 g) were, from day 40 post first feeding, offered six diets in which 10% or 30% of the dietary protein was hydrolysed with (a) pepsin (P), (b) pepsin + trypsin (PT) or (c) pepsin trypsin + chymotrypsin (PTC). In addition, a diet without hydrolysed protein was offered, and enriched Artemia was fed as control. The amount of soluble protein increased progressively with the enzyme treatments P, PT and PTC and with higher inclusion levels of hydrolysed protein. Survival was highest among the larvae offered Artemia (83 +/- 0%) or the diet 10P (10% pepsin hydrolysed protein; 67 +/- 4%). The diet 10P supported survival significantly better than the more hydrolysed diets 10PTC, 30P, 30PT and 30PTC, but not significantly better than the non-hydrolysed diet and 10PT. Specific growth rate (SGR) was 1.76 +/- 0.20 in average for all groups of larvae and was not significantly affected by the diets. Still, the larvae offered pepsin hydrolysed diets tended to have better growth (2.10 +/- 0.05 SGR; P < 0.06) than the larvae offered the other hydrolysed diets. The larvae offered the formulated diets did not differ in chemical composition.

b Abstract Considering that amino acids constitute an important energy fuel during early life of the Atlantic halibut (Hippoglossus hippoglossus L.), it is of interest to understand how the nitrogenous end products are handled. In this study we focused on the kinetics and fates of ammonia, urea and uric acid. The results showed that ammonia (T :N HqNH ), and q Amm 3 4 urea - N contents increased during final oocyte maturation. Urea - N excretion dominated the total nitrogenous end product formation in early embryos. Later, yolk T levels increased in embryos and ammonia excretion was low. In Amm the last part of the embryonic stage T accumulation dominated, and was apparently due to yolk storage. Around Amm hatching, the larval body tissues (larva with yolk-sac removed) accounted for 68% of whole animal urea - N accumulation, while T levels increased predominately by yolk accumulation. Afterwards, ammonia excretion dominated and uric Amm acid accumulation accounted for less than 1%. Urea, synthesised either through the ornithine - urea cycle, argininolysis or uricolysis, accounted for approximately 8% of total nitrogenous end product formation in yolk-sac larvae. The results suggested that a sequence occurred regarding which nitrogenous end products dominated and how they were handled. Urea excretion dominated in early embryos (-7 dPF), followed by yolk ammonia accumulation ( 7-1 2 dPF), and finally, ammonia excretion dominated in later embryonic and yolk-sac larval stages ()12 dPF). 2002 Elsevier Science Inc. All rights reserved.

The possibility of using fish skin as model membrane tostudy drug permeation and penetration enhancement by cyclodextrins was investigated.The permeability of the skin from four species of fish, Anarhichas lupus (catfish),Pleuronectes platessa (Plaice), Hippoglossus hippoglossus (Halibut)and Anarhichas minor (Spotted catfish), was compared in a Franz diffusion cell set-up using 1% hydrocortisone aqueous solution as a donor phase. The drug fluxthrough fish skin was more than 100 times faster than the flux through hairless mouse skin and more than 10 000 times faster than through snake skin. Catfishskin was most easily accessible and was therefore used for further study. The octanol-water partition coefficient did not affect the transmembrane flux of small molecules whereas the aqueous diffusion coefficient could be correlated with the flux.The hydrocortisone flux of from aqueous hydroxypropyl-β-cyclodextrin solutions, which were saturated with the drug, increased with increasing cyclodextrinconcentration. From these and other observations it was concluded that small moleculesare transported through fish skin in aqueous channels. The properties of thesechannels resemble the properties of the aqueous diffusion layer present in human andanimal skin and other types of biological membranes. Previous studies have shown thatcyclodextrins will enhance drug delivery by increasing aqueous diffusion rate. Catfish skin can therefore be a good model membrane to study penetration enhancementby cyclodextrins.

The effect of temperature (8, 12, 15 and 18 degrees C) on a variety of non-specific defence and haematological parameters was examined in three geographically distinct reared strains (Canadian, Icelandic, Norwegian) of Atlantic halibut. The results indicate that temperature exerts a considerable influence on some blood parameters (packed cell volume and the percentage population of leucocytes in peripheral blood) and on some humoral parameters (serum lysozyme activity and serum protein levels) of halibut. A high temperature of 18 degrees C caused a decrease in the number of circulating blood cells and an increase in serum lysozyme levels; effects consistent with those reported within the literature for stress. The different strains of halibut exhibited differing responses with respect to differential counts of peripheral blood lymphocytes and thrombocytes, and to serum protein concentrations, serum lysozyme activity, serum iron content, unsaturated iron binding capacity of serum and O2- production by kidney macrophages.

This is the first description of a persistent subclinical nodavirus infection in the Atlantic halibut Hippoglossus hippoglossus. Juvenile fish (1 to 5 g) were sampled at 4, 5 and 8 mo of age at a fish farm in Norway during and after weaning. None showed clinical signs of viral encephalopathy and retinopathy (VER) or other disease. Pathological changes and/or nodavirus were detected by light microscopy, immunohistochemistry, reverse transcriptase polymerase chain reaction (RT-PCR) and transmission electron microscopy in all fish examined. High numbers of virus particles were found in macrophage-like cells in the central nervous system, including brain and retina (CNS). The virus particles displayed the icosahedral shape and size (approximately 25 nm) characteristic of noda- viruses. The virus-infected cells formed focal cell aggregates and were seen in all regions of the brain and all nuclear cell layers of the retina. The cytoplasm of the infected cells was filled with membrane- enclosed inclusions packed with virus particles. Some virus particles lay along membranes and formed membrane-bound necklace-like arrangements. The virus-infected cells of the retina also contained pigment granula located generally inside virus inclusions and sometimes forming a coating around the virus particles. All frontal parts with the eyes and brain and 50% of the mid-parts, which included the abdominal organs, were found positive for nodavirus with RT-PCR. Pathological changes in these persistently nodavirus-infected fish differ from earlier descriptions in Atlantic halibut during out- breaks of VER. Vertical transmission from infected spawners is believed to be a major route for nodavirus infection. Detection of nodavirus in subclinical infected fish and a better understanding of its pathogenesis are important in order to prevent the spread of nodavirus in the fish-farming industry.

Atypical strains of Aeromonas salmonicida are the causal agent of atypical furunculosis or ulcer disease in various fish species, including spotted wolffish Anarhichas minor, which is a promising species in the Norwegian fish-farming industry. Isolates of atypical A. salmonicida comprise a very heterogenous group showing large variety in biochemical, molecular and virulence characteristics. The genetic variability among atypical isolates from wolffish was characterised using amplified fragment length polymorphism analysis: AFLP-fingerprinting. Additional isolates from halibut Hippoglossus hippoglossus, turbot Scophthalmus maximus, cod Gadus morhua and several salmonid fishes were included for assessment of variability and relatedness among a total of 56 atypical isolates of A. salmonicida. They were compared to reference strains of A. salmonicida subspecies and to other Aeromonas species pathogenic in fishes. AFLP-fingerprints subjected to similarity analysis yielded a grouping of the isolates into several clusters, revealing genetic heterogeneity among the isolates. There seems to be a correlation between genetic similarity among isolates and the fish host. The Icelandic isolates, mainly from cod, formed a very homogeneous subcluster, which was closely related to the wolffish isolates. All atypical isolates from spotted and common wolffish grouped together in a large cluster and appear to be very homogeneous, even though they had been isolated over a period of 8 yr at different locations in Norway. On the other hand, most of the isolates from turbot and halibut grouped together into 2 different clusters, while the 9 atypical isolates from salmonids appeared in 4 different clusters. Thus, the atypical isolates of A. salmonicida from halibut, turbot and salmonid fishes seem to be more genetically diverse than those from wolffish and cod.

Turbot and Atlantic halibut are highly valued fish species. However,very little is known about fillet shelf-life characteristics associated withboth species. Thus, fillet α-tocopherol content and proximate compositionof wild turbot (1.5 kg) and Atlantic halibut (1.1 kg)caught off the south coast of Ireland and the north-west coast of Iceland,respectively, were investigated. In addition, the susceptibility of fillets, storedunder retail conditions, to lipid oxidation and colour change was studied.Proximate composition analysis showed that turbot had significantly highermoisture (P < 0.001) and lower protein (P < 0.001) contents compared toAtlantic halibut. Atlantic halibut incorporated significantly higher (P

The atypical strains of Aeromonas salmonicida isolated from spotted wolffish Anarhichas minor O., show biochemical and molecular variations, but it is not known whether these variations affect bacterial virulence or antigenicity of possible protective antigens. In this study, protection against atypical furunculosis was compared in spotted wolffish vaccinated with monovalent or multivalent vaccines containing different strains of atypical A. salmonicida. In addition, these vaccines were compared to a vaccine containing an atypical A. salmonicida isolate from Atlantic halibut, Hippoglossus hippoglossus L. Vaccinated fish were challenged with three different A. salmonicida strains from spotted wolffish. Significant protection was obtained against both homologous and heterologous challenge. The best protection, regardless of strain used for challenge, was obtained with the multivalent vaccine composed of three different bacterial strains. Also, some of the monovalent vaccines resulted in significant protection, but they seemed to differ in protection against one particular strain of atypical A. salmonicida. In addition, significant cross-protection was observed with a vaccine containing a strain isolated from halibut, while a commercial salmon vaccine against furunculosis based on A. salmonicida subsp. salmonicida, failed to protect against an atypical strain from spotted wolffish.

Larval Atlantic halibut fed Artemia has previously been shown to contain lower levels of Vitamin A compared to larvae fed zooplankton. The two types of live prey contain small or no amounts of vitamin A, but high levels of carotenoids that can be converted to vitamin A in other fish species. The purpose of this study was to investigate the ability of Atlantic halibut juveniles to convert β-carotene, astaxanthin and canthaxanthin to vitamin A. Three levels of each carotenoid and retinyl acetate were fed to Atlantic halibut juveniles for 60 days. A vitamin A and carotenoid deficient diet was fed in triplicate as control. A HPLC method modified from Noll (1996) and validated for fish matrix was used to quantify both all-trans-retinol and 3,4-didehydro retinol. By comparing regression coefficients we observed that the increasing levels of carotenoids in the diets were reflected in increasing levels of vitamin A in both whole fish and liver samples. All carotenoids were converted to vitamin A, but to different degrees. Retinyl acetate and β-carotene resulted in whole fish vitamin A levels significantly higher than canthaxanthin and astaxanthin. 3,4-didehydro retinol was not detected when the overall level of all-trans-retinol was low. When 3,4-didehydro retinol appeared, it was always in lower levels than all-trans-retinol.

One of the main problems facing Atlantic halibut hatcheries is the high mortality in the early stages of larval development. Several factors could be involved, for example: water quality, diseases or abnormalities, such as deformities occurring in the yolk sac larvae prior to exogenous feeding. The aim of this study was to identify differences in bacterial flora associated with yolk sac larvae with oral deformity. We also aimed to establish whether there is any relationship between bacterial strains and the "gaping jaws" syndrome. During our study, 74 bacterial isolates were obtained using three different nutrient media: Marine Agar, R2A and TCBS. Some of these bacteria were characterized using Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP) and 16S rRNA sequencing. The immune response in larvae exhibiting the "gaping jaws" condition was measured by real time PCR. Our results showed significant differences in bacterial flora between normal and gaping larvae. The gaping yolk sac larvae were predominantly colonized by members of the families Vibrionaceae and Flavobacteriaceae. Bacteria belonging to the Bacillus and Pseudoalteromonas genera were also present but less frequent. It was not possible to associate a type or group of bacteria directly related to "gaping". Strikingly, larvae with gaping jaws had an increase in the expression of two immune related genes, like hepcidin and chemokine (MIP-1B). These results indicate activation of the immune response in larvae with "gaping jaws" syndrome and this response could be related to bacteria isolated from gaping condition. Uno de los mayores problemas que enfrentan los criaderos de halibut del Atlántico es la alta mortalidad en estados tempranos del desarrollo larval. Distintos factores pueden estar involucrados, e.g., calidad del agua, enfermedades o anormalidades tales como deformidades que ocurren en las larvas en estado de saco vitelino antes de la alimentación exógena. El objetivo de este estudio fue identificar diferencias en la flora bacteriana asociada a larvas con saco vitelino con deformidad oral. Además se buscó establecer si existió relación entre alguna cepa bacteriana y el síndrome de "gaping jaws". Durante el estudio, 74 cepas bacterianas fueron aisladas usando diferentes medios de cultivo: agar marino, R2A y agar TCBS. Algunas de las bacterias fueron caracterizadas usando reacción en cadena de la polimerasa con análisis del polimorfismo de los fragmentos de restricción (PCR-RFLP) y secuenciación del gen ribosomal 16S. La respuesta inmune de larvas con el síndrome de "gaping jaws" fue medida por PCR en tiempo real. Los resultados evidenciaron diferencias significativas en la flora entre larvas normales y "gaping jaws". Las larvas con "gaping" fueron principalmente colonizadas por miembros de las familias Vibrionaceae y Flavobacteriaceae, mientras que bacterias pertenecientes a los géneros Bacillus y Pseudoalteromonas se observaron en baja frecuencia. No fue posible asociar directamente a un grupo bacteriano con el desarrollo del "gaping", sin embargo fue llamativa la respuesta inmune de larvas con "gaping", ya que mostraron un aumento en la expresión de los genes de la hepdicina y la citoquina (MIP-1B). Estos resultados indican la activación de la respuesta inmune de las larvas con el síndrome de "gaping" y podría estar relacionada con las bacterias aisladas para la condición de "gaping".

Ocean acidification and warming are both primarily caused by increased levels of atmospheric CO2, and marine organisms are exposed to these two stressors simultaneously. Although the effects of temperature on fish have been investigated over the last century, the long-term effects of moderate CO2 exposure and the combination of both stressors are almost entirely unknown. A proteomics approach was used to assess the adverse physiological and biochemical changes that may occur from the exposure to these two environmental stressors. We analysed gills and blood plasma of Atlantic halibut (Hippoglossus hippoglossus) exposed to temperatures of 12 °C (control) and 18 °C (impaired growth) in combination with control (400 µatm) or high-CO2 water (1000 µatm) for 14 weeks. The proteomic analysis was performed using two-dimensional gel electrophoresis (2DE) followed by Nanoflow LC-MS/MS using a LTQ-Orbitrap. The high-CO2 treatment induced the up-regulation of immune system-related proteins, as indicated by the up-regulation of the plasma proteins complement component C3 and fibrinogen β chain precursor in both temperature treatments. Changes in gill proteome in the high-CO2 (18 °C) group were mostly related to increased energy metabolism proteins (ATP synthase, malate dehydrogenase, malate dehydrogenase thermostable, and fructose-1,6-bisphosphate aldolase), possibly coupled to a higher energy demand. Gills from fish exposed to high-CO2 at both temperature treatments showed changes in proteins associated with increased cellular turnover and apoptosis signalling (annexin 5, eukaryotic translation elongation factor 1γ, receptor for protein kinase C, and putative ribosomal protein S27). This study indicates that moderate CO2-driven acidification, alone and combined with high temperature, can elicit biochemical changes that may affect fish health.

One of the main problems facing Atlantic halibut hatcheries is the high mortality in the early stages of larval development. Several factors could be involved, for example: water quality, diseases or abnormalities, such as deformities occurring in the yolk sac larvae prior to exogenous feeding. The aim of this study was to identify differences in bacterial flora associated with yolk sac larvae with oral deformity. We also aimed to establish whether there is any relationship between bacterial strains and the "gaping jaws" syndrome. During our study, 74 bacterial isolates were obtained using three different nutrient media: Marine Agar, R2A and TCBS. Some of these bacteria were characterized using Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP) and 16S rRNA sequencing. The immune response in larvae exhibiting the "gaping jaws" condition was measured by real time PCR. Our results showed significant differences in bacterial flora between normal and gaping larvae. The gaping yolk sac larvae were predominantly colonized by members of the families Vibrionaceae and Flavobacteriaceae. Bacteria belonging to the Bacillus and Pseudoalteromonas genera were also present but less frequent. It was not possible to associate a type or group of bacteria directly related to "gaping". Strikingly, larvae with gaping jaws had an increase in the expression of two immune related genes, like hepcidin and chemokine (MIP-1 beta). These results indicate activation of the immune response in larvae with "gaping jaws" syndrome and this response could be related to bacteria isolated from gaping condition.

Muller [1] was the first to describe a monogenean, collected from the skin of the halibut (Hippoglossus hippoglossus). However, he regarded the parasite as a leech and named it Hirudo hippoglossi. It was not until 1858 that its status as a monogenean was established by van Beneden [2] and named Epibdella (now Entobdella) hippoglossi. Van Beneden published a detailed and accurate description of the parasite and one of his excellent illustrations is reproduced here in Figure 1. Entobdella hippoglossi is one of the largest monogeneans, measuring up to 2 cm in length. It has a smaller relative, measuring 5 to 6 mm in length, which was described by van Beneden and Hesse in 1864 [3] and named Phyllonella (now Entobdella) soleae from the skin of the Dover or common sole, Solea solea (Figures 2 and 3). This parasite is now perhaps the best known monogenean in terms of its biology [4-6].

The correlation between positive phototaxis and feeding incidence at first-feeding for groups of Atlantic halibut larvae ranging in age from 210 to 240 degreedays, post hatch, was examined. Phototactic response was measured as the fraction of larvae that responded by horizontal swimming towards a light source, and the median travel distance for the responding fraction of larvae. Within the ranges investigated, larval age, size or proportion of deformed larvae had no significant effect on the phototactic response, or on the feeding incidence (deformed larvae excluded from analysis). The fraction of larvae responding phototactically and the median travelling distance for responding larvae were significantly correlated. Feeding incidences after 24 and 48 h were also significantly correlated with both measures of phototactic response. The possibility of using phototactic response in quality assessment is discussed.

Most molecular studies on the visual system in fish have been performed on freshwater teleosts such as goldfish and zebrafish where cones and rods appear simultaneously. Many marine fishes have long larval phase in the upper pelagic zone before transformation into a juvenile and a benthic life style. The retina at the larval stages consists of only single cone cells; later during metamorphosis double cones and rods develop. The flatfish Atlantic halibut (Hippoglossus hippoglossus) is a typical example of a marine species with such a two-step retina development. In this study, we have cloned five different opsins from Atlantic halibut larvae and juvenile retinas. Sequence comparisons with other opsins and phylogenetic analysis show that the five genes belong to the opsins of long-wavelength sensitive (L); middle-wavelength sensitive, MCone and MRod; and short-wavelength sensitive, SBlue and SUltraviolet, respectively. In situ hybridization analysis reveals expression in double cone (L and MCone), single cone (SBlue and SUltraviolet), and rod (MRod) types of photoreceptor cells in juvenile halibut retina. The visual system in Atlantic halibut seems therefore to have all four types of cone photoreceptors in addition to rod photoreceptors. This work shows for the first time molecular isolation of a complete set of retinal visual pigment genes from a marine teleost and describes the first cloning of an ultraviolet-sensitive opsin type from a marine teleost.

European larviculture techniques for commercially farmed marine finfish species are reviewed with reference to recent developments in the fields of health management, larval nutrition and system operation. Specific rearing methods and critical aspects of production are described for sea bream Sparus aurata, sea bass Dicentrarchus labrax, turbot Scophthalmus maximus, and Atlantic halibut Hippoglossus hippoglossus. The European industry is dominated by rapidly expanding production of juvenile sea bream and sea bass (approximately 450 million fry in 1999). These species are mainly reared intensively in green water, using Brachionus and Artemia as prey, although hyper-intensive clear water techniques and extensive/semi-intensive rearing methods are also used. Large “industrial” hatcheries, incorporating system automation and standardized operating procedures, now account for a high proportion of total European sea bass and sea bream production. Intensive larviculture techniques are also now prevalent for the production of turbot and Atlantic halibut in Europe, although several key operators retain the use of extensively reared zooplankton, owing to pigmentation abnormalities associated with Brachionus and Artemia prey. Output of these flatfish species is relatively low and rising slowly (circa 5 million and 750,000 fry, respectively, in 1999). Future development of the European marine fish hatchery sector is considered in terms of increasing output levels and production efficiency for existing farmed species and adapting hatchery facilities and techniques to enable species diversification.

The appearance and distribution of cholecystokinin (CCK)-producing cells were investigated in the digestive tract of developing larvae of cultured Atlantic halibut, Hippoglossus hippoglossus. The CCK-producing cells were detected immunohistochemically, by use of a primary antiserum against CCK cloned for the Japanese flounder, Paralichthys olivaceus. No CCK-immunoreactive (IR) cells were detected in first-feeding larvae (33 days after hatching, DAH). Forty-five DAH or 12 days after first feeding, there were a few scattered CCK-IR cells in the epithelium of the anterior midgut in about 30% of the examined larvae. All larvae older than 52 DAH had CCK-IR cells in the anterior midgut, particularly frequent in the most anterior region adjacent to the pyloric caeca. No CCK-IR cells were detected in the foregut, the hindgut, or the midgut posterior to the first curvature. The CCK-IR cells spanned the intestinal epithelium from the basal lamina to the lumen and were triangular in shape, with the nucleus in the basal part and a thin apex toward the lumen. The mechanisms controlling release of bile, pancreatic enzymes, and peristalsis during the 12 days between first feeding and the first detection of CCK-IR cells remain to be clarified.

A diet for Atlantic halibut-larvae was formulated taking into account the fact that marine-fish larvae have a limited ability to assimilate protein and lipid. Dietary protein consisted of a free amino-acid premix (7.2% of crude protein), predigested-squid mantle (7.2%), squid mantle (8.6%) and cod-muscle mince (77.0%). Lipid sources were soyabean lecithin (33% of crude lipids), crude phospholipids extracted from cod roe (10%) and sardine oil (57%). Larvae were weaned onto the experimental diet at wet-body weights of 0.07, 0.10 or 0.16 g, respectively. The experimental diet was fed for 31, 25 or 17 days, respectively, and the experiment was terminated on the same calendar day for all groups. A control group was fed with Artemia nauplii enriched with DHA Selco™ from 0.07 g. Survivals ranged from 78% in larvae transferred at 0.10 g to 96% in those transferred at 0.16 g and in the control group. Daily specific-growth rates (SGR) were 3.1 ± 0.07, 3.3 ± 0.11 and 2.2 ± 0.01% day−1 in larvae transferred at 0.07, 0.10 and 0.16 g, respectively, while growth in the control group was 5.1% day−1. It was concluded that weaning of Atlantic-halibut larvae is feasible from 0.7 g (approximately 20 days post first-feeding) when the formulated diet contains predigested protein and ample amounts of phospholipids.

Juvenile Atlantic halibut (Hippoglossus hippoglossus L.) and turbot (Scophthalmus maximus L.) acclimated to 8–9 °C sea water were transferred to sea water at 1 °C for 7 days. An immediate spasm-like response was noticed in the turbot after the transfer, but not in the halibut. None of the fish died. The turbot suffered from substantial physiological disturbances after the transfer. Their blood plasma Cl– concentration increased and muscle water content decreased markedly. The plasma glucose concentration was several times higher than the control level from day one onwards. A decrease in the haematocrit was recorded some hours after transfer, followed by a marked increase on day 7. In halibut, the plasma Cl– concentration fell somewhat during the first days, but returned thereafter to its pretransfer level. The muscle water content was unchanged. Both haematocrit and plasma glucose concentration were unchanged until day 7, when both were significantly higher. The differences in the response to low-temperature challenge between halibut and turbot are probably related to genetic differences in temperature tolerance, which reflect differences in the distribution of the species.

The commercialization of Atlantic halibut, Hippoglossus hippoglossus , aquaculture has been hampered by a failure to obtain consistently high fertilization rates. The principal goal of this research was to determine the optimum sperm density for successful fertilization of Atlantic halibut eggs. Sperm densities ranged from 2×10 11 to 6×10 11 spermatozoa/ml at 23% and 99% spermatocrit, respectively, for 36 milt samples collected from 17 males. Regression analysis showed a significant positive linear relationship between sperm density and spermatocrit, supporting the use of spermatocrit as a rapid estimator of sperm density in this species. There was no relationship between sperm density and sperm motility (defined as time elapsed from activation until 8 to 9.4×10 5 spermatozoa/egg, respectively.

Small raceways were used in a weaning experiment with Atlantic halibut (Hippoglossus hippoglossus L.) larvae. The size of the tanks was 1.0 × 0.4 m with a 1- to 2-cm water level. Duplicate larval groups were transferred to the raceways from circular first feeding tanks at 0.07, 0.10 and 0.16 g wet weight, while recommended weaning size of this species is 0.2–0.3 g. During the first 7 days of weaning, Artemia was used as a food supplement in combination with the formulated dry feed. Thereafter only dry feed was used. The dry feed used in this experiment was produced by a special heat technique. The 0.07, 0.10 and 0.16 g larval groups were evaluated after 31, 25 and 17 days respectively (same date) The corresponding average survival was 81.4%, 78.0% and 96.6% and the specific daily growth rate was 3.18%, 3.17% and 2.38% respectively. In the Artemia control group, a survival rate of 96.0% and a growth rate of 5.28% was achieved. To evaluate the weaning success, the groups were followed in a 22-day post-weaning period on a commercial dry diet. Higher growth rates, 5.8–6.9%, were then obtained in all experimental groups, except control. The survival here averaged approximately 80% in the three experimental groups, but showed some differences between replicates. One hundred per cent survival was achieved during weaning in the former Artemia group. From the start of weaning to the end of the post-weaning period, the survival rates averaged 64% for the 0.07 and 0.10 g groups, approximately 80% for the 0.16 g group and 96% in the Artemia control group. Higher variance (CV) through the experiment and highest growth of the 75% quartiles of the fish groups compared with the 50% and 25%, indicated suppressed growth of the smaller fish. The successful weaning at these small sizes considerably reduced the live food period. Based on the present knowledge of the energetic demands of this species, it is calculated that weaning at 0.07 g compared with 0.25 g will reduce the amount of Artemia needed by at least 60%. No differences in pigmentation or degree of completed eye migration were detected between groups, indicating that this is determined at earlier developmental stages.

Physical damage among farmed Atlantic halibut, comprising injuries to eyes, pectoral fins and tails, first becomes apparent post-weaning. This study examined social interactions in farmed halibut, to determine whether potentially serious aggression occurred in fish of different age classes in which injury has been reported. Specific aims were to understand the context in which such behaviour happens and to examine individual variations in aggressiveness. Behavioural data were collected via direct observation and video footage from halibut of six size classes in Norway and the UK. Food was delivered to experimental tanks, one pellet at a time, to enable the identification of consumers and the registration of all social activity. In five size classes (45–3000 g), potentially damaging contact among fish, occurred solely during feeding and comprised targeted aggression (nips and chases) and collisions due to misdirected feeding attempts. Feed intake was positively correlated with levels of aggression, and fish were most aggressive early in a feeding session. A mean 85% of attacks were directed at fish that had won pellets from aggressors. The frequency of aggression declined markedly with fish size, and no aggression was observed in the largest size class (2.5–10 kg). Our studies indicate that the majority of aggression occurs early in hand feeding sessions, when fish are particularly hungry. To reduce the occurrence of this behaviour, we suggest that feed should be delivered by a combination of hand and automatic feeders. In this way, feed can be well-dispersed and provided throughout the day, thereby preventing a build-up of hunger levels and ensuring that fish are fed to satiation.

Ultrasonography is a non-invasive method that can be used to assess sex and maturity of fish. This study was conducted to examine the feasibility of using ultrasound images to sex juvenile and mature halibut Hippoglossus hippoglossus (Linnaeus), mature winter Pleuronectes americanus (Walbaum) and yellowtail flounder Pleuronectes ferruginea (Storer) and mature haddock Melanogrammus aeglefinus (Linnaeus). In females, both immature and mature ovaries were relatively easy to distinguish, but in males only mature testes were consistently distinguishable. Ovarian maturation of haddock and ovulatory cycles of halibut may also be assessed using ultrasound.

This review is aimed atelucidating the mechanisms that regulate growthin cultured juvenile Atlantic halibut (Hippoglossus hippoglossus) and turbot (Scophthalmus maximus) by evaluating thesignificance of environmental factors(temperature, photoperiod) and the interactionsbetween them. In addition, we examine growthproperties in three populations of juvenilehalibut and turbot in order to elucidate anygeographical differences in growth and growthefficiency in these species. Both temperatureand photoperiod have a significant andpersistent effect on growth rate in bothspecies. Temperature tolerance, demonstrated bya wide temperature range supporting maximalgrowth, increased with size. Fish subjected tocontinuous light exhibited faster growth thanthose experiencing a natural photoperiod or aconstant short day. Moreover, when thephotoperiod increased naturally with day-lengthor when fish were abruptly switched from beingreared on short-day conditions to continuouslight, a subsequent increase in growth rate wasobserved. This growth enhancing effect ofextended photoperiods was more apparent in ashort time scale in Atlantic halibut than inturbot, but both species show significantlong-term effects of extended photoperiods. Enhanced growth in fish in continuous light waspartly explained by higher growth efficiency.In both species, there was a significantinteraction between temperature andphotoperiod, suggesting that thegrowth-enhancing effect of continuous light isrelatively stronger at lower temperatures.Growth rate in both species was alsosignificantly influenced by the interaction oftemperature and fish size, as the optimaltemperature for growth decreased rapidly withincreasing fish size. Differences in growth,food intake, food conversion efficiency,metabolism, ammonia excretion and RNA/DNAratios in white muscle were observed betweendifferent strains of halibut and turbot, withthe best growth properties being observed inthe northern populations. These findings onhalibut and turbot partly support the theory ofcountergradient variation in growth, suggestingthat populations from high latitudes havehigher growth capacity than populations fromlow latitudes.

Theaim of this study was to determine whether it is possible to influence thespecies composition of the gut microflora in 70-days old halibut larvae(Hippoglossus hippoglossus L.) by addition of bacteriabioencapsulated in Artemia franciscana in two shortpulses.Two Vibrio strains, PB 1-11 and PB 6-1 were used. Twotreatments received each strain alone, a third treatment received the twobacterial strains in mixture, whereas in a control treatment no bacteria wereadded. Five fish tanks were included in each treatment. The bacteria werebioencapsulated in Artemia franciscana metanauplii duringashort-term incubation in bacterial suspensions of the specific strains, andwereadded in two pulses, on day 1 and day 10 of the experiment. Addition ofbacteriadid not increase the total number of colony-forming units (CFU) in the larvalgut. The total CFU in the water was lower in tanks added bioencapsulatedbacteria than in the control treatment (p 0.05). The addition of bioencapsulated bacteria intwo pulses within a ten days period was therefore not sufficient to influencethe species composition of the microflora of the halibut larvae.

The absorption, distribution and elimination of 14C-labelled flumequine were studied using whole body autoradiography and liquid scintillation counting. Flumequine was administered to eel Anguilla anguilla, turbot Scophthalmus maximus and halibut Hippoglossus hippoglossus intravenously and orally as a single dose of 5 mg kg(-1), corresponding to 0.1 mCi kg(-1). The turbot and halibut studies were performed in salt water (salinity of 32%) at temperatures of 16 +/- 1 degrees C (turbot) and 9.5 +/- 0.5 degrees C (halibut). The eel study was conducted in fresh water at 23 +/- 1 degrees C. In the intravenously administered groups flumequine was rapidly distributed to all major tissues and organs. After oral administration flumequine also appeared to have rapid and extensive absorption and distribution in all 3 species. After the distribution phase, the level of flumequine was higher in most organs and tissues than in the blood, except in muscle and brain. The most noticeable difference between the species was the slow elimination of flumequine from eel compared to turbot and halibut. In orally administered eels, substantial amounts of flumequine remained in all major organs/tissues for 7 d. At 28 d significant levels of flumequine were present in liver, kidney and skin (with traces in muscle), and at the last sampling point (56 d) in eye, bone, bile and posterior intestine. In orally administered turbot significant levels of flumequine were observed over 96 h in bile, urine, bone, skin, intestine and eye, and traces were detected over 28 d in bone and eye in addition to a significant level in bile. In orally administered halibut, significant levels of flumequine were observed in bile, skin, intestine and eye over 96 h. Traces were present in skin and eye over 7 d. The maximal flumequine concentrations in blood were calculated to be 2.5 mg equivalents l(-1) (eel at 12 h), 0.8 mg l(-1) (turbot at 6 h) and 0.6 mg l(-1) (halibut at 6 h) after oral administration.

Studies were conducted with juvenile (28 g) Atlantic cod (Gadus morhua) and halibut (Hippoglossus hippoglossus) fed fish meal (FM) based diets with partial (0%, 25%, 50% and 75%) or total (100%) replacement of FM with freeze-dried krill (FDK). Addition of dietary FDK significantly stimulated feeding activity in both species and resulted in higher feed consumption and intakes of digestible protein (DP) and energy (DE). This is a significant finding on the feed attractant properties of FDK because it occurred despite having 2-4 MJ kg⁻¹ higher levels of DE in the FDK diets which could have had an intake suppression effect. In addition, it indicates that the soluble protein fraction of krill thought to be responsible for its attractant properties and is routinely lost or greatly reduced during commercial processing of krill meals, is retained by freeze-drying. Supplementation of FDK significantly improved growth rates, digestibility and nutrient utilization. Levels exceeding 25-50% FM replacement, however, generally did not significantly increase growth performance or nutrient utilization. In addition, feed formulations with high FDK (>75% replacement) are likely to be cost-prohibitive as long as the market price remains higher than FM.

Fish nodaviruses belonging to the genus betano-davirus are the aetiological agents of viral nervousnecrosis (VNN), also referred to as viral encephalo-pathy and retinopathy (VER), or fish encephalitis(Mori, Nakai, Arimoto, Mushiake & Furasawa1992; Comps, Pepin & Bonami 1994). This dis-ease is characterized by the development of avacuolating encephalopathy and retinopathy associ-ated with arrays of virus-like particles in infectedneurons. Since the first report of the disease in seabass

Apparent astaxanthin (3,3'-dihydroxy-beta,beta-carotene-4,4'-dione) digestibility coefficients (ADC) and carotenoid compositions of the muscle, liver, whole kidney and plasma were compared in Atlantic salmon (Salmo salar) and Atlantic halibut (Hippoglossus hippoglossus) fed a diet supplemented with 66 mg astaxanthin kg(-1) dry matter for 112 days. The astaxanthin source consisted of 75% all-E-, 3% 9Z- and 22% 13Z-astaxanthin, of (3R,3'R)-, (3R,3'S; meso)-, and (3S,3'S)-astaxanthin in a 1:2:1 ratio. The ADC of astaxanthin was significantly higher in Atlantic halibut than in Atlantic salmon after 56 and 112 days of feeding (P < 0.05). The ADC of all-E-astaxanthin was significantly higher than ADC of 9Z-astaxanthin (P < 0.05). Considerably more carotenoids were present in all plasma and tissue samples of salmon than in halibut. Retention of astaxanthin in salmon muscle was 3.9% in salmon and 0 in halibut. All-E-astaxanthin accumulated selectively in the muscle of salmon, and in plasma of salmon and halibut compared with diet. 13Z-astaxanthin accumulated selectively in liver and whole kidney of salmon and halibut, when compared with plasma. A reductive pathway for astaxanthin metabolism in halibut similar to that of salmon was shown by the presence of 3',4'-cis and trans glycolic isomers of idoxanthin (3,3',4'-trihydroxy-beta,beta-carotene-4'-one) in plasma, liver and whole kidney. In conclusion, the higher ADC of astaxanthin in halibut than Atlantic salmon may be explained by lower feed intake in halibut, and the lower retention of astaxanthin by a higher capacity to transform astaxanthin metabolically.

Juvenile Atlantic halibut ( Hippoglossus hippoglossus L., initial weight (SD) 31.1 g (±5.8 g)) were reared for 147 days at a constant temperature of 11°C and subjected to four different light regimes from February 10: continuous light (LD 24:0), simulated natural photoperiod of Bergen (60°25′N, LDN), constant 8 h light:16 h dark (LD 8:16) and LD 8:16 switched to continuous light on May 4 (LD 8:16–24:0). Fish exposed to LD 24:0 had a significantly higher specific growth rate (1.05% day −1 ) than those on LDN (0.98% day −1 ). Fish exposed to LD 8:16 had the poorest growth (0.80% day −1 ). The group on LD 8:16–24:0 (overall growth rate 0.94% day −1 ) grew poorly at first but subsequently increased growth rate after having been exposed to continuous light. This response was delayed by at least 21 days, suggesting that the fish needed some time to adapt to the change in photoperiod. Sensitivity to continuous light became less pronounced under summer photoperiods (above approximately 18 h daylength), which suggested that continuous light could be used to improve growth in juvenile halibut when the natural daylength is less than approximately 18 h. Overall mortality was 11.9%, with significantly higher mortality being observed in the LDN (17.4%) and LD 8:16–24:0 (17.1%) groups than in the LD 24:0 (6.2%) and LD 8:16 (6.8%) groups. Size-dependent mortality was seen in LD 8:16–24:0 in all periods from May 4 to July 6, with mortality being higher amongst small fish.

The presence of carbamoyl phosphate synthetase III (CPSase III), catalyzing the first step of the urea cycle in fish, in Atlantic halibut ( Hippoglossus hippoglossus L.) yolk-sac larvae and adult white muscle has been established using gel filtration chromatography to separate the CPSase III from the pyrimidine-pathway related CPSase II. The results are consistent with the hypothesis that teleostean fish express urea cycle enzymes during early development and with recent observations of low levels of CPSase III in muscle tissue. The presence of CPSase III in crude extracts could not be established using sensitive assay conditions to discriminate between CPSase III and CPSase II. However, kinetic characterization after chromatographic separation identified each as typical CPSase II and CPSase III activities, respectively. The CPSase III was less sensitive to activation by N -acetyl- l -glutamate and had a higher K m for ammonia than CPSase III found in other species. These results suggest that precise quantitation of low levels of CPSase III in the presence of CPSase II by assaying crude extracts may be difficult unless the enzymes are first separated and the kinetic properties of CPSase III are determined; the results indicate that assaying larval extracts of Atlantic halibut in the presence of uridine triphosphate results in CPSase activity that reflects mostly CPSase III and can, therefore, be used to measure changes in CPSase III activity.

A higher number of epidermal mucous cells was observed in Atlantic halibut ( Hippoglossus hippoglossus L.) yolk sac larvae kept in seawater from 50 m depth, containing 7.5×10 4 –10.3×10 4 bacteria/ml (DAPI), as compared to larvae kept in seawater from 200 m depth with 3.3×10 4 –5.0×10 4 bacteria/ml. The affected mucous cells were identified as saccular cells. The number of saccular cells was also higher in larvae kept in incubators with seawater that were supplemented with Lactobacillus plantarum (about 10 6 bacteria/ml). Thus, it appeared that the number of bacteria in the seawater could affect the proliferation of saccular cells in halibut larvae. Furthermore, survival of the halibut larvae was affected by pre-incubation of eggs and hatching larvae with different strains of commensal bacteria isolated from fish. A larval survival of 94.4% was obtained following pre-incubation with an apathogenic Vibrio salmonicida strain, whereas V. iliopiscarius sp. nov. reduced survival to 63.4% in the first 12 days, compared to 81% survival in the control group. In the same period, survival following incubation with L. plantarum was 95.8%. Survival 32 days post-hatching was significantly higher in the groups of halibut larvae pre-incubated with L. plantarum (68.4%) and V. salmonicida (72.8%), as compared to the control group (58.2%). Pre-incubation with V. iliopiscarius resulted in 41.8% survival at day 32, which was significantly lower than the control group. Thus, pre-incubation of halibut eggs and hatching larvae with strains of commensal bacteria may subsequently affect larval survival, and in the present study, an indigenous apathogenic strain closely resembling the marine pathogen V. salmonicida resulted in the highest survival.

Amino acid and proximate composition of the whole body tissue of juvenile Atlantic halibut, yellowtail flounder and Japanese flounder were determined. Higher protein and lower lipid levels were found in whole body of Japanese flounder than the other two species. The amino acid composition was, however, not significantly different among species when expressed as g/100 g amino acids. The values were also found to be very similar to those of rainbow trout and Atlantic salmon. The A/E ratios [(each essential amino acid content/total essential amino acid content including cystine and tyrosine)×1000] based on the essential amino acid composition of whole body tissue, suggest that the amino acid requirements of flatfish may not be greatly different among species.

Experimental bath challenge of juvenile and sub-adult Atlantic halibut with Vibrio anguillarum induced severe mortalities of 47 and 80%, respectively. However, animals vaccinated with a commercial V. anguillarum vaccine demonstrated excellent protection against the disease (100% RPS). This study also describes the gross pathology and histological changes associated with this infection. A loss of coordination, haemorrhage at the fin base and splenomegaly were frequent findings. Serum agglutinating activity demonstrated a rise following vaccination, the mean log2 titre rising from 3.8 to 8.4. This was associated with a significant rise in antibody-mediated complement killing ability of immune serum when compared to non-immune serum.

Growth rate of individually tagged medium-sized (249 ± 6.9 g) juvenile halibut was 18% lower when medium sized fish were reared alone (treatment Mm) as compared with rearing with either large/dominant (382 ± 12.1 g) (Ml) or small/subordinate (158 ± 3.1 g) (Ms) conspecifics. The coefficient of variation of weight of medium-sized fish increased with weight in both the Mm and the Ml group whereas it was stable in the Ms group. Size rank correlation between initial and final weight was highest in the Mm group and lowest in Ms. A negative rank correlation was found also between initial weight and overall growth rate for the Ms treatment groups but not the other groups. It is hypothesized that interactions between similar-sized individuals in the Mm treatment group had an inhibiting effect on growth as social hierarchies were being resolved.

The problem of determining when larvae should be offered food is particularly difficult in a species such as Atlantic halibut that has a long yolk-sac period (280–320 degree days, dd). In order to help determine at what age Atlantic halibut larvae are able to digest food, we compared the activities of key digestive enzymes in four yolk-sac stages at an age interval that has been recommended for initiation of feeding, i.e., 161–276 dd. We tested the hypothesis that digestive enzyme activities reach highest levels near the end of this age interval. Activities of trypsin, amylase, lipase and alkaline phosphatase were determined spectrophotometrically in whole yolk-sac larvae at 161, 179, 230, and 276 dd. The activities of the same enzymes were measured in metamorphic larvae (660 dd) and in their Artemia prey to provide reference levels from a fully developed digestive system and to estimate the importance of exogenous enzymes for Atlantic halibut larvae. Our results showed significant (P

Atypical furunculosis caused by atypical Aeromonas salmonicida is an increasing problem in commercial halibut farming, and a potential problem in farming of spotted wolffish in Norway. Halibut, Hippoglossus hippoglossus L., and spotted wolffish, Anarhichas minor L., vaccinated with oil-emulsified vaccines against atypical furunculosis, demonstrated a relative percent survival (RPS) of approximately 90 when challenged with homologous isolates. Bath and aqueous injection vaccines failed to protect against disease when challenged 10 weeks post-vaccination. High antibody titres were produced in both species after vaccination with oil-emulsified vaccines, and the major antibody response were against A-layer, LPS and some minor outer membrane (OM) proteins.

Higher growth capacity and food conversion efficiency was observed in populations of juvenile halibut from high Hippoglossus hippoglossus compared lower latitudes. In addition, temperature adaptation shown by the lower temperature optimum for growth in the Norwegian population (mean ± S.E. 12.9 ± 0.1° C) compared with the Icelandic and Canadian populations (14.2 ± 0.2 and 13.9 ± 0.3° C respectively), seems to occur. Overall the data support the hypothesis of countergradient variation in growth These results have implications firstly for selection focusing on growth performance in halibut culture; and secondly, for safe prediction of growth, since if countergradient variation in growth performance occurs one cannot assume automatically that a species will respond to the same set of physiologica parameters in the same way throughout its range.

The combined effect of temperature and photoperiod on the growth of juvenile Atlantic halibut Hippoglossus hippoglossus (initial weight 11.6 g) was investigated under different temperature (6 °C and 12 °C) and photoperiod (continuous light and simulated natural photoperiod) combinations by studying the effects on fish weight, specific growth rate (SGR), feed consumption (F%), feed conversion (FCE) and oxygen consumption and ammonia excretion under routine and fasting conditions. SGR was greater at the higher temperature, and growth was faster under continuous light than under natural photoperiod, irrespective of temperature, SGRs being 0.83% and 0.98%, and 1.61% and 1.71% at 6 °C and 12 °C respectively. The weight data revealed a significant interaction between temperature and light, and provided evidence of a stronger growth-enhancing effect of continuous light at the lower temperature. F% increased with temperature but was independent of photoperiod, whereas FCE was higher among fish exposed to continuous light, irrespective of temperature. Independently of photoperiod, oxygen consumption and ammonia excretion increased with temperature, but the differences in these rates in relation to photoperiod became more pronounced at the lower temperature, where continuous light gave the highest rates. Higher oxygen consumption during darkness suggested that light may suppress activity in halibut. It was proposed that reduced activity and anabolic effects of photoperiod contribute to explain the increased growth and growth efficiency in fish subjected to continuous light.

Radiolabelled bacterial lipopolysaccharide (3H-LPS) obtained from Aeromonas salmonicida subsp. salmonicida was added to the petri dishes containing yolk sac larvae of Atlantic halibut (Hippoglossus hippoglossus L.). The larvae were exposed either to 6.25, 12.5, 25, 50 or 100 micrograms 3H-LPS ml-1. The uptake was both dependent on the LPS concentration and the time of exposure. After 5 days of exposure, each larva contained 1.8-7.4 ng 3H-LPS dependent on the initial concentration. After 10 days of exposure each larva contained 7.0-12.4 ng LPS and after 15 days they contained 18.3-34.9 ng 3H-LPS. Fluorescence microscopic analysis of sections obtained from larvae exposed to FITC-LPS (25, 50 and 100 micrograms ml-1) for 5, 10 and 15 days, revealed fluorescence in intestinal epithelial cells, cells in the connective tissue adjacent to the intestine, in cells located between the integumental layer and yolk sac, and in some epithelial cells in the integument. By use of immunohistochemical techniques, LPS was confined to intestinal epithelial cells, lumen of excretory duct and in numerous cells in the epidermal layer. Control specimens did not contain fluorescence or were immunohistochemically negative for LPS. In groups of larvae exposed to 12.5, 25, 50 and 100 micrograms LPS ml-1, the survival was significantly increased after exposure to 50 and 100 micrograms LPS ml-1 from day 20 (96 d degree) and throughout the yolk sac period compared to untreated larvae.

In two separate spawning seasons, spermiating male Atlantic halibut were implanted with pellets containing gonadotrophin-releasing hormone agonist (GnRHa). Males were bled repeatedly, and milt samples were collected. Blood samples were assayed for free and conjugated steroids: testosterone, 11-ketotestosterone, 17,20α-dihydroxy-4-pregnen-3-one (17,20α-P), 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P), 17,20β,21-trihydroxy-4-pregnen-3-one and steroids with a 17,20β configuration. Towards the end of the first season, pellets were implanted into three wild-caught and three hatchery-reared males. No control fish were available. The major progestogen in plasma was identified as sulphated 5β-pregnane-3β,17,20β-triol (3β,17,20β-P-5β-S). Concentrations of this steroid were stimulated by the GnRHa. Sulphated 17,20β-P was also identified in the plasma, but at 10-fold lower concentrations than 3β,17,20β-P-5β-S. In the middle of the second season, pellets were implanted into five hatchery-reared males; five unimplanted males were used as controls. Levels of androgens fell following GnRHa treatment, levels of progestogens rose briefly, and there was a significant increase in the fluidity of the milt. Of all the measured steroids, free and sulphated 17,20α-P showed the best correlation with milt fluidity.

Isolation in cell culture of nodavirus from Atlantic halibut Hippoglossus hippoglossus suffering from viral encephalopathy and retinopathy (VER) is described. The cell line SSN-1 was inoculated with tissue material from affected juveniles (60 d after first feeding). Extensive cytopathic effects (CPE) developed approximately 5 d after inoculation, and were also observed after several passages in the same cell line. Cells from infected cultures showed reactivity with an antiserum against sea bass Dicentrarchus labrax nodavirus in an indirect immunofluorescence test. Analysis of infected cells with reverse transcriptase-polymerase chain reaction (RT-PCR) resulted in a product of the predicted size using primers specific for striped jack Pseudocaranx dentex nodavirus. Electron micrographs of infected SSN-1 cells demonstrated virus particles that were approximately less than 30 nm. Challenge of Atlantic halibut larvae (4 d post-hatching) with supernatants from infected SSN-1 cells resulted in development of VER as verified by immunohistochemistry performed on larvae sampled from Day 9 after challenge. The present results show that a nodavirus from Atlantic halibut has been isolated using the SSN-1 cell line and that virus propagated in cell culture retained virulence.

The aim of the present study was to investigate the pattern of food selection by larvae of Atlantic halibut fed Brachionus plicatilis and different size classes of Artemia franciscana, including short-term enriched (ST) A. franciscana nauplii. A second aim was to study the digestibility of different size classes of A. franciscana by halibut larvae. The larvae did not show any significant preference for B. plicatilis or nauplii of A. franciscana during the first 2 days after initiating first feeding at 285 day degrees. From day 3 and onwards, they selected significantly (P

After 7 d of ice storage, the muscle pH and flesh color (CIE L*, a*, b*) was measured in 118 Atlantic halibut (Hippoglossus hippoglossus) slaughtered at different times of the year. All fish originated from the same stock and the fish were farmed under natural and continuous light and slaughtered fed or starved, representing a wide distribution of fish undergoing different stages of growth, maturation, and sizes (1.5 to 5.9 kg). Results show a considerable variation of end pH ranging from 5.7 to 6.8. The color correlated significantly with muscle pH with R = −0.63, 0.61, and −0.57 for L*, a*, and b*, respectively, while fillet weight and sampling position on the fillet had less influence on color. We conclude that the end pH of the muscle is an important factor for the visual quality of fish, and in particular for Atlantic halibut. Therefore, measures should be taken to avoid high glycogen levels at death, as this will have a large impact on the muscle pH, with negative consequences for flesh color.

Variation in growth and reproduction dynamics of fish populations substantially affects production in wild as well as domesticated fish populations. Identifying the intrinsic and environmental factors which influence the dynamics of these traits is important for stock assessment methodology and management of wild fish populations as well as for improving production in aquaculture. In this paper the relationship between growth and maturation of cod (Gadus morhua) and Atlantic halibut (Hippoglossus hippoglossus) is described using data from spawning locations along the Norwegian coast. Both species show an inverse relationship between immature growth and age/size at first spawning. Long term trends may be connected to climatic changes as well as to the increased exploitation experienced in this century. Shortterm variation may be caused by abrupt changes in the conditions for growth, e.g. due to temperature shifts and/or variation in prey abundance.

To analyse the role of thyroid hormones in development of the halibut Hippoglossus hippoglossus, a cDNA library was constructed from mRNA isolated from metamorphosing halibut larvae and approximately 500 000 recombinants were screened using a probe derived from the thyroid hormone receptor (TR)-α gene of the frog Xenopus laevis. A single positive clone was isolated and sequenced. It was found to contain the entire coding sequence of the halibut TR-α gene. Expression of the TR-α gene during halibut development was monitored using reverse-transcriptase polymerase chain reaction, which showed that there is a several-fold increase in the level of TR-α expression around the time of hatching but that the level of TR-α expression is relatively constant thereafter.

To develop a compact farming system, a shallow raceway has been tested on five flatfish, three bottom-dwelling species and three open-water species. The main effort has been on turbot Scophthalmus maximus L., Atlantic halibut Hippoglossus hippoglossus L. and spotted wolffish Anarhichas minor (Olafsen). The system is almost a standard raceway, but with a very low water level (7 mm for fish at about 100 mg; 25 cm for fish above 2 kg). The system is a package with some crucial preconditions. These include high fish density (often 100–500 kg m− 3), no countercurrent in the levelled raceways (no jet currents), adjustment of water intake with the most remote fish in mind and feeding with floating pellets. Self-cleaning is easily obtained in the system. The system has been tested for a wide size range of raceways (0.07–80 m2) and fish sizes (2 mg to 10 kg), normally with growth and survival rates as good as with traditional rearing systems. There seems to be a ‘learning window’; the earlier the fish are introduced to shallow raceways, the better they perform. The results indicate that a variety of fish species can be produced in shallow raceways; these can be stacked in racks, which facilitate re-use of water from level to level. A very compact and cost-effective farming system will be the outcome.

Ascorbic acid (AA) and α-tocopherol (α-TOH) levels in whole Atlantic halibut larvae were constant during the yolk sac stage at 170 and 131 ng individual−1, respectively. At hatching c. 80% of the AA and 97% of the α-TOH were contained within the yolk-sac compartment. With development, AA and α-TOH levels in the yolk decreased, at different rates. At first feeding (at 200 day degrees post hatch, D°PH)>95% of AA but 800 ng individual−1 at 483 D°PH.

In a first feeding experiment, halibut larvae were offered either short term enriched (ST) 1-day old Artemia franciscana or A. franciscana of successively increasing size (ST, 2-, 3- and 4-day old juveniles), from day 0 to day 60. No differences, either in growth (approximately 6.5% daily weight increase, DWI) or in survival (ca. 25%) between the two treatments were observed, but both were satisfactory. However, feeding increasing sizes of A. franciscana increased the number of completely pigmented and metamorphosed larvae from 4 to 20%. Protein/lipid ratio in the feed seems to be important for pigmentation and metamorphosis, provided the levels of essential fatty acids (esp. DHA) are sufficient. Juvenile A. franciscana may therefore be a better live feed than ST A. franciscana for halibut larvae. The fact that a relatively high load of bacteria was introduced by 4-day old A. franciscana , may have had a negative influence on the growth of the larvae. Tests indicated that a great part of the bacteria exhibited hemolytic activity, and many of the bacteria were apparently Vibrios . Strict microbial control in the production of live feed is very likely just as important as control of the nutritional composition of the larval feed. This is a challenge for further work.

Four experimental diets were fed to triplicate groups of Atlantic halibut in a 2×2 factorial design, testing the effects of protein source and the use of an attractant. The diets contained 61% fish meal (FM) or 37% FM and 28% soy protein concentrate (SPC) with or without a coating of squid meal. In the SPC diets, 44% of nitrogen was supplied from the SPC. The mean initial fish weight was 633 g, and at the end of the 12-week trial, the mean fish weight was 874 g. There was no significant effect of dietary treatment on specific growth rate (0.43±0.02% body weight d−1, mean±S.E.M.). Feed efficiency ratio was significantly lower in the group fed the SPC diet (1.22±0.02) compared with those fed the FM diet (1.28±0.02). Fish fed the SPC diets had a higher feed intake (0.64±0.02%) than those fed the FM diet (0.59±0.02%). Addition of squid had no effect on feed intake. The groups fed diets not supplemented with squid were used for determination of whole body chemical composition, and the remaining fish from these groups were subsequently stripped for feces for digestibility estimations. There were no effects of protein source on digestibility or protein or energy retention in whole fish. The fish fed the diet containing soy protein had a slightly higher dry matter (DM) content (P