The inter-tissue metabolic responses to hypoxia were determined in lungfish (Protopterus aethiopicus), and trout (Salmo gairdneri). Lungfish respond to hypoxia with a reduction in metabolic rate. It was intended to determine which tissue, or tissues exhibit decreased metabolic rates during hypoxia, and then compare the results with the metabolic reactions observed in trout, which are not reported to reduce metabolic rate during hypoxia. The metabolic potentials of the heart, brain, white muscle and liver in the African lungfish were estimated using enzymatic data. Metabolic effects of a 12 hr submergence were monitored using metabolite measurements. Heart was the most oxidative tissue, but also showed the greatest anaerobic potential. The brain displayed relatively low oxidative capabilities. White muscle remained almost inert. Although high energy phosphate concentrations in brain and heart did not fall during submergence, glycolysis was activated as indicated by cross-over plots, depletion of endogenous glycogen stores, and lactate accumulation. Blood-tissue lactate and glucose gradients indicated (1) that the heart and brain released lactate throughout submergence, (2) that after 12 hr of submersion the brain and heart were probably obtaining all their required glucose from the blood (3) that the liver released glucose throughout submergence, and (4) the white muscle was metabolically isolated from the rest of the body during submergence. The lack of measurable changes in white muscle metabolite concentrations coupled with the low enzyme activities leads to the suggestion that the most significant adaptation to hypoxia in these fishes may not be the capacity for increased anaerobic energy production. Instead, it is likely that the ability of the muscle to prevent the activation of glycolysis during hypoxic dysoxia is the key to the animal's survival. Histochemical and ultrastructural studies were done on the axial musculature of the lungfish. The small wedge of red coloured muscle evident upon gross examination was shown by histochemical demonstrations of lactate and succinate dehydrogenases, of adenosine triphosphatases, and of lipid to be composed of a mosaic of red and intermediate fibres. Respectively, these fibres measured 23.6 and 34.3 microns in average diameter. The bulk of the myotome is composed of white fibres having an average diameter of 67.3 microns. Mitochondrial density, capillarity and lipid content were very low for all fibres. These data suggest that the axial musculature is geared primarily for anaerobic function. The relatively large percentage of white muscle indicates that the overall metabolic rate of the axial muscle is low. The capacity of the muscle to exist with a reduced rate of ATP turnover (as was suggested above) may be related to the large proportion of white fibres present in the myotome. Tissue metabolites were measured in a hypoxia sensitive organism, the Rainbow trout (Salmo qairdneri), before and after exposure for 3 hr to inspired oxygen tensions of 20 torr (at 4°C). There were small changes in the brain but the energy status was maintained. The red muscle was the least affected. White muscle creatine phosphate was depleted. Various data indicate that the white muscle is the major user of glycolytic substrates and the major producer of lactate. The heart is stressed as indicated by a decline in glycogen, ATP, CrP, and the total adenylate pool. The liver exhibited declines in every indicator of metabolic homeostasis. The liver concentrations of glycogen did not decline. The fact that anaerobic glycolysis has been activated in the white muscle, while the muscle remains in metabolic communication with the other tissues via the blood, supports the suggestion that the metabolism of the white muscle will have a pronounced effect on the metabolic status of the whole animal. The trout is maintaining its rate of oxygen uptake while activating anaerobic glycolysis in the attempt to maintain 'normal' rates of energy utilization. The turnover rates of glucose and lactate were measured in trout subjected to the same hypoxic stress as above. Glucose turnover did not change while lactate turnover increased from 2.8 ± 0.4 µmoles/min./kg to 20.6 ± 6.8 µmoles/min./kg. The lack of increase in glucose turnover was attributed to the observation that liver glycogen concentrations do not change and so there is no increase in glucose flux. The increase in lactate turnover emphasizes the fact that anaerobic glycolysis is activated and that some tissues are oxidizing lactate. The problem of when a cell becomes hypoxic and the reactions of the cell to that stress is addressed. The cell (tissue, organ, animal) has two options if oxygen supply drops to a level which prevents oxidative metabolism from supplying all of the requirements for ATP synthesis. The cell may exhibit a decline in requirements, in which case the rate of ATP production need not be as high as in the oxidative state or, conversely, anaerobic energy production may increase in the attempt to maintain ATP production rates. The lungfish muscle appears to be capable of the former, thus preserving substrates for other tissues and reducing the rate of end-product formation. The trout white muscle, on the other hand, exerts a major influence upon the other tissues when the animal is stressed with hypoxia. The term 'energy conformer' is applied to animals which do not maintain oxygen uptake in the face of a declining supply, and which allow ATP production to decline concomittantly by not activating glycolysis to a marked degree. An energy regulator would activate glycolysis in the attempt to maintain oxidative rates of ATP production. The trout is more of an energy regulator than is the lungfish with the main difference in this capacity being in the white muscle. / Science, Faculty of / Zoology, Department of / Graduate

Rainbow trout (Salmo gairdneri) from two wild stocks native to British Columbia, and a non-native domestic strain were reared under varying conditions to examine effects of growth rate and body size, genetic stock, and photoperiod on the incidence and timing of testis development. The ratio of gonad weight to body weight (Gonadosomatic index) was found to be useful for separating mature and immature male fish and for determining the onset of gonadal development. In Premier lake fish testis development began one year before the expected date of spawning. At this time, signs of maturation were evident primarily among males that reached a body weight of between ten and twenty grams, whereas most of the fish smaller than this "critical" size remained immature (i.e. no testis developmment). Apparently, this critical size must be reached by a certain time of the year. Thus, a time 'window' exists wherein fish achieving a certain size begin preparation for spawning the following year. Altering photoperiod regime during the time window had no effect on the incidence of sexual precociousness, but did delay spermatogenesis by an undetermined length of time. / Science, Faculty of / Zoology, Department of / Graduate

Juvenile rainbow trout (Salmo gairdneri Richardson) from the Lardeau River in southeastern British Columbia were studied in both their natural environment and stream tanks. In the river, highest densities of recently emerged trout occurred in areas with overhanging cover, shallow depth and minimal current. Older, underyearling trout were generally found in regions with smaller bottom particle size than yearlings; both age classes avoided fast current areas. Young trout migrated from the Lardeau River to Kootenay Lake during spring and summer. Except during these seasons trout were larger and more abundant in the upper versus the lower river; this was probably a result of better rearing habitat and higher biological productivity in the upper river. Trout fed almost exclusively on drifting organisms. Lake origin drift was an important food to riverine trout during summer; kokanee (Oncorhynchus nerka) eggs and fry were significant during fall and spring respectively. In stream tanks, where it was possible to manipulate prey populations, live prey were consumed significantly more than the same species when dead, suggesting that prey body movement was an important prey characteristic. In both the field and stream tanks, trout consumed prey within a discrete size range with larger fry generally consuming bigger prey than smaller fry. Further work is suggested which would improve our understanding of the feeding ecology of stream rearing trout. / Science, Faculty of / Zoology, Department of / Graduate

Memoria para optar al título profesional de Ingeniero Agrónomo / No disponible a texto completo / El objetivo general de este ensayo fue evaluar el efecto de la modificación de la composición en ácidos grasos de la dieta, sobre el contenido de ácidos grasos ω-3 eicosapentaenoico (EPA), docosahexaenoico (DHA), ácidos grasos ω-6 Linoleico y Araquidónico; comportamiento productivo y características de calidad de canal, en trucha arcoíris (Oncorhynchus mykiss). Para modificar la composición en ácidos grasos de la dieta el aceite de soja, que es la principal fuente de grasas vegetales en las dietas comerciales actuales, fue reemplazado parcialmente por aceite de lino. Los peces con un peso vivo promedio inicial de 1126 g fueron alimentados por 10 semanas, mientras permanecían en 6 balsas-jaula en condiciones comerciales. Los peces fueron muestreados al azar cada 5 semanas para medir peso vivo, longitud horquilla, peso canal, peso vísceras, peso hígado y se identificó el sexo. Al mismo tiempo, se tomaron 9 filetes por cada tratamiento los que se mantuvieron congelados a -20°C hasta su análisis. El incremento de ácido α-linolénico en la dieta, resultó en un incremento significativo (P<0,05) de este ácido graso en el filete (0,74 vs. 0,60 control); sin embargo, no se observó diferencias significativas (P>0,05) en el contenido de EPA (0,41 vs. 0,43 control) ni DHA (0,49 vs. 0,55 control). Existió un menor contenido (P<0,05) de ácido linoleico en los filetes del grupo control (4,44 vs. 5,00 control), valores expresados como g de cada ácido graso en 100 g de filete Se concluye que la adición de un 20 % de aceite de lino en reemplazo de aceite de soja en la dieta de trucha arcoíris, disminuye significativamente el contenido de ácido linoleico y aumenta significativamente el contenido de ácido α-linolénico en el filete luego de 10 semanas de alimentación; determinando una mayor relación ω-3/ω-6 (P<0,05) en el grupo con aceite de lino (0,44±0,06 vs 0,37±0,05), sin modificar significativamente el contenido de EPA y DHA. / The objective of the present study was to evaluate the effects of changing the proportions of dietary fatty acids ω-3 EPA, DHA and ω-6 linoleic, arachidonic acid over rainbow trout (Oncorhynchus mykiss) fatty acid profile, productive performance and fillet composition. Soybean oil, the main source of vegetable fat in current commercial diets, was partially replaced for linseed oil to modify the fatty acid profile of the diet. Fish were kept in sea-cages and reared under regular commercial conditions; initial fish live weight was 1126 g and the experimental diets were supplied during 10 weeks. Fish samples were randomly taken from each cage every 5 weeks to measure live weight, fork length, carcass weight, guts and liver weight, sex was identified, at the same time 9 fish fillets were collected and frozen (-20ºC) until further analysis. The increase in dietary α-linolenic acid generated a significant (p<0.05) elevation of this fatty acid in fish fillet (0.74 vs 0.60); however, EPA and DHA levels were not affected by treatment and showed similar values when compared to the control (0.41 vs. 0.43 and 0.49 vs. 0.55). A minor contents (P < 0.05) of linoleic acid in the fillet existed of the control (4.44 vs. 5.00 control), when expressed as grams of fatty acid per 100 grams of fillet. In conclusion, replacing 20% soybean oil by linseed oil in rainbow trout diet, significantly decreased linolenic acid and significantly increased α-linolenic acid concentrations in fish fillets after 10 weeks of feeding; determining a higher ω-3 / ω-6 proportion in the fillet of fish fed linseed oil (0,44±0,06 vs 0,37±0,05), with no significant effect on EPA and DHA content.

<p> The development of Na+ and Ca++ transport, ammonia excretion, and respiration in larval rainbow trout were studied over the period of yolk sac absorption and shortly after the onset of exogenous feeding. The gills and the yolk sac epithelium of larvae were examined for any indication of the presence of mitochondria-rich cells and/ or Na-K-ATPase activity. </p> <p> Whole body Na+ content of the larvae increased throughout yolk sac absorption and after the onset of exogenous feeding. During this period there was an increase in Na+ influx, efflux, transporter capacity Umax> and transporter affinity (Km). Jmax increased 4.6 times during yolk sac absorption, and increased another 2.8 times 248 degree·days after the onset of exogenous feeding. Km decreased over the first 168 degree·days and stayed constant for the remainder of the experimental period. The gills were the primary site of Na+ efflux. Na+ accumulation from the water began the first day post hatch. </p> <p> Ca++ transport capacity increased throughout yolk sac absorption, but then decreased after the onset of exogenous feeding. Jmaxca++ exhibited a 15.5 fold increase over 257 degree·days, which then decreased to 30% of that value after exogenous feeding commenced. There was a 162 fold increase in Ca++ Km over yolk sac absorption, which then decreased 15.6 times after the onset of exogenous feeding. Whole body Ca++ began to increase approximately 200 degree·days post-hatch and exhibited a large increase after the onset of exogenous feeding. It is possible that during yolk sac absorption the larvae does not begin to accumulate Ca++ from the external environment until its internal reserves are depleted, and with the onset of feeding the larvae then obtains its Ca++ from its diet. </p> <p> Ammonia excretion in larval trout increased 3.6 times over yolk sac absorption, and increased another 2.8 times after exogenous feeding commenced. There was no evidence of a link between Na+ uptake and NJ-:4+ excretion in larval fish as NJ-:4+ excretion was not stimulated in response to increasing external Na+ concentrations. Measurements of the ammonium concentration at different surfaces of the larvae revealed a larger ammonium concentration next to the gills than next to the yolk sac and skin. This may indicate that the gills are the primary site for ammonia excretion in the larval fish. </p> <P> Initially, branchial and cutaneous surfaces contributes to respiration, but as the gills grow and develop and the yolk sac decreases in size and the skin thickens, the contribution of the gills to this function increases. </p> <p> The gills of first day hatchlings are comprised of gill arches and filaments. Lamellae do not begin to develop until a few days after hatching. The gill filaments contain mitochondria-rich cells and have Na-K-ATPase activity as determined through binding of the fluorescent dye anthroylouabain. The yolk sac epithelium contained cells with concentrated regions of mitochondria, but significant Na-K-ATPase activity was not detected when compared to branchial staining. </p> / Thesis / Master of Science (MSc)

This study investigated the regulation of Na⁺ Ca²⁺ Cl⁻ during development in embryonic rainbow trout (Oncorhynchus mykiss). Because there is a close relationship between pH regulation and ion uptake mechanisms in adult teleosts, pH, pCO₂, and NH₄⁺ levels in unstirred layers (USLs) adjacent to whole eggs and dechorionated embryos were determined using double-barrel ion-selective microelectrodes (ISMEs). Whole eggs accumulated Na⁺, Ca²⁺ and Cl⁻ during the last 20 days of embryonic development, suggesting an ionoregulatory ability prior to hatching. Na⁺ uptake by whole eggs was linearly related to external [Na⁺], suggesting that Na⁺ crosses the chorion by diffusion. The uptake by dechorionated embryos was saturable, indicating the presence of active transport or facilitated diffusion mechanisms on the surface of embryos prior to hatching. Ca²⁺ uptake by whole eggs and dechorionated embryos was saturable, suggesting that specific pathways or binding sites are present in the chorion, and that active transport or facilitated diffusion mechanisms are present at the surface of embryos. The much higher Jₘₐₓ for whole eggs than dechorionated embryos suggests a role of the perivitelline fluid (pvf) or chorion in ion uptake. Jₘₐₓ was lower in dechorionated embryos than in hatchlings suggesting that Ca²⁺ uptake mechanisms may not be fully developed in dechorionated embryos. Low pH and high [NH₄⁺] were measured in the USLs adjacent to whole eggs and dechorionated embryos, relative to the bulk water. A high Pcₒ₂ measured adjacent to the gills suggests that CO₂ excretion was the primary source of USL acidification. pH was lower in the USL adjacent to the gills and trunk than in the USL adjacent to the yolk sac. [NH₄⁺] was highest adjacent to the gills and trunk than adjacent to the yolk sac. There was no change in the extent of USL acidification or in [NH₄⁺] over the last half of embryonic development. After hatching, however, there was a significant increase in [NH₄⁺] adjacent to the gills, which was not accompanied by a change in USL pH. It is possible that a change in permeability of the transporting surfaces (i.e. gills, trunk, yolk sac) occurs after hatching. / Thesis / Master of Science (MS)

Flavobacterium psychrophilum est l’agent pathogène des flavobactérioses d’eau froide touchant essentiellement les salmonidés dont la truite arc-en-ciel Oncorhynchus mykiss d’élevage. Cette bactérie Gram négative a un très fort impact économique en aquaculture car elle peut causer jusqu'à 70 % de mortalité dans les bassins d’élevage. La flavobactériose se décline sous deux formes pathologiques : la maladie de l’eau froide touchant les poissons adultes et le syndrome de l’alevin de truite arc-en-ciel touchant les juvéniles.Au cours de se travail, une méthode de PCR quantitative a été proposée. Elle permet en moins de trois heures de détecter et de quantifier un nombre de copies du gène codant l’ARNr 16S de la bactérie dans les tissus du poisson. Cette méthode a été testée sur différentes suspensions bactériennes (F. psychrophilum, autres flavobactéries, autres pathogènes) afin d’en valider la spécificité. La sensibilité de la méthode de détection a été évaluée à 1 et 2 bactéries par PCR en fonction de la matrice biologique utilisée.Une étude écotoxicologique a été menée et montre d’une part que F. psychrophilum est une bactérie hyper-sensible au cadmium comparée aux autres bactéries Gram négatives. Sa croissance est diminuée d’un facteur 2 en présence d’une contamination au Cd à 0,4 µM. D’autre part, nous avons constaté qu’une contamination de truites juvéniles par 1 µg CdCl2/L (2 mois) et une injection de 5 × 107 flavobactéries par individu (1 mois) ne provoque aucune mortalité. L’expression génique mesurée sur ses poissons démontre que le cadmium peut avoir des effets contradictoires sur le système immunitaire du poisson, pouvant soit exacerber ou diminuer la réponse immunitaire selon l’organe considéré. Un travail comparatif de la prévalence de la flavobactérie dans 7 sites aquacoles d’Aquitaine a démontré que la flavobactérie est omniprésente et que sa pathogénicité est contrôlée par le système immunitaire des poissons en bonne santé apparente. L’expression génique mesurée sur les poissons malades et apparemment sains nous apporte deux informations importantes : 1/ les gènes codant pour la métallothionéine A et l’interleukine 1-β sont de bons bio-marqueurs de la maladie et 2/ la répression des gènes codant pour le complexe majeur d’histocompatibilité 2-β, le facteur de croissance transformant β, le cluster de différentiation 8-α et l’immunoglobuline T dans la rate des poissons malades montre un effondrement du système immunitaire acquis nous permettant d’émettre l’hypothèse que ce phénomène déclenche l’apparition de la maladie. Ainsi, F. psychrophilum aurait un comportement de pathogène à virulence latente.Afin d’imaginer de nouvelles mesures prophylactiques et pour mieux comprendre la pathogénicité de la bactérie, une analyse du protéome de la membrane externe couplée à l’annotation du génome séquencé a été effectuée. Il a été identifié entre autres 1/ des protéines d’adhésion et d’invasion des tissus et 2/ des protéines d’acquisition de métabolites de l’hôte. De plus, un nombre important de protéines immunogènes chez la truite potentiellement utilisable dans un cocktail vaccinant a été détecté. Afin de chercher un vecteur pour ce cocktail, des nanoparticules d’acide γ-glutamique et phénylalanine d’environ 100 à 200 nm de diamètre ont été synthétisées. Ces dernières constituent une approche séduisante pour vacciner les poissons avec des antigènes de F. psychrophilum encapsulés puis incorporés dans la nourriture. / Flavobacterium psychrophilum is the causative agent of cold water flavobacteriosis, a condition affecting mostly salmonid fish, including the farmed rainbow trout Oncorhynchus mykiss. This Gram negative bacterium can cause up to 70% mortality in breeding tanks and has a very strong economic impact on the fish farming industry. Flavobacteriosis can take two pathological forms: the cold water disease affecting adult fish and the rainbow trout fry syndrome affecting juveniles.In the present study, a method of quantitative PCR was devised that allowed for the detection and the quantification, within three hours, of the 16S rRNA copy number in fish tissues. This method’s specificity was confirmed through the use of various bacterial suspensions (F.psychrophilum, others flavobacteria and others pathogens) and its detection limit was estimated to be 1 and 2 bacteria in broth and in biological matrices, respectively.An ecotoxicological study was then performed that showed that, on the one hand, F. psychrophilum is cadmium hypersensitive compared to others Gram negative bacteria because its growth rate, compared to a control, is decreased by a factor 2 at a cadmium concentration of 0,4 µM. On the other hand, we observed that subjecting rainbow trout juveniles to a concentration of 1 µg CdCl2/L for2 months prior to an injection of 5 × 107 F. psychrophilum by fish didn’t lead to any mortality. The gene expression which was measured on these fish demonstrated that cadmium can have contradictory effects on the immune system of fish, which could enhance or decrease the immune response depending of the organ. A comparative work of the prevalence of flavobacteria in 7 fish farms within the Aquitaine region (France) demonstrated that the bacterium was endemic and present in asymptomatic fish. Gene expression levels were measured on diseased and asymptomatic fish and demonstrated that the genes metallothionein A and interleukine 1-β were good biomarkers of the disease and that repression of the genes major histocompatibility complex 2-β, transforming growth factor -β, cluster of differentiation α and immunoglobulin T in the spleen of diseased fish was indicative of a collapse of the acquired immune system. We therefore hypothesized that this event marked the beginning of the disease and that F. psychrophilum is mostly an opportunistic pathogen.To prepare the development of new prophylactic techniques and to understand better the bacterium pathogenicity, an analysis of the outer membrane proteome coupled with sequencing of the bacterial genome was also performed. Furthermore, a significant number of immunogenic proteins were identified as good candidates for the preparation of a vaccine. Finally, γ-glutamic acid and phenylalanine nanoparticles of about 100 - 200 nm in diameter were synthesized to serve as potential vector for this vaccine. These nanoparticles should be tested to administrate F. psychrophilum antigens to fish through the digestive route.

The aim of this thesis was to examine the influence of nitrite on fish. Influence of nitrite was evaluated on the basis of acute and sub-chronic toxicity tests results on rainbow trout (Oncorhynchus mykiss). The effects of nitrite were also observed in wels catfish {--} albino (Silurus glanis). On the basis of the results of acute toxicity tests, values of lethal concentration of nitrite for rainbow trout (Oncorhynchus mykiss) (96hLC50 = 11.2 mg.l-1 NO2-) and for wels catfish {--} albino (Silurus glanis) (96hLC50 = 15.8 mg.l-1 NO2-) were calculated. Using results of acute toxicity test for rainbow trout (Oncorhynchus mykiss) and with respect to legislative requirements, concentrations of nitrite for sub-chronic toxicity test were selected. The results of sub-chronic toxicity test showed that nitrite concentration of 3 mg.l-1 NO2- during 28-day exposition caused 65 % fish mortality. This concentration also caused growth rate decrease compared to control. Growth rate among fish exposed to concentrations lower then 3 mg.l-1 NO2- was comparable to control. Among fish exposed to nitrite concentration of 0.6 mg.l-1 NO2- and higher nitrite accumulation in muscle and in blood plasma was observed. Haematological examination showed statistically significant decrease of haematocrit value and concentration of haemoglobin and increase of the number of leukocytes in experimental fish. Other measured haematological parameters (Ery, MCV, MCH and MCHC) were comparable with control.

The Bonneville Power Administration (BPA) Fish and Wildlife Program mitigates for impacts of hydroelectric dams on ESA-listed salmon and steelhead populations in the Columbia River Basin (CRB). Considering the sizable investments in mitigation and the diversity of stream habitats within the CRB, there has been a pointed effort to develop and identify meaningful metrics relating to fish populations and trends in their habitat across the CRB. The Integrated Status and Effectiveness Monitoring Program (ISEMP) was developed in 2003 specifically for this purpose, and is tasked with developing and testing strategies for determining the status and trend of salmonid populations and their habitats in the CRB. This thesis was funded by the BPA, ISEMP, Eco Logical Research Inc., the Snake River Salmon Recovery Board, and the Intensively Monitored Watershed project in the Asotin Creek basin with the purpose of investigating the efficacy of foraging modeling as part of a large fish habitat monitoring program. The primary objectives were i) to assess a foraging model's ability to predict fish density in study sites involved in long-term monitoring and ii) to evaluate energetic implications of restoration design and progress after implementation using a foraging model. To assess the foraging model's ability to predict fish density (objective i), we collected topography, drift, temperature, discharge, and fish population information to support foraging modeling, and we simulated flow patterns, drift, foraging, swimming costs, carrying capacity, and density. We then compared observed and predicted densities in 22 study sites from the John Day and Asotin Creek watersheds: Linear regression between observed and predicted fish densities was significant (R2 = 0.61, p < 0.001). When assuming spatially uniform drift densities and small fish territories, carrying capacity predictions were related to the number of foraging locations simulated, suggesting the model is highly sensitive to territory size assumptions. To evaluate restoration design and monitor restoration progress (objective ii), we simulated foraging before restoration, after a virtual restoration (carried out using the restoration designs in a GIS environment), and again followinfg restoration implementation. We used raster differencing to compare the "before" results to the virtual restoration results and then the "before" results to the "after" results. Hydraulic and foraging models suggested: Mean net energy intake increased following both simulated and actual restoration. Restoration structures generally slowed water's progress through the study site or caused pooling, both resulting in an increase in energetically favorable areas. Generally, more areas shifted from another state to having an acceptable energy balance than to an unacceptable energy balance.

Fluoxetine (FLX), the active ingredient of Prozac™, is a member of the selective serotonin reuptake inhibitor (SSRI) class of anti-depressant drugs and is present in aquatic environments worldwide. Previous studies reported that FLX is an endocrine disruptor in fish, bioconcentrating in tissues including the brain. Evidence implicates that serotonin influences the activity of the hypothalamo-pituitary-interrenal (HPI) stress axis, thus exposure to FLX may disrupt the teleost stress response. This study examined in vitro cortisol production in rainbow trout (Oncorhynchus mykiss) head kidney/interrenal cells exposed to FLX and 14C-pregnenolone metabolism in head kidney microsome preparations of FLX-exposed trout. Results indicated that cells exposed in vitro to increasing concentrations of FLX had lower cortisol production and cell viability (versus control) and microsomes isolated from trout exposed to 54 μg/L FLX had higher pregnenolone metabolism versus those of control and low FLX-exposed (0.54 μg/L) trout.