Antarctic fish from the Perciform suborder Notothenioidei inhabit arguably the most thermally stable ocean environment on earth. In order to populate the subzero environment Antarctic fish have evolved numerous adaptations. However, specialisation to -1.9°C has incurred a trade off, thermal flexibility is lost likely due to modifications to the cold and as a result Notothenioidei are extremely stenothermic. Climate change mediated warming is predicted to increase the ocean temperature surrounding the Antarctic continent by 2°C within the next century. This increase is projected to affect individuals, populations and the community structures of those inhabiting the area and therefore the physiological study of the acclimation ability and thermal limitations of Antarctic fish is an area scientific interest. The present study is a series of discrete experiments relating to one species, Trematomus bernacchii, a circumpolar benthic Notothenioidei found in nearly all inshore waters surrounding the Antarctic coastline. These studies included investigation of the response of this species to both chronic and acute temperature exposure prior to and following a feeding event, a reduction in environmental oxygen and an exhaustive exercise event, as well as examination of T. bernacchii ability to recovery from these challenges. T. bernacchii demonstrated variable success when acclimated to +3°C. Failure appeared to be determined by the recovery period following capture and aquarium housing, 7 days housing following capture resulted in 100% mortality, conversly 3 months resulted in 100% survival. Following successful acclimation T. bernacchii showed physiological adjustment as acclimated resting metabolic rate mirrored that of T. bernacchii tested at environmental temperature, 20.63 ± 1.3 compared to 22.38 ± 1.02 mg. O₂. kg⁻¹. h⁻¹. The previously undefined specific dynamic action response (SDA), in T. bernacchii was characteristic of polar species. At environmental temperatures SDA scope was small 14.52 ± 3.52 mg O₂. kg⁻¹. h⁻¹, and lengthy ,72 hours; SDA duration was reduced to 9 hours in acclimated fish. Resting metabolic rate was elevated following acute exposure to +3°C, 34.27 ± 2.35 mg O₂. kg⁻¹. h⁻¹, masking the SDA response and associated parameters. T. bernacchii were relatively sensitive to hypoxia, Pcrit over four acute temperature exposures, ranged between 69 and 102mmHg, higher than the average range for teleosts (40 – 60 mmHg). Above -1°C Pcrit increased, rising with acute temperature exposure. Ventilation rate was temperature dependent and completely absent at +4 and +6°C. A bradycardia (beginning at 60 and 70mmHg) was observed at all temperature exposures, this response was consistent as all heart rates reduced by 25%. Recovery from both hypoxia and acute temperature exposure was rapid. Following an exhaustive exercise event aerobic Scope of T. bernacchii was constrained over an acute temperature increase, reducing from 38.58 ± 5.64 to 24.41 ± 4.92 mg.O₂. kg⁻¹.h⁻¹ over a 7°C temperature increase, respiratory scope too was reduced such that at +4 and +6°C scope was absent. Heart rate of T. bernacchii was highly constrained at -1°C, increasing by 2.54 ± 0.9 bpm following exercise. Acute temperature increase resulted in an increase in cardiac scope, maximum 6.29 ± 1.2 bpm at +2°C, due likely to a thermally mediated loss of cholinergic tonus following exhaustive exercise. Recovery of all parameters was temperature dependent and rapid upon return to -1°C. The present study is the first to quantify and assess the effect of acute and chronic temperature exposure on the SDA response of T. bernacchii. Furthermore, it supplements the current literature on acclimation ability, acute temperature exposure, aerobic scope and hypoxia tolerance for this species. This work will be of use in future investigations of the effects of rapid climate change on Antarctic notothenioid fish and the interconnected ecosystem.

<p>Knowledge on fuel use and muscle metabolism in high altitude mammals is very limited. Yet, as the oxidation of carbohydrates offers an oxygen-saving advantage over the oxidation of fatty acids (15-30% more energy produced per oxygen used), one possible adaptation to maintain performance at high altitude is to elevate the use of carbohydrates as a fuel source for energy metabolism. To test this hypothesis, I performed intraspecific and interspecific comparisons of whole-body fuel use and muscle metabolism in closely related high (4000-4500 m) and low altitude (100-300 m) native mice (genus <em>Phyllotis</em>), which I collected at different locations in Andean and coastal regions of Peru. My results show a higher proportional use of carbohydrates when oxygen becomes limited in high altitude <em>Phyllotis</em> in comparison to their low altitude counterparts. This phenotype does not seem to result from similar phylogenetic history or from a chronic exposure to hypobaric hypoxia during development or adulthood. Accordingly, this thesis provides the first compelling evidence of enhanced carbohydrate utilization as an adaptation to high altitude, a hypothesis proposed nearly 30 years ago. The mechanisms responsible for this shift in fuel use are unknown. There were no strong indications of a greater capacity for carbohydrate oxidation in skeletal and cardiac muscles of high altitude <em>Phyllotis</em> mice. Finally, as this thesis provides the first report of whole-body fuel use in mice, a comparison with other mammalian species (rats, dogs and goats) revealed that the current model of mammalian fuel selection, which is thought to be conserved among mammals, does not apply to small mammals. I thus revisited the current model and proposed a new one general to all mammals. This thesis thus provides significant advancements not only in the field of high altitude physiology but also in the field of mammalian energetics.</p> / Doctor of Philosophy (PhD)

La sélection des habitats est un comportement important reliant des individus aux conditions environnementales de leur habitat. Elle est généralement étudiée pour faire des inférences sur les patrons de distribution des populations. Or, la sélection des habitats peut varier entre individus d’une même population et cette variation peut excéder la variation observée entre les populations. D’une part, si la sélection des habitats est adaptative, on peut supposer que les individus sélectionneront des habitats leur permettant de maximiser leur performance. D’autre part, les conditions environnementales dans les habitats peuvent affecter les performances individuelles, impliquant ainsi que la sélection des habitats peut avoir des conséquences physiologiques. Par ailleurs, l’environnement social peut influencer la performance physiologique des individus. L’objectif général de la thèse est l’étude des déterminants et des conséquences physiologiques de la sélection des habitats chez les poissons. Dans un premier temps, nous avons créé et comparé la capacité prédictive de modèles de sélection des habitats pour l’achigan à petite bouche Micropterus dolomieu intégrant la variabilité individuelle. Nos résultats ont démontré que l’intégration de la variabilité individuelle permettait d’identifier les variables influençant la sélection des habitats au niveau individuel, des groupes et de la population. Les modèles incluant les variables représentant la présence de refuges dans les habitats avaient un meilleur pouvoir prédictif que ceux qui ne les incluaient pas. Par ailleurs, des groupements d’individus présentant des similitudes dans leur sélection d’habitats ont été identifiés. Malgré tout, la variabilité dans la sélection des habitats entre les individus était nettement plus grande que la variabilité entre les groupes. Nous avons démontré que la présence de refuge était la variable la plus importante à considérer dans les modèles de sélection d’habitats pour les achigans à petite bouche. Nous avons ensuite investigué si la présence de refuge pouvait influencer différents traits métaboliques des achigans à petite bouche grâce à des expériences de respirométrie en laboratoire. La présence de refuge a diminué les taux métaboliques au repos (RMR) des achigans provenant d’un lac alors qu’il n’y a pas eu d’effet sur les achigans provenant d’une rivière. En considérant la position hiérarchique des individus, nous avons noté que les individus dominants avaient un temps de récupération plus court en présence de refuge alors que la présence de refuge n’a rien changé pour les individus soumis. Finalement, nous avons étudié si l’environnement social, en particulier la taille du groupe social, pouvait influencer l’estimation des taux métaboliques des poissons en présence ou en absence de refuge. Nous avons cette fois mené des expériences sur des vairons Phoxinus phoxinus, des poissons très sociaux. Les vairons gardés en petits groupes avaient des taux métaboliques plus élevés que ceux gardés en grands groupes. La présence de refuge a diminué les taux métaboliques indépendamment de la taille des groupes. Nos résultats ont démontré que la taille des groupes peut influencer les dépenses énergétiques des individus, ce qui souligne l'importance de comprendre le rôle des dynamiques sociales sur les variations dans les traits métaboliques. Les résultats de la thèse démontrent l’importance de tenir compte de l’environnement physique et de l’environnement social pour mieux comprendre les conséquences physiologiques de la sélection des habitats. / Habitat selection is an important behaviour that relates individuals to the environmental conditions in their habitat, and is generally studied to infer population-level patterns of distributions. Habitat selection varies among individuals and there is growing evidence that individual differences often exceed population differences in habitat selection. On the one hand, if habitat selection is adaptive, it could be hypothesized that individuals would select habitats that would maximize their fitness. On the other hand, environmental conditions in habitats can have physiological consequences, which can be amplified or masked by the social environment. Therefore, the general objective of this thesis was to better understand the determinants and physiological consequences of habitat selection. We created and compared the predictive capacity of habitat selection models for smallmouth bass Micropterus dolomieu integrating individual variability. Our results show that by integrating individual variability, we could identify variables influencing individual-, group-, and population-level habitat selection. Models that included variables referring to presence of shelter had the best predictive capacity. Further, we identified groups of individuals defined by their habitat selection. Nevertheless, variation in habitat selection among individuals was higher than that among groups. Presence of shelter was the main correlate of habitat selection for smallmouth bass. We then we tested whether presence of shelter could influence smallmouth bass metabolic traits estimated during respirometry trials. In presence of shelter, resting metabolic rates (RMR) were lower than in absence of shelter for smallmouth bass from a lake population. There was no difference in RMR for smallmouth bass from a river population. Further, dominant individuals showed reduced recovery time (RT) in presence of shelter, while no difference was observed in subordinate individuals. We investigated how social group size and availability of shelter could influence metabolic rate. This project was conducted on Eurasian minnow Phoxinus phoxinus, a highly social fish. Fish held in smaller groups had higher standard metabolic rate as compared to that of fish held in larger groups. Presence of shelter during respirometry trials was associated with reduced metabolic rates, regardless of group size fish were held in. Our results suggest that social group size may directly influence energy demands of individuals, highlighting the importance of understanding the role of group size on variations in physiological traits associated with energy expenditure. Our results highlight the importance of considering the physical and social environment to better understand the physiological consequences of habitat selection.

Fish encounter daily cycles of hypoxia in the wild, but the physiological strategies for coping with repeated cycles of normoxia and hypoxia (intermittent hypoxia) are poorly understood. Contrastingly, the physiological strategies for coping with continuous (constant) exposure to hypoxia have been studied extensively in fish. The main objective of this thesis was to understand how Fundulus killifish cope with a diurnal cycle of intermittent hypoxia, an ecologically relevant pattern of aquatic hypoxia in the natural environment. To do this, I characterized the effects of intermittent hypoxia on hypoxia tolerance, oxygen transport, metabolism, and the oxidative stress defense system of killifish, and compared these effects to fish exposed to normoxia, a single cycle of hypoxia-normoxia, and constant hypoxia. Specifically, I studied the following topics: (i) how acclimation to intermittent hypoxia modifies hypoxia tolerance, and the hypoxia acclimation response of Fundulus heteroclitus (Chapter 2), (ii) metabolic adjustments occurring during a hypoxia-reoxygenation cycle (Chapter 3), (iii) how acclimation to intermittent hypoxia alters O2 transport capacity and maximal aerobic metabolic rate (Chapter 4), (iv) the effects of hypoxia and reoxygenation on reactive oxygen species and oxidative stress (Chapter 5), and (v) variation in hypoxia tolerance and in the hypoxia acclimation responses across Fundulus fishes (Chapter 6). Killifish rely on a unique and effective physiological strategy to cope with intermittent hypoxia, and that this strategy is distinct from both the response to a single bout of acute hypoxia-reoxygenation (12 h hypoxia followed by 6 h reoxygenation) and to chronic exposure to constant hypoxia (24 h hypoxia per day for 28 d). Key features of the acclimation response to intermittent hypoxia include (i) maintenance of resting O2 consumption rate in hypoxia followed by a substantial increase in O2 consumption rate during recovery in normoxia, (ii) reversible increases in blood O2 carrying capacity during hypoxia bouts, (iii) minimal recruitment of anaerobic metabolism during hypoxia bouts, and (iv) protection of tissues from oxidative damage despite alterations in the homeostasis of reactive oxygen species and cellular redox status. Of these features, (i) is unique to intermittent hypoxia, (ii) also occurs in fish exposed to acute hypoxia-reoxygenation, and (iii) and (iv) are observed in both fish acclimated to intermittent hypoxia as well as those acclimated to constant hypoxia. This is the most extensive investigation to date on how fish cope with the energetic and oxidative stress challenges of intermittent hypoxia, and how these responses differ from constant hypoxia. This thesis adds substantial insight into the general mechanisms by which animals can respond to an ecologically important but poorly understood feature of the aquatic environment. / Dissertation / Doctor of Philosophy (PhD) / Oxygen levels in the aquatic environment are dynamic. Many fishes routinely encounter changes in oxygen content in their environment. However, we have very little understanding of how cycles between periods of low oxygen (hypoxia) and periods of high oxygen (normoxia) affect the physiology of fish. This thesis investigated how Fundulus killifish cope with daily cycles between hypoxia and normoxia (intermittent hypoxia) by modifying oxygen transport, metabolism, and oxidative stress defense systems. I found that killifish rely on a unique and effective physiological strategy to cope with intermittent hypoxia, and that this strategy is distinct from how they respond to a single bout of hypoxia (followed by normoxia) and to a constant pattern of only hypoxia. This is the most extensive investigation to date on how fish respond to the challenges of intermittent hypoxia, an understudied but ecologically important type of aquatic hypoxia.