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Boreogadus Saida Et Arctogadus Glacialis Vie Larvaire Et Juvénile De Deux Gadidés Se Partageant L’Océan Arctique

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Boreogadus Saida Et Arctogadus Glacialis Vie Larvaire Et Juvénile De Deux Gadidés Se Partageant L’Océan Arctique Boreogadus saida et Arctogadus glacialis Vie larvaire et juvénile de deux gadidés se partageant l’océan Arctique Thèse Caroline Bouchard Doctorat interuniversitaire en océanographie Philosophiae doctor (Ph.D.) Québec, Canada © Caroline Bouchard, 2014 Résumé Le très abondant Boreogadus saida occupe au sein de l’écosystème marin arctique une place prépondérante, ce qui lui vaut une attention croissante des scientifiques. Arctogadus glacialis, commun dans toutes les mers arctiques, est beaucoup moins étudié. Les deux espèces et leurs jeunes stades cohabitent mais ces derniers sont pratiquement impossibles à différencier. Seuls des outils génétiques, ou une méthode utilisant la taille du noyau de l’otolithe développée dans cette thèse, peuvent distinguer les deux espèces. Ces méthodes d’identification ont permis d’étudier pour la première fois l’écologie des jeunes stades d’Arctogadus et d’estimer la proportion de cette espèce dans des échantillons de gadidés arctiques. À la lumière des observations faites en mer de Beaufort, il apparait que les jeunes Arctogadus ont une abondance environ vingt fois moindre, une taille à l’éclosion supérieure, un taux de croissance similaire, et un taux de mortalité inférieur aux jeunes Boreogadus. Pour Boreogadus, l’hypothèse selon laquelle certaines larves éclosent en hiver près des panaches des fleuves, a été testé, d’abord en comparant la saison d’éclosion dans six régions de l’océan Arctique caractérisées par différents apports d’eau douce. Conformément à cette hypothèse, l’éclosion commence en hiver dans les mers recevant de forts apports fluviaux alors que l’éclosion débute au printemps dans les régions aux apports d’eau douce limités. Les larves qui éclosent en hiver profitent d’une longue saison de croissance leur permettant d’atteindre des tailles pré-hivernales largement supérieures aux larves qui éclosent en été, ce qui favoriserait leur survie. Cette même hypothèse a ensuite été testée en comparant la composition chimique des otolithes de Boreogadus provenant de ces six régions, et les différences observées semblent appuyer l’hypothèse. Les tendances actuelles au devancement de la débâcle, au réchauffement des eaux de surface et à l’augmentation du débit des fleuves pourraient favoriser le recrutement de Boreogadus, et possiblement aussi celui d’Arctogadus. Découle de cette thèse une connaissance accrue de l’écologie de gadidés habitant un océan confronté à une pléthore de changements. iii Abstract The very abundant polar cod (Boreogadus saida) plays a preponderant role in the Arctic marine ecosystem and consequently has received significant attention in recent years. The ice cod (Arctogadus glacialis), a common species in all Arctic seas, is much less studied. Both species co-occur on Arctic continental shelves and their early life stages are often found together in ichthyoplanktonic collections. However, larvae and juveniles of polar cod and ice cod are almost impossible to differentiate. Only genetic tools, or a method using the size of the otolith nucleus developed in this thesis, can distinguish the two species. These identification methods allowed to study for the first time ice cod early life stage ecology and estimate the proportion of this species in Arctic gadids samples. In light of observations made in the Beaufort Sea, it seems that young ice cod are about twenty times less abundant, hatch at a larger size, grow at the same rate, and have a mortality rate inferior to young polar cod. For polar cod, the hypothesis that some larvae hatch in winter near river plumes, was tested, first by comparing the hatching season in six regions of the Arctic characterized by different freshwater inputs. Consistent with this hypothesis, hatching starts in winter in seas receiving large river discharge while hatching starts in spring in regions with limited freshwater inputs. The larvae hatched in winter benefit from a long growth season allowing them to reach larger pre-winter size than larvae hacth in summer, a condition that likely favors their survival. This same hypothesis was further tested by comparing the otolith chemistry of polar cod juveniles from those six regions, and the differences observed seem to support the hypothesis. On-going trends of earlier ice break-up, warmer surface layer, and increased river discharge could favor polar cod, and possibly also ice cod, recruitment. Arise from this thesis an increased knowledge of the ecology of gadids living in an Ocean facing a plethora of changes. v Table des matières Résumé ................................................................................................................................. iii Abstract .................................................................................................................................. v Table des matières ............................................................................................................. vii Liste des tableaux ................................................................................................................. xi Liste des figures ................................................................................................................. xiii Remerciements ................................................................................................................ xxiii Avant-Propos ..................................................................................................................... xxv Chapitre 1 – Introduction générale ..................................................................................... 1 1.1 Les écosystèmes marins arctiques en changement ....................................................... 1 1.2 Boreogadus saida ......................................................................................................... 5 1.3 Arctogadus glacialis ..................................................................................................... 8 1.4 La survie larvaire : importance de l’eau douce ........................................................... 11 1.5 Objectifs ...................................................................................................................... 16 1.6 Approches utilisées ..................................................................................................... 17 1.6.1 Microstructure des otolithes................................................................................. 18 1.6.2 Chimie des otolithes............................................................................................. 19 1.6.3 Génétique des populations ................................................................................... 22 Chapitre 2 – Circum-arctic comparison of the hatching season of polar cod Boreogadus saida: A test of the freshwater winter refuge hypothesis ............................ 25 2.1 Résumé ........................................................................................................................ 25 2.2 Abstract ....................................................................................................................... 26 2.3 Introduction ................................................................................................................. 27 2.4 Materials and methods ................................................................................................ 28 2.4.1 Study areas ........................................................................................................... 28 2.4.2 Sampling of fish larvae and juveniles .................................................................. 32 2.4.3 Discriminating Boreogadus saida and Arctogadus glacialis larvae and juveniles ...................................................................................................................................... 34 2.4.4 Validation of the ageing of young polar cod ....................................................... 36 2.4.5 Regional Hatch-date frequency distribution (HFD) of polar cod ........................ 37 2.4.6 Regional long-term average surface salinity and temperature ............................. 39 2.5 Results ......................................................................................................................... 39 vii 2.5.1 Boreogadus saida versus Arctogadus glacialis .................................................. 39 2.5.2 Validation of the daily deposition of increments in the otoliths of polar cod ..... 40 2.5.3 Length-age relationships and growth rates.......................................................... 45 2.5.4 Interannual variability in regional hatch-date frequency distributions ............... 45 2.5.5 Average hatching season in relation to regional river input................................ 48 2.5.6 Pre-winter size ..................................................................................................... 48 2.6 Discussion .................................................................................................................. 53 2.6.1 The true age of the true polar cod ....................................................................... 53 2.6.2 The winter thermal refuge hypothesis ................................................................. 54 2.6.3 Winter hatching and the pre-winter size of polar cod juveniles .......................... 55 2.6.4 The food of polar cod larvae under the ice in winter .......................................... 57 2.6.5 Climate change and the hatching season of polar
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