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110 Vertical distribution during ontogeny . . . but induces some rare retention events were chosen because an ontogenetic shift was significant and larvae were abundant enough to estimate the probability density function of z cms in the three ontogenetic stages. After a larva was released, the straight-line distance between its starting point and current position was computed at each time step. These distances were compared to determine whether shifts in vertical distribution facilitate or impede drift. After six days, passive particles were on average 80 km away from their point of origin (past this point, some particles reached the domain boundaries and biased the estimation of drift distance). In all cases, vertical migration reduced the mean drift distance, albeit by very little: 2.3 km for Acanthuridae, 3.3 km for Serranidae, 4.7 km for Labridae, and 3.6 km for a vertical movement corresponding to that of the global coral reef fish community. These differences were even smaller when particles were allowed to move randomly within the depth range defined by the PDF, i.e. not enforcing smooth vertical migration. Passive particles were all advected away more or less at the same pace. They were only slowed down by their entrapment in transient eddies. By contrast, some migrating particles were brought at depth, behind the island, in areas where retention was much higher. As shown in Figure 5.7, some particles were actually retained within 10 km of the island for the whole eight days. For those few, the drift trajectory was very different from that of passive particles. Figure 5.7 Trajectories of vertically migrating particles advected around an isolated deep oceanic island for eight days. The island was 20 km in diameter. Particles were constrained in the upper 100 m. Most particles were advected away but a few were retained at depth, behind the island.
Discussion 111 5.5 Discussion 5.5.1 Vertical distribution The z cm analysis first highlighted that different families had contrasting vertical distributions and that it was the main force structuring the vertical assemblages of coral reef fish larvae. The only documented hypothesis regarding these taxonomic differences is that they could be caused by taxonomic variations in the minimum intensity of light required to feed 65 . However, based on those requirements alone, Apogonidae should be deeper in the water column than Pomacentridae for example, because their light sensitivity is higher. Yet, the opposite was observed here (Figure 5.3). Furthermore, Apogonidae were on average slightly older than Pomacentridae (pre-flexion, flexion, post-flexion ratios in Apogonidae: 15% 40% 45%, and Pomacentridae: 22% 52% 26%) so they should have been even deeper because visual sensitivity increases with age. The opposite position observed here is therefore not confounded by ontogeny. Light intensity may not be as prominent as is was supposed to be in shaping the vertical patterns of distribution in those families. Although there is currently no published information regarding the diet of coral reef fish larvae, all species are probably quite specific in their preferences (J. Llopiz, unpublished) and not likely to eat the same prey. If those prey are distributed differently, fish larvae would probably accumulate where their prey are abundant. Finally, not all species have the same swimming abilities or larval duration, and these various ecological strategies may also show through their vertical placements, because it affects dispersal trajectories 71,84,196 . Analyses were mostly inconclusive when conducted at taxonomic levels under family. Probably because larvae were difficult to identify to these levels, hence the sample sizes were small. Only more extensive sampling or other identification techniques (such as genetic barcoding 220 ) would have allowed to overcome this limitation. When these analyses were possible, however, they highlighted possible intra-family differences (in Acanthuridae and Serranidae). Such differences are to be expected because other behavioural characteristics, such as swimming speed, are known to be species-specific 221 . Beyond these taxonomic differences, physical factors usually observed to influence vertical positioning (depth of clines, time of day) had little influence here. The absence of significance in diel vertical migration is particularly intriguing given its prevalence in the literature 36,194 . Of course, the sampling strategy was not designed to capture daily migration of a specific group of individuals — it would have been more appropriate to sample repeatedly a single patch throughout one or several days. So the results here may be obscured by inter-patch variability. Furthermore, diel vertical migration was often described for late stage larvae or juvenile fishes 210,215 and may only occur in larvae older than those caught here, Yet, it was not detected more clearly in Large taxonomic variability Possible intra-family differences Not strong evidence for diel vertical migration
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À mon étoile.
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vi Résumé La plupart des organism
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viii Abstract Most demersal marine
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x Table des matières 1.3.3 Simple
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xii Table des matières 6.2.3 Examp
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xiv Liste des figures 4.6 Distribut
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Liste des tableaux 1.1 Potential or
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2 Introduction La survie des larves
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4 Introduction Limitation par le re
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6 Introduction Les courants déterm
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8 Introduction où m est le taux de
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10 Introduction biologique simple.
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12 Introduction Le milieu lui-même
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14 Introduction Figure I.6 Schémat
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16 Introduction obstacles technique
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18 Introduction Figure I.8 Prédict
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20 Introduction I.5 La biologie des
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22 Introduction du fonctionnement d
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24 Introduction L’objectif de cet
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26 Behaviour in models 1.1 Introduc
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28 Behaviour in models Using mean p
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30 Behaviour in models 1.3 Vertical
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32 Behaviour in models . . . bring
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34 Behaviour in models of measured
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36 Behaviour in models Additive dis
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38 Behaviour in models In situ stud
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40 Behaviour in models Operational
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42 Behaviour in models Effects on p
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44 Behaviour in models begins is no
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46 Behaviour in models 1.10 Conclus
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48 Larvae orientation in situ Diver
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50 Larvae orientation in situ . . .
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52 Larvae orientation in situ optio
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54 Larvae orientation in situ if >
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56 Larvae orientation in situ Table
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58 Larvae orientation in situ that
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Page 80 and 81: 62 Behaviour of settling fishes at
Page 87 and 88: Chapter 4 Biophysical correlates in
Page 89 and 90: Methods 71 4.2 Methods 4.2.1 Sampli
Page 91 and 92: Methods 73 Three rotations were com
Page 93 and 94: Methods 75 test really focuses on w
Page 95 and 96: Results 77 Figure 4.3 Mean of insta
Page 97 and 98: Results 79 Figure 4.5 Perspective v
Page 99 and 100: Results 81 Table 4.1 Abundances of
Page 101 and 102: Results 83 Figure 4.8 Distribution
Page 103 and 104: Results 85 Figure 4.10 Concentratio
Page 105 and 106: Discussion 87 Figure 4.11 Compariso
Page 107 and 108: Discussion 89 But the most likely e
Page 109 and 110: Acknowledgements 91 This evidence a
Page 111 and 112: Chapter 5 Ontogenetic vertical “m
Page 113 and 114: The statistical analysis of vertica
Page 119 and 120: Methods 101 80, 55, 30 m (Figure 4.
Page 121 and 122: Methods 103 same station were likel
Page 123 and 124: Results 105 Figure 5.2 Univariate r
Page 125 and 126: Results 107 5.4.3 Ontogenetic shift
Page 127: Results 109 Figure 5.5 Violin plot
Page 131 and 132: Discussion 113 expression of differ
Page 133 and 134: Chapter 6 Oceanography vs. behaviou
Page 135 and 136: Introduction 117 a good description
Page 137 and 138: A general modelling framework for l
Page 145 and 146: The balance between feeding and pre
Page 155 and 156: Oriented swimming and passive advec
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General discussion 161 such integra
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General discussion 163 6.5.2 Why se
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General discussion 165 such as the
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General discussion 167 energeticall
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Acknowledgements 169 Therefore: •
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172 Conclusion Un environnement pé
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174 Conclusion global sur la connec
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176 Conclusion l’intérêt est po
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178 Conclusion La différence entre
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180 Conclusion Mieux décrire la di
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182 Conclusion Comme tout comportem
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184 Undescribed Chaetodonthidae Fig
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186 Undescribed Chaetodonthidae Fig
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188 Bibliographie [12] Cushing DH.
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190 Bibliographie [39] Johnson MW.
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192 Bibliographie [71] Paris CB, Co
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194 Bibliographie [99] Lara MR. Mor
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196 Bibliographie [127] Masuda R, S
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198 Bibliographie [157] Holbrook SJ
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200 Bibliographie [187] Bowen B, Ba
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202 Bibliographie [218] Oxenford HA
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204 Bibliographie [246] Wellington
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206 Bibliographie [276] Sargent RC,
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208 Bibliographie [308] Greenberg D
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210 Remerciements solides pour ces
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212 Remerciements tri des larves de
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214 Remerciements Évidemment, tout
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