par Bette, Simon 
Président du jury Aron, Serge
Promoteur Gilbert, Thomas
Co-Promoteur Deneubourg, Jean-Louis
Publication Non publié, 2019-09-13
Président du jury Aron, Serge
Promoteur Gilbert, Thomas
Co-Promoteur Deneubourg, Jean-Louis
Publication Non publié, 2019-09-13
Thèse de doctorat
| Résumé : | Collective motion in fish shoals have been studied for many years. Multiple shoal structures and configurations have been observed and indexed since the early 20th century. In the last decades, the development of tracking techniques allowed precise measurements of the shoal dynamics. Nevertheless, most of the studies based on those measurements mainly focus on few configurations (usually an organized and mobile configuration named school and a disorganized and static one named swarm) and do not integrate the variety of structures a shoal can adopt. Moreover, their discrimination methods for these configurations are not adapt to an accurate study of the alternation dynamics between these.Fish shoals take many advantage of the heterogeneities found in their environment (shelters, visual cues, meeting points... ). These heterogeneities can largely influence the shoal dynamics. Among these, the floating objects are the most studied due to economic issues related to the use of FADs (fish aggregating devices) in fishery. Nevertheless, only sparse data are collected in the field as many technical challenges has to be be tackled. The lack of precise measurements makes difficult to study the shoal dynamics under and around floating objects. Similar mechanisms can underlie the floating objects association of many fish species. Furthermore, small species in laboratory experiments associate with floating objects as well. This thesis presents a study of the influence of the floating objects and the population size on the shoal dynamics. In this way, a set of laboratory experiments has been conducted on zebrafish (Danio rerio) varying the number of individuals (from 5 to 25) and the number of floating objects (from 0 to 2). A tracking technique has been developed to detect the fishes under the objects and rebuild fish trajectories when one fish hides others. A thorough analysis of shoal measurements (alignment, speed,... ) allowed to establish a discrimination methodology of multiple shoal configurations (a total of seven) and to measure their frequencies and durations. Several configurations are less frequent than the school and the swarm that are the most observed. All the configurations are allocated to either a dynamic state (corresponding to a shoal displacement) or a static state. This division of the dynamics in two states allowed to investigate the influence of the different conditions on the lifetimes of the states and their alternation. Different factors influence the dynamics of the shoal states: the shoal size, the time and the floating objects.Regarding the shoal size, our results show that population size greatly influences the durations of the shoal states. The most populated shoals stay longer in every state and the fraction of time spent in the static state increases with population. Regarding the time, remarkably, the static and dynamic states' duration respectively increases and decreases over time so that the total duration of two successive events (a static state followed by a dynamic state, or vice versa) does not vary over time. Furthermore, during the dynamic state, the shoal speed decreases over time. We also observed that the individuals adopt a preferential direction in our circular tank although they often change direction. In the presence of a floating object, individuals tend to interrupt the dynamic state and favour a transition to the static state beneath it. A retention effect of the object is observed only for populated shoals with an increase of it with population. However, in the presence of two floating objects, the lifetime of the static states under the floating object is shorter and we observe a relatively fast alternation of the shoal location between the two objects. Furthermore, few shoal divisions between the two objects are observed. Although they rarely occur, these divisions are as stable in time as a stop of the whole shoal under a unique object. The methodology introduced here allows a precise study of the fish shoal dynamics in different social and environmental conditions. This thesis highlights numerous effects of the population, the floating objects and time on the shoal dynamics. Several new questionings emerge in conclusion and pave the way to multiple research perspectives. |
