Collective fish swimming in altered environment

Overview & Synopsis

This thesis is focused on collective motions in schools of fish. In particular, weattempted to shed light on how variations in environmental parameters can influence interactions between individuals within a group, and consequently the organisation of that group. Using simplified laboratory experiments, we varied theseenvironmental parameters in a controlled manner and observed the motions of groups of fish in response to these perturbations. The model species chosen forthese experiments is Hemigrammus rhodostomus, a small gregarious tropical fish.We were able to show that light plays a major role in the emergence of collective motion: in the absence of lighting, schools of around 50 fish were unable to swim in a coordinated fashion. Conversely, as the light intensity gradually increases, theschool goes through di_erent behavioural phases, with an initially highly polarised organisation followed by a state of rotation (milling) that is stable over time above a light level threshold. The limits of the swimming range also play a part in determining the collective state observed for a group of interacting agents. In the case of schools of 10 to 70fish, it has been shown that reducing the available swimming area and increasing the number of fish in the school have equivalent e_ects: the greater the confinement(measured by the density of fish per unit area), the greater the probability of moving from a polarised state to a rotating state. Finally, the behaviour of rheotaxis was studied, i.e. the aptitude of a fish or a group of fish to swim against the current. In particular, the ability to maintain a stable position for single fish and pairs of fish, considered as the minimum constitutive cell of a school, was measured during forced swimming experiments in aswim tunnel. The tests were repeated in illuminated and dark environments. Theresults suggest that flow sensing is used by fish in unlit conditions, but is not essential in lit conditions. In summary, this research underscores the role of light and spatial constraints in shaping collective behaviors but also sheds light on the interplay between environmental factors and group dynamics in schools of fish. These insights contribute to a deeper understanding of the mechanisms governing collective motion in complexbiological systems.