Abstract
Group-living is widespread in fish, and mono- or multi-specific assemblages can be found in a variety of environmental contexts. In streams, drift-feeding fish either display territorial behaviour or form shoals. They inhabit dynamic and heterogeneous three-dimensional environments, where micro-scale hydraulic features and food patchiness make spatial positioning critical for individual fitness. In this con- text, the spatial structure displayed by such groups is likely to have high ecological significance, as it would reflect mechanisms underlying intra-specific competition and resource distribution. In territorial fish such as salmonids, there is still an on- going debate about how intra-specific competition is underlying density-dependent growth and mortality. On another hand, empirical studies on intra-specific competition within non-territorial (or shoaling) species are very scarce, as the accurate description of such dynamic systems is challenging, in terms of collecting and analysing data. Here, I used a digital imaging technique (VidSync) to manually monitor movements and behaviour of individual fish, in 3-D, and at a high spatiotemporal level of resolution. I developed tools to study the statistical property of space-use of each individual, enabling the description and quantification of key features of the spatial structure of drift-feeding fish. I studied links between individual space-use strategy, feeding and social behaviour in order to investigate how the spatial structure of group-living drift-feeding fish could be related to its social organisation. This study was carried out on free-ranging shoals of juvenile drift-feeding fish (Galaxias anomalus) and on juvenile brown trout (Salmo trutta). The aim of this study was to investigate mechanisms underlying patterns of intra-specific competition within groups of territorial and non-territorial species, under different environmental contexts. Results highlights interspecific differences in competition regulation, but also provide unique insights on the economic underlying the spatial and social structure of these species.