Body Size and Local Density Explain Movement Patterns in Stream Fishes

Movement is a fundamental process in structuring communities, distributing species, and mediating gene flow. Both extrinsic (e.g., density of species) and intrinsic factors (e.g., body size) influence movement patterns, ultimately driving the spatial organization of ecological communities. However, these extrinsic and intrinsic factors are often assessed in isolation, limiting our ability to understand how multiple factors combine to shape movement patterns in nature. Here, we evaluate whether body size (intrinsic) and intra- and interspecific densities (extrinsic) have an impact on the movement rates of four fish species (<i>Nocomis leptocephalus</i> bluehead chub, <i>Semotilus atromaculatus</i> creek chub, <i>Lepomis cyanellus</i> green sunfish, and <i>L. auritus</i> redbreast sunfish) in a small stream. We employed a capture-mark-recapture framework to individually track movements, defined as the difference between locations on consecutive (re)captures. We then applied a dispersal-observation model that accounts for detectability, survival, and emigration when inferring movement processes. We found that larger individuals of creek chub and green sunfish were more likely to move, which may be explained by their greater physical ability to balance the energetic cost of moving in tandem with greater competitive ability during settlement. The effect of density on movement was mixed. Green sunfish moved away from areas with high density of creek chub, but movement declined when bluehead chub density was high. Bluehead chub responded reciprocally to green sunfish, with less movement at high green sunfish density. Movement also declined for creek chubs in the presence of bluehead chub. This may suggest that certain species interact due to predator-prey interactions either directly or indirectly. Collectively, our results suggest that intrinsic (body size) and extrinsic factors (density) influence movement patterns, but their relative importance is species-specific. Further exploring the mechanistic relationship behind drivers of movement will provide greater insights into spatial community dynamics.