Much of my past work has focused on studying the interactions between food and space competition in stream-dwelling salmonid fish populations. Through a combination of observational field work and multi-factor experiments, I have examined how food and space requirements influence the demographic characteristics of a population. Population density, growth, emigration and the size structure of individuals in a population are often strongly food and density-dependent. However, my observations indicate that populations of animals that experience dramatic changes in mean body size, will also experience declines in density due to allometric increases in individual food and space requirements. This phenomenon describing a decrease in population density due to increasing space demands, known as self-thinning, has been widely studied in plant populations. Animal ecologists have only recently recognized the potential importance of this concept. My work provides the first experimental evidence supporting the idea of self-thinning in a mobile animal population that I know of. In conjunction with my colleagues from Quebec and New Brunswick, we recently published a conceptual paper on how the idea of self-thinning might be applied to wild populations and why biologists should consider space requirements when measuring the abundance of salmonid fishes in streams.