DENSITY-DEPENDENT PROCESSES IN STRUCTURED FISH POPULATIONS: INTERACTION STRENGTHS IN WHOLE-LAKE EXPERIMENTS

Abstract

The goal of this study is to identify the mechanisms and measure the strengths of interactions within and among size classes in experimental populations of rainbow trout, Onchorynchus mykiss. The metric that we used to assess the density-dependent effects was based on consumptive allometry and predator–prey theory. We demonstrate that the interactions among size classes were asymmetrical, favoring larger-bodied individuals. Descriptions of diet and spatial resource use, measures of prey availability, and risk to intra-specific interactions allowed assessment of the relative contributions of exploitative and interference competitive interactions among size classes. Growth of the larger classes was strongly density-dependent and driven primarily by exploitative competition. Growth of the smallest size class was controlled by a combination of exploitative competition within and among size classes and interference competition with larger-bodied conspecifics. This combination of interactions among size classes within populations resulted in a body-size-based asymmetry favoring the larger size classes. Survival of all size classes was positively related to both body size and growth rate. We speculate that the net result of these processes within size-structured populations is compensatory, leading to stable population dynamics.