Ideal gas model adequately describes movement and school formation in a pelagic freshwater fish

Abstract

Movement is a fundamental aspect of the population and community ecology of many organisms, yet, until recently, it has been difficult to measure in the wild. Consequently, simple assumptions are often used to represent movement; a key assumption found in many classic theoretical ecological models (e.g., predator–prey interactions) is that organisms move like ideal gas particles. Here, we test whether this assumption adequately describes the movement of the Cisco (Coregonus artedi) and its schools using fisheries acoustic surveys and mathematical models. We find that several of the individual components of an ideal gas model (IGM) have some inconsistencies with Cisco behavior, yet overall patterns of school formation are close to IGM expectations. For both individual fish and schools: 1) the spatial distributions were random or slightly clumped; 2) the swimming speed distributions were unimodal but significantly different from normal; 3) horizontal movement was more frequent than depth changes; and 4) movement trajectories across the acoustic beam sometimes deviated from straight lines. However, including the average individual and school swimming speeds and known nighttime densities in an IGM generated values that were similar to the observed values for: 1) the time required for schools to form in the morning and 2) school encounter rates.