Juvenile rainbow trout were studied to see how their territory size changes with population density. The researchers set up experiments with different amounts of food and found that territory size decreased as population density increased. In one experiment, territory size leveled off at a minimum size of 20-30 cm radius. This suggests that there is a limit to how many rainbow trout can live in a certain area, as they need a minimum amount of space to establish their territory. This could help regulate the population size of rainbow trout in streams.