Predator fish are randomly distributed throughout the river initially, but move in such a way that habitat preferences are expressed. The model runs for a number of time steps to allow predators to develop their habitat preferences before outmigrant simulations begin. These migrants drift downstream following the river flow velocities and their own random swimming velocity. Higher random swimming velocities provide more dispersion of the migrants. If a migrant passes within the capture radius of a predator, it dies.
Migrants obtain the river velocity from the hydraulic model node that is closest to their location.
The random swimming component of migrant velocity is user-determined and has a constant magnitude. The frequency with which migrants change the random direction of their swimming velocity is independent of the movement time step. The user specifies the number of seconds that pass before a migrant chooses a new random swimming direction. Predation is a three-dimensional phenomenon. Each migrant and predator has a depth, and a migrant must be within a predator's capture radius over all three dimensions to be killed.