  We consider the stirring of an inviscid fluid caused by the locomotion of
bodies through it. The swimmers are approximated by non-interacting cylinders
or spheres moving steadily along straight lines. We find the displacement of
fluid particles caused by the nearby passage of a swimmer as a function of an
impact parameter. We use this to compute the effective diffusion coefficient
from the random walk of a fluid particle under the influence of a distribution
of swimming bodies. We compare with the results of simulations. For typical
sizes, densities and swimming velocities of schools of krill, the effective
diffusivity in this model is five times the thermal diffusivity. However, we
estimate that viscosity increases this value by two orders of magnitude.
