  We derive macroscopic dynamics for self-propelled particles in a fluid. The
starting point is a coupled Vicsek-Stokes system. The Vicsek model describes
self-propelled agents interacting through alignment. It provides a
phenomenological description of hydrodynamic interactions between agents at
high density. Stokes equations describe a low Reynolds number fluid. These two
dynamics are coupled by the interaction between the agents and the fluid. The
fluid contributes to rotating the particles through Jeffery's equation.
Particle self-propulsion induces a force dipole on the fluid. After
coarse-graining we obtain a coupled Self-Organised Hydrodynamics (SOH)-Stokes
system. We perform a linear stability analysis for this system which shows that
both pullers and pushers have unstable modes. We conclude by providing
extensions of the Vicsek-Stokes model including short-distance repulsion,
finite particle inertia and finite Reynolds number fluid regime.
