  Observations of quasars at redshifts z > 6 reveal that 10^9 Msol supermassive
black holes (SMBHs) had already formed when the Universe was < 0.9 Gyr old. One
hypothesis for the origins of these SMBHs is that they grew from the remnants
of the first generation of massive stars, which formed in low-mass (~ 10^5 to
10^6 Msol) dark matter minihaloes at z > 20. This is the regime where baryonic
streaming motions--the relative velocities of baryons with respect to dark
matter in the early Universe--most strongly inhibit star formation by
suppressing gas infall and cooling. We investigate the impact of this effect on
the growth of the first SMBHs using a suite of high-fidelity,
ellipsoidal-collapse Monte Carlo merger-tree simulations. We find that the
suppression of seed BH formation by the streaming motions significantly reduces
the number density of the most massive BHs at z > 15, but the residual effect
at lower redshifts is essentially negligible. The streaming motions can reduce
by a factor of few the number density of the most luminous quasars at z ~
10-11, where such objects could be detected by the James Webb Space Telescope.
We conclude, with minor theoretical caveats, that baryonic streaming motions
are unlikely to pose a significant additional obstacle to the formation of the
observed high-redshift quasar SMBHs. Nor do they appreciably affect the heating
and reionization histories of the Universe or the merger rates of nuclear BHs
in the mass and redshift ranges of interest for proposed gravitational-wave
detectors.
