  We study nuclear shadowing in $J/\psi$ and $\Upsilon$ production in
hadron-nucleus interactions and in nucleus-nucleus collisions at the
Relativistic Heavy Ion Collider and the Large Hadron Collider. %We define the
regions in $x_f$ where nuclear shadowing begins %to set in for \jp\ and \up. As
a consequence of the perturbative $Q^2$-dependence of gluon shadowing, we
predict that $\Upsilon$ production is less suppressed than the $J/\psi$. We
show that antishadowing leads to enhanced \jp\ production at $x_f \lesssim 0$,
an effect reduced for $\Upsilon$ production.
