  A rational map $\phi: \mathbb{P}_k^m \dashrightarrow \mathbb{P}_k^n$ is
defined by homogeneous polynomials of a common degree $d$. We establish a
linear bound in terms of $d$ for the number of $(m-1)$-dimensional fibers of
$\phi$, by using ideals of minors of the Jacobian matrix. In particular, we
answer affirmatively Question~11 in arXiv:1511.02933v2.
