  Suppose that $X$ is a torus bundle over a closed surface with homologically
essential fibers. Let $X_K$ be the manifold obtained by Fintushel--Stern knot
surgery on a fiber using a knot $K\subset S^3$. We prove that $X_K$ has a
symplectic structure if and only if $K$ is a fibered knot. The proof uses
Seiberg--Witten theory and a result of Friedl--Vidussi on twisted Alexander
polynomials.
