Given a fibration of complex projective manifolds $f:X\rightarrow Y$ with general fiber $F$, if the stable base locus of $-K_X $ is vertical then a theorem of Chang establishes the inequality $\kappa(-K_X)\leq \kappa(-K_Y) +\kappa(-K_F)$. In this talk I am going to discuss a generalization of this result to fibrations in positive characteristic satisfying certain tameness conditions. This is based on a joint project with Marta Benozzo and Chi-Kang Chang.