A note on Iitaka's conjecture $C_{3,1}$ in positive characteristics

Let $f:X\to Y$ be a fibration from a smooth projective 3-fold to a smooth projective curve, over an algebraically closed field $k$ of characteristic $p >5$. We prove that if the generic fiber $X_{\eta}$ has big canonical divisor $K_{X_{\eta}}$, then $$\kappa(X)\ge\kappa(Y) + \kappa(X_{\eta}).$$