Let the weights in kilograms of the head and body of the fish be $h$ and $b$ respectively. Then $h=9+\frac{1}{3}b$ and $b=h+9$. So $b=9+\frac{1}{3}b+9$, that is, $\frac{2}{3}b=18$, which gives $b=27$. Hence $h=18$, so the whole fish weighed 54kg.