Shell correlations in the SU_q(2) rotor model

An SU_q(2) description of nuclear rotational spectra is compared with one previously proposed in the framework of the hybrid model energy formula. As a result an analytical correlation between the quantum algebraic parameter tau and the number of nucleon pairs in the valence shells N is established. It is shown that such a correlation actually applies in rotational regions 150<A<190 and A>220 and justifies the physical interpretation of tau in connection with nuclear softness.