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Summary form only given. Influence of Coulomb collisions on the wave propagation in plasmas is considered. It is demonstrated that the nonlinear coupling of the nonresonant wave with electric field fluctuations resonant with the plasma particles as well as contribution of the system nonstationarity induced by the collisions is crucial to obtain the correct conservation equation for the wave occupation number. In contrast to the occupation number, the canonical wave energy changes because of the nonstationarity of the system. Since the definition of the wave energy is not necessarily unique and depends on separation into background and wave subsystems, the result for the change of the canonical wave energy is not universal. Alternative way of separation into background and wave subsystems is discussed.